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Sustainable biochar to mitigate global climate change

Sustainable biochar to mitigate global climate change ARTICLE DOI: 10.1038/ncomms1053 Received 29 Oct 2009 | Accepted 14 Jul 2010 | Published 10 Aug 2010 Sustainable biochar to mitigate global climate change 1 2 1 3 4 Dominic Woolf , James E. Amonette , F. Alayne Street-Perrott , Johannes Lehmann & Stephen Joseph Production of biochar (the carbon (C)-rich solid formed by pyrolysis of biomass) and its storage in soils have been suggested as a means of abating climate change by sequestering carbon, while simultaneously providing energy and increasing crop yields. Substantial uncertainties exist, however, regarding the impact, capacity and sustainability of biochar at the global level. In this paper we estimate the maximum sustainable technical potential of biochar to mitigate climate change. Annual net emissions of carbon dioxide (CO ), methane and nitrous oxide could be reduced by a maximum of 1.8 Pg CO -C equivalent (CO -C ) per 2 2 e year (12 % of current anthropogenic CO -C emissions; 1 Pg = 1 Gt), and total net emissions 2 e over the course of a century by 130 Pg CO -C , without endangering food security, habitat or 2 e soil conservation. Biochar has a larger climate-change mitigation potential than combustion of the same sustainably procured biomass for bioenergy, except when fertile soils are amended while coal is the fuel being offset. 1 2 School of the Environment and Society, Swansea University, Singleton Park , Swansea SA2 8PP , UK . Chemical and Materials Sciences Division, Pacifi c Northwest National Laboratory , Richland , Washington 99352, USA . Department of Crop and Soil Sciences, College of Agriculture and Life Sciences, Cornell University , Ithaca , New York 14853 , USA . The School of Materials Science and Engineering, University of New South Wales , Sydney, New South Wales 2052 , Australia . Correspondence and requests for materials should be addressed to J.E.A. (email: jim.amonette@pnl.gov ) . NATURE COMMUNICATIONS | 1:56 | DOI: 10.1038/ncomms1053 | www.nature.com/naturecommunications 1 © 2010 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1053 ince 2000, anthropogenic carbon dioxide (CO ) emissions able bioenergy; it can improve agricultural productivity, particularly have risen by more than 3 % annually , putting Earth ’ s eco- in low-fertility and degraded soils where it can be especially useful Ssystems on a trajectory towards rapid climate change that is to the world ’ s poorest farmers; it reduces the losses of nutrients and both dangerous and irreversible . To change this trajectory, a timely agricultural chemicals in run-off ; it can improve the water-holding 17,18 and ambitious programme of mitigation measures is needed. Several capacity of soils; and it is producible from biomass waste . Of the studies have shown that, to stabilize global mean surface temperature, possible strategies to remove CO from the atmosphere, biochar is cumulative anthropogenic greenhouse-gas (GHG) emissions must notable, if not unique, in this regard. be kept below a maximum upper limit, thus indicating that future Biochar can be produced at scales ranging from large industrial 2 – 6 19 net anthropogenic emissions must approach zero . If humanity facilities down to the individual farm , and even at the domestic oversteps this threshold of maximum safe cumulative emissions level , making it applicable to a variety of socioeconomic situations. (a limit that may already have been exceeded ), no amount of Various pyrolysis technologies are commercially available that yield emissions reduction will return the climate to within safe bounds. diff erent proportions of biochar and bioenergy products, such as Mitigation strategies that draw down excess CO from the atmos- bio-oil and syngas. Th e gaseous bioenergy products are typically phere would then assume an importance greater than an equivalent used to generate electricity; the bio-oil may be used directly for low- reduction in emissions. grade heating applications and, potentially, as a diesel substitute Production of biochar, in combination with its storage in soils, aft er suitable treatment . Pyrolysis processes are classifi ed into two has been suggested as one possible means of reducing the atmos- major types, fast and slow, which refer to the speed at which the pheric CO concentration (refs 8 – 13 and see also Supplementary biomass is altered. Fast pyrolysis, with biomass residence times of Note for a history of the concept and etymology of the term). Bio- a few seconds at most, generates more bio-oil and less biochar than char ’ s climate-mitigation potential stems primarily from its highly slow pyrolysis, for which biomass residence times can range from 14 – 16 recalcitrant nature , which slows the rate at which photosyntheti- hours to days. cally fi xed carbon (C) is returned to the atmosphere. In addition, Th e sustainable-biochar concept is summarized in Figure 1 . CO biochar yields several potential co-benefi ts. It is a source of renew- is removed from the atmosphere by photosynthesis. Sustainably CO removed CO by returned photosynthesis IMPACT OUTPUTS APPLICATIONS INPUTS Biofuel CO emissions Bio-oil Rice Syngas Energy Process heat Other cereals Sugar cane PROCESS Avoided Avoided Avoided Manures fossil CO soil 2 biomass decay emissions Avoided CH /N O Biomass crops 4 2 Pyrolysis Soil Biochar Stored C amendment Agroforestry Oxidation, soil C, tillage, transport Felling loss Enhanced primary productivity Figure 1 | Overview of the sustainable biochar concept. The fi gure shows inputs, process, outputs, applications and impacts on global climate . Within each of these categories, the relative proportions of the components are approximated by the height / width of the coloured fi elds. CO is removed from the atmosphere by photosynthesis to yield biomass. A sustainable fraction of the total biomass produced each year, such as agricultural residues, biomass crops and agroforestry products, is converted by pyrolysis to yield bio-oil, syngas and process heat, together with a solid product, biochar, which is a recalcitrant form of carbon and suitable as a soil amendment. The bio-oil and syngas are subsequently combusted to yield energy and CO . This energy and the process heat are used to offset fossil carbon emissions, whereas the biochar stores carbon for a signifi cantly longer period than would have occurred if the original biomass had been left to decay. In addition to fossil energy offsets and carbon storage, some emissions of methane and nitrous oxide are avoided by preventing biomass decay (see Supplementary Table S5 for example) and by amending soils with biochar. Additionally, the removal of CO by photosynthesis is enhanced by biochar amendments to previously infertile soils, thereby providing a positive feedback. CO is returned to the 2 2 atmosphere directly through combustion of bio-oil and syngas, through the slow decay of biochar in soils, and through the use of machinery to transport biomass to the pyrolysis facility, to transport biochar from the same facility to its disposal site and to incorporate biochar into the soil. In contrast to bioenergy, in which all CO that is fi xed in the biomass by photosynthesis is returned to the atmosphere quickly as fossil carbon emissions are offset, biochar has the potential for even greater impact on climate through its enhancement of the productivity of infertile soils and its effects on soil GHG fl uxes. 2 NATURE COMMUNICATIONS | 1:56 | DOI: 10.1038/ncomms1053 | www.nature.com/naturecommunications © 2010 Macmillan Publishers Limited. All rights reserved. Biomass crops, Agricultural agroforestry residues Net avoided emissions NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1053 ARTICLE procured crop residues, manures, biomass crops, timber and for- production. Of primary importance is the conversion of land to estry residues, and green waste are pyrolysed by modern techno- generate feedstock. In addition to its negative eff ects on ecosystem logy to yield bio-oil, syngas, process heat and biochar. As a result conservation, land clearance to provide feedstock may also release of pyrolysis, immediate decay of these biomass inputs is avoided. carbon stored in soils and biomass, leading to unacceptably high Th e outputs of the pyrolysis process serve to provide energy, avoid carbon-payback times before any net reduction in atmospheric CO is emissions of GHGs such as methane (CH ) and nitrous oxide achieved (ref. 25, Supplementary Methods and Supplementary (N O), and amend agricultural soils and pastures. Th e bioenergy Fig. S2 ). For example, we fi nd that a land-use change carbon debt − 1 is used to off set fossil-fuel emissions, while returning about half of greater than 22 Mg C ha (an amount that would be exceeded by the C fi xed by photosynthesis to the atmosphere. In addition to the conversion of temperate grassland to annual crops ) will result in GHG emissions avoided by preventing decay of biomass inputs, a carbon-payback time that is greater than 10 years. Clearance of soil emissions of GHGs are also decreased by biochar amendment rainforests to provide land for biomass-crop production leads to to soils. Th e biochar stores carbon in a recalcitrant form that can carbon payback times in excess of 50 years. Where rainforest on increase soil water- and nutrient-holding capacities, which typi- peatland is converted to biomass-crop production, carbon-payback cally result in increased plant growth. Th is enhanced productivity times may be in the order of 325 years. We therefore assume that no is a positive feedback that further enhances the amount of CO land clearance will be used to provide biomass feedstock, nor do we removed from the atmosphere. Slow decay of biochar in soils, include conversion of agricultural land from food to biomass-crop together with tillage and transport activities, also returns a small production as a sustainable source of feedstock, both because of the amount of CO to the atmosphere. A schematic of the model used negative consequences for food security and because it may indirectly to calculate the magnitudes of these processes is shown as Sup- induce land clearance elsewhere . Some dedicated biomass-crop plementary Figure S1 . production on abandoned, degraded agricultural soil has been Even under the most zealous investment programme, biochar included in this study as this will not adversely aff ect food security 28,29 production will ultimately be limited by the rate at which biomass and can improve biodiversity . We further assume that extraction can be extracted and pyrolysed without causing harm to the bio- rates of agricultural and forestry residues are suffi ciently low to sphere or to human welfare. Globally, human activity is responsible preclude soil erosion or loss of soil function, and that no industrially for the appropriation of 16 Pg C per year from the biosphere, which treated waste biomass posing a risk of soil contamination will be used. corresponds to 24 % of potential terrestrial net primary productiv- Other constraints on biochar production methods arise because ity (NPP) . Higher rates of appropriation will increase pressure on emissions of CH , N O, soot or volatile organic compounds combined 4 2 global ecosystems, exacerbating a situation that is already unsus- with low biochar yields (for example, from traditional charcoal kilns tainable . or smouldering slash piles) may negate some or all of the carbon- Th e main aim of this study is to provide an estimate of the theo- sequestration benefi ts, cause excessive carbon-payback times or be retical upper limit, under current conditions, to the climate-change detrimental to health. Th erefore, we do not consider any biochar pro- mitigation potential of biochar when implemented in a sustain- duction systems that rely on such technologies, and restrict our analy- able manner. Th is limit, which we term the maximum sustainable sis to systems in which modern, high-yield, low-emission pyrolysis technical potential (MSTP), represents what can be achieved when technology can feasibly be used to produce high-quality biochar. the portion of the global biomass resource that can be harvested Within these constraints, we derived a biomass-availability sce- sustainably (that is, without endangering food security, habitat or nario for our estimate of MSTP, as well as two additional scenarios, soil conservation) is converted to biochar by modern high-yield, Alpha and Beta, which represent lower demands on global biomass low-emission, pyrolysis methods. Th e fraction of the MSTP that is resources ( Table 1 ). Attainment of the MSTP would require sub- actually realized will depend on a number of socioeconomic fac- stantial alteration to global biomass management, but would not tors, including the extent of government incentives and the relative endanger food security, habitat or soil conservation. Th e Alpha sce- emphasis placed on energy production relative to climate-change nario restricts biomass availability to residues and wastes available mitigation. Aside from assuming a maximum rate of capital invest- using current technology and practices, together with a moderate ment that is consistent with that estimated to be required for cli- amount of agroforestry and biomass cropping. All three scenarios mate-change mitigation , this study does not take into account any represent fairly ambitious projects, and require progressively greater economic, social or cultural barriers that might further limit the levels of political intervention to promote greater adoption of sus- adoption of biochar technology. tainable land-use practices and increase the quantity of uncontami- Our analysis shows that sustainable global implementation of nated organic wastes available for pyrolysis. We do not consider any biochar can potentially off set a maximum of 12 % of current anthro- scenarios that are not ambitious in this study, as the intention is to pogenic CO -C equivalent (CO -C ) emissions (that is, 1.8 Pg C O - investigate whether biochar could make a substantial contribution 2 2 e 2 C per year of the 15.4 Pg CO -C emitted annually), and that over to climate-change mitigation— an aspiration that certainly will not e 2 e the course of a century, the total net off set from biochar would be be accomplished by half-hearted measures. Th e range of mitigation 130 Pg CO -C . We also show that conversion of all sustainably results reported thus refers only to the scenarios considered and does 2 e obtained biomass to maximize bioenergy, rather than biochar, pro- not encompass the full range of less-eff ective outcomes correspond- duction can off set a maximum of 10 % of the current anthropogenic ing to varying levels of inaction. Th e scenarios are based on current CO -C emissions. Th e relative climate-mitigation potentials of bio- biomass availability ( Supplementary Methods and Supplementary 2 e char and bioenergy depend on the fertility of the soil amended and Tables S1 and S2 ), the composition and energy contents of diff erent the C intensity of the fuel being off set, as well as the type of biomass. types of biomass and the biochar derived from each ( Supplementary Locations at which the soil fertility is high and coal is the fuel being Tables S3 and S4 ), and the rate of adoption of biochar technology off set are best suited for bioenergy production. Th e climate-miti- ( Supplementary Fig. S3 ). How this biomass resource base changes gation potential of biochar (with combined energy production) is over the course of 100 years will depend on the potential eff ects of higher for all other situations. changing climate, atmospheric CO , sea level, land use, agricultural practices, technology, population, diet and economic development. Results Some of these factors may increase biomass availability and some Sustainable biomass-feedstock availability . To ensure that our may decrease it. A full assessment of the wide range of possible future estimates represent a sustainable approach, we use a stringent scenarios within plausible ranges of these factors remains outside the set of criteria to assess potential feedstock availability for biochar scope of this study. NATURE COMMUNICATIONS | 1:56 | DOI: 10.1038/ncomms1053 | www.nature.com/naturecommunications 3 © 2010 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1053 Table 1 | Annual globally sustainable biomass feedstock availability. Biomass available in scenario (Pg C per year) Maximum sustainable technical Alpha Beta potential Rice 0.22 0.25 0.28 Rice husks and 70 % of paddy rice straw Rice husks and 80 % of paddy rice straw Rice husks and 90 % of paddy rice not used for animal feed not used for animal feed straw not used for animal feed Other cereals 0.072 0.13 0.18 8 % of total straw and stover (assumes 14 % of total straw and stover (35 % 20 % of total straw and stover (45 % 25 % extraction rate of crop residues minus extraction rate minus animal feed) extraction rate minus animal feed) quantity used as animal feed) Sugar cane 0.09 0.11 0.13 Waste bagasse plus 25 % of fi eld trash Waste bagasse plus 50 % of fi eld trash Waste bagasse plus 75 % of fi eld trash Manures 0.10 0.14 0.19 12.5 % of cattle manure plus 50 % of pig 19 % of cattle manure plus 70 % of pig and 25 % of cattle manure plus 90 % of and poultry manure poultry manure pig and poultry manure Biomass crops 0.30 0.45 0.60 50 % of potential production of abandoned, 75 % of potential production of abandoned, 100 % of potential production of degraded cropland that is not in other use degraded cropland that is not in other use abandoned, degraded cropland that is not in other use Forestry residues 0.14 0.14 0.14 44 % of difference between reported fellings and extraction Agroforestry 0.06 0.34 0.62 17 Mha of tropical silvopasture 85 Mha of tropical grass pasture converted 170 Mha of tropical grass pasture to silvopasture converted to silvopasture Green / wood waste 0.029 0.085 0.14 75 % of low-end estimate of yard- Alpha plus mid-range estimate of yard Beta plus high-end estimate of trimmings production and wood-milling trimmings plus urban food waste, including global yard trimmings and food residues 40 % of waste sawnwood (legislation waste, including 80 % of waste required to ensure that this fraction of waste sawn wood wood is free of harmful contaminants) Total 1.01 1.64 2.27 Avoided GHG emissions . Results for the three scenarios are Of the adverse feedbacks, biochar decomposition is the largest expressed below as a range from the Alpha scenario fi rst to the (8 – 17 Pg CO -C ), followed by loss of soil organic carbon due to diver- 2 e MSTP last. Th e model predicts that maximum avoided emissions sion of biomass from soil into biochar production (6 – 10 Pg CO -C ), 2 e of 1.0 – 1.8 Pg CO -C per year are approached by mid-century and and transport (1.3 – 1.9 Pg CO -C , see Supplementary Fig. S7 ). Contri- 2 e 2 e that, aft er a century, the cumulative avoided emissions are 66 – 130 Pg butions to the overall GHG budget from tillage (0.03 – 0.044 Pg CO - CO -C ( Fig. 2 ). Half of the avoided emissions are due to the net car- C ) and reduced N-fertilizer production (0.2 – 0.3 Pg CO -C ) are neg- 2 e e 2 e bon sequestered as biochar, 30 % to replacement of fossil-fuel energy ligible (although their fi nancial costs may not be). by pyrolysis energy and 20 % to avoided emissions of CH and N O. Th e relative importance of all these factors to the GHG budget 4 2 Cumulative and annual avoided emissions for the individual gases varies considerably among feedstocks. Notably, rice residues, green CO , CH and N O are given in Supplementary Figures S4 – S6 . waste and manure achieve the highest ratios of avoided CO -C emis- 2 4 2 2 e A detailed breakdown of the sources of cumulative avoided sions per unit of biomass-carbon (1.2 – 1.1, 0.9 and 0.8 CO -C / C, 2 e GHG emissions over 100 years is given in Figure 3 . The two respectively) because of the benefi ts of avoided CH emissions. most important factors contributing to the avoided emissions from biochar are carbon stored as biochar in soil (43 – 94 Pg Sensitivity and Monte Carlo analyses . Sensitivity and Monte Carlo CO -C ) and fossil-fuel offsets from coproduction of energy analyses with respect to reasonable values of key variables were used 2 e (18 – 39 Pg CO -C ). to estimate the uncertainty of the model results; they suggest areas 2 e Of the benefi cial feedbacks, the largest is due to avoided CH in which future research is most needed and provide guidance on emissions from biomass decomposition (14 – 17 Pg CO -C ), pre- how biochar production systems might be optimized ( Fig. 4 ). 2 e dominantly arising from the diversion of rice straw from paddy Th e strongest sensitivity is to the half-life of the recalcitrant frac- fi elds (see Supplementary Table S5 for estimate of the mean CH tion of biochar (see also Supplementary Table S6 ). Net avoided GHG emission factor). Th e next largest positive feedbacks, in order emissions vary by − 22 % to + 4 % from that obtained using the base- of decreasing magnitude, arise from biochar-enhanced NPP on line assumption of 300 years. However, most of this variation occurs cropland, which contributes 9 – 16 Pg CO -C to the net avoided for half-life < 100 years, in which range we fi nd (in agreement with 2 e emissions (if these increased crop residues are converted to bio- previous work ) that sensitivity to this factor is high. Conversely, char), followed by reductions in soil N O emissions (4.0 – 6.2 Pg for a more realistic half-life of the recalcitrant fraction ( > 100 years), CO -C ), avoided N O emissions during biomass decomposition sensitivity to this factor is low because biochar can be produced 2 e 2 (1.8 – 3.3 Pg CO -C ) and enhanced CH oxidation by dry soils much more rapidly than it decays. As currently available data sug- 2 e 4 (0.44 – 0.8 Pg CO -C ). gest that the half-life of biochar ’ s recalcitrant fraction in soil is in the 2 e 4 NATURE COMMUNICATIONS | 1:56 | DOI: 10.1038/ncomms1053 | www.nature.com/naturecommunications © 2010 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1053 ARTICLE 2.0 default values assumed in this study up to the higher rate of 5 % 33,34 Annual suggested by more recent work . Th is would increase the net avoided GHG emissions by up to 8 % . Uncertainty in the response of crop yields to biochar amend- 1.5 ment results in estimated range of − 6 % to + 7 % in the impact of enhanced NPP of cropland on net avoided GHG emissions. Sensitivities to the pyrolysis energy effi ciency ( ± 5 % ), to the half- life of the biochar ’ s labile fraction ( − 4 % to + 1 % ) and to its impact 1.0 on soil CH oxidation ( ± 1 % ) are small. Th e net eff ect of covariance of the above factors was assessed using the Monte Carlo analysis ( n = 1,000, Supplementary Table S7 ). Despite limited data on the decomposition rate of biochar in soils 0.5 and the eff ects of biochar additions on soil GHG fl uxes, sensitiv- ity within realistic ranges of these parameters is small, resulting in an estimated uncertainty of ± 8 to 10 % ( ± 1 s.d.) in the cumulative 0 avoided GHG emissions for the three scenarios. Cumulative Comparison of biochar and bioenergy approaches . Th e mitiga- tion impact of the renewable energy obtained from both biochar MSTP-Biochar production and biomass combustion depends on the carbon inten- MSTP-Combustion sity (that is, the mass of carbon emitted per unit of total energy Beta-Biochar produced) of the off set energy sources . At our baseline carbon inten- Beta-Combustion − 1 sity (17.5 kg C GJ ; see Methods section), the model predicts that, on Alpha-Biochar an average, the mitigation impact of biochar is 27 – 22 % (14 – 23 Pg Alpha-Combustion CO -C ) larger than the 52 – 107 Pg CO -C predicted if the same sus- 2 e 2 e tainably procured biomass were combusted to extract the maximum amount of energy ( Fig. 2 ). Th is advantage of biochar over bioenergy is largely attributable to the benefi cial feedbacks from enhanced crop yields and soil GHG fl uxes ( Fig. 3 , Supplementary Fig. S8 ). Because the principal contribution of biomass combustion to avoided GHG emissions is the replacement of fossil fuels ( Fig. 3 ), the bioenergy approach shows a considerably higher sensitivity to 0 20406080 100 carbon intensity than does biochar ( Fig. 5 ). Th e carbon intensity of off set energy varies from near-zero for renewable and nuclear energy Time (y) − 1 35 to 26 kg C GJ for coal combustion . Mean cumulative avoided emis- sions from biochar and biomass combustion are equal in our sce- Figure 2 | Net avoided GHG emissions. The avoided emissions are − 1 attributable to sustainable biochar production or biomass combustion over narios when the carbon intensity of off set energy is 26 – 24 kg C GJ ( Fig. 5 ). In the MSTP scenario, this corresponds to an energy mix to 100 years, relative to the current use of biomass. Results are shown for the three model scenarios, with those for sustainable biochar represented which coal combustion contributes about 80 % , whereas in the Alpha scenario, the mean mitigation benefi t of biochar remains higher than by solid lines and for biomass combustion by dashed lines. The top panel shows annual avoided emissions; the bottom panel, cumulative avoided that of bioenergy, even when 100 % coal is off set. Th e cumulative avoided emissions from both strategies decrease as the carbon inten- emissions. Diamonds indicate transition period when biochar capacity of the top 15 cm of soil fi lls up and alternative disposal options are needed. sity of the off set energy mix decreases, but the rate of decrease for biomass combustion is 2.5 – 2.7 times greater than that for biochar. As expected, the cumulative avoided emissions for biomass combustion millennial range (see Supplementary Methods , Supplementary Table are essentially zero when the carbon intensity of the energy mix is also S6 and refs 8, 15, 16, 31), the contribution of its decay to the net zero. In contrast, the cumulative avoided emissions for biochar are GHG balance over centennial timescales is likely to be small. still substantial at 48 – 91 Pg CO -C . 2 e Th e next largest sensitivity is to the pyrolysis carbon yield ( − 9 % Given that much of the increased climate mitigation from bio- to + 11 % ), indicating the importance of engineering to optimize for char relative to biomass combustion stems from the benefi cial high yields of biochar rather than for energy production. Th is will feedbacks of adding biochar to soil, and that these feedbacks will be constrained, however, by the sensitivity to the labile fraction of be greatest on the least fertile soils, the relative mitigation poten- the biochar ( − 7 % to + 4 % ), which indicates the importance of opti- tials will vary regionally with soil type (see Supplementary Meth- mizing for production of recalcitrant biochar rather than for higher ods , Supplementary Fig. S9 and Supplementary Tables S8 – S11 for yields of lower-quality biochar. an account of how these feedbacks are calculated). Th e distribution Aft er carbon yield, the next largest sensitivity is to the carbon of soils of varying fertility on global cropland is shown in Figure intensity of the fuel off set by pyrolysis energy production, with net 6 . Globally, 0.31 Gha of soils with no fertility constraints are in use avoided emissions varying by − 4 % from the baseline assumption as cropland, as well as 0.29 Gha of cropland with few fertility con- when natural gas is the fuel being off set and by + 15 % when coal straints, 0.21 Gha with slight constraints, 0.32 Gha with moderate is off set. constraints, 0.18 Gha with severe constraints, 0.13 Gha with very Varying the impact of biochar amendment on soil N O emissions severe constraints and 0.09 Gha of cropland on soils categorized as from zero to the largest reported reduction (80 % ; ref. 32) produces unsuitable for crop production. Th e amount of biomass produced a sensitivity of − 4 % to + 11 % . Further variability in the impact in soils of diff erent fertilities is shown in Supplementary Table S12 . of biochar on N O emissions arises from adjusting the fraction Figure 7 shows how the climate mitigation from biochar varies rela- of biomass-N that (if left to decompose) would be converted to tive to biomass combustion when both soil fertility and the carbon N O-N, from the Intergovernmental Panel on Climate Change intensity of energy off sets are considered. Th e relative benefi t of NATURE COMMUNICATIONS | 1:56 | DOI: 10.1038/ncomms1053 | www.nature.com/naturecommunications 5 © 2010 Macmillan Publishers Limited. All rights reserved. Annual net avoided emissions (Pg CO -C ) 2 e Enhanced NPP Green/wood waste Agroforestry Forestry residues Biomass crops Manures Sugar cane Other cereals Rice ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1053 CH Biomass CH Soil N O Biomass N O Soil Fossil fuel offset 4 4 2 2 Soil organic C Transportation and tillage C in Biochar Biochar decomposition 40 200 MSTP Biochar 32 160 24 120 16 80 8 40 0 0 -8 -40 Beta 32 160 24 120 16 80 8 40 32 160 Alpha 24 120 16 80 8 40 0 0 -8 -40 Totals Figure 3 | Breakdown of cumulative avoided GHG emissions (Pg CO -C ) from sustainable biochar production. The data are for the three model 2 e scenarios over 100 years by feedstock and factor. The left side of the fi gure displays results for each of eight feedstock types and the additional biomass residues that are attributed to NPP increases from biochar amendments; the right side displays total results by scenario for both biochar (left column) and biomass combustion (right column). For each column, the total emission-avoiding and emission-generating contributions are given, respectively, by the height of the columns above and below the zero line. The net avoided emissions are calculated as the difference between these two values. Within each column, the portion of its contribution caused by each of six emission-avoiding mechanisms and three emission-generating mechanisms is shown by a different colour. These mechanisms (from top to bottom within each column) are (1) avoided CH from biomass decay, (2) increased CH oxidation by 4 4 soil biochar, (3) avoided N O from biomass decay, (4) avoided N O caused by soil biochar, (5) fossil fuel offsets from pyrolysis energy production, 2 2 (6) avoided CO emissions from carbon stored as biochar, (7) decreased carbon stored as soil organic matter caused by diversion of biomass to biochar, (8) CO emissions from transportation and tillage activities and (9) CO emissions from decomposition of biochar in soil. 2 2 producing biochar compared with biomass combustion is greatest feedstocks ( Fig. 7b ), because avoided CH emissions from the use of when biochar is added to marginal lands and the energy produced manure, green waste and rice residues occur regardless of whether by pyrolysis is used to off set natural gas, renewable or nuclear these other feedstocks are used for energy or biochar. energy. When biochar is added to the most infertile cropland to off set the current global primary energy mix ( M ), which has a car- Discussion − 1 bon intensity of 16.5 kg C GJ , the relative benefi t from biochar is as Our analysis demonstrates that sustainable biochar production much as 79 – 64 % greater than that from bioenergy ( Fig. 7 , Supple- (with addition to soils) has the technical potential to make a substan- mentary Fig. S10 ). Th is net benefi t diminishes as more coal is off set tial contribution to mitigating climate change. Maximum avoided and as biochar is added to soils with higher fertility. Nevertheless, emissions of the order of 1.8 Pg CO -C annually, and of 130 Pg 2 e with the exception of those geographical regions having both natu- CO -C over the course of a century, are possible at current levels 2 e rally high soil fertility and good prospects for off setting coal emis- of feedstock availability, while preserving biodiversity, ecosystem sions (in which bioenergy yields up to 16 – 22 % greater mitigation stability and food security. impact than biochar), biochar shows a greater climate-mitigation Th e biochar scenarios described here, with their very high levels potential than bioenergy. Th e relative benefi t of producing biochar of biomass utilization, are not compatible with simultaneous imple- compared with bioenergy is greatest when biomass crops are used as mentation of an ambitious biomass energy strategy. Th e opportunity 6 NATURE COMMUNICATIONS | 1:56 | DOI: 10.1038/ncomms1053 | www.nature.com/naturecommunications © 2010 Macmillan Publishers Limited. All rights reserved. Cumulative avoided emissions (Pg CO -C ) 2 e Biochar Combustion Cumulative avoided emissions (Pg CO -C ) 2 e NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1053 ARTICLE 100 200 300 Half-life recalcitrant C (y) 50 1000 MSTP Biochar Pyrolysis C yield (%) 40 62 Combustion -1 15 26 C Intensity of fuel offset (kg C GJ ) Decrease in soil N O emissions (%) 0 80 Beta Cropland NPP (% yield response) 50 150 30 5 Labile-C fraction (%) Alpha Global N O emission factor (%) 1.05 5 65 85 Pyrolysis energy efficiency (%) 20 50 1 25 Half-life labile C (y) 0 200 Soil CH oxidation (mg CH per m 4 4 per year) -20 -10 0 10 20 Deviation from reported estimate (%) 5 1015202530 -1 C Intensity of fuel offset (kg C GJ ) Figure 4 | Sensitivity of the model to key variables. Sensitivity is expressed as a percentage deviation from the reported value of cumulative Figure 5 | Cumulative mitigation potential (100 years) of biochar and net avoided GHG emissions over 100 years for each scenario. Top (blue), biomass combustion as a function of carbon intensity of the type of middle (yellow) and bottom (red) bars for each variable correspond to energy being offset. The black vertical dashed line labelled M on the Alpha, Beta and MSTP scenarios. Minimum and maximum values for upper x axis refers to the carbon intensity of the baseline energy mix each variable are at the ends of the bars (with additional sensitivities to − 1 assumed in this study. Grey vertical dashed lines at 15, 19 and 26 kg C GJ recalcitrant carbon half-life of 100 and 200 years shown); baseline values denote the carbon intensity of natural gas, oil and coal, respectively. The of the key variables used in this study correspond to 0 % deviation. − 1 carbon intensity of renewable forms of energy is close to 0 kg C GJ . See also Supplementary Table S7 . cost of this forgone energy resource must be taken into account in feedstock procurement, transport, pyrolysis, energy production, soil incorporation, soil GHG fl ux, soil fertility and fertilizer use (see also Supplementary Table S13 ), an economic comparison of the two strategies. However, in terms and biomass and biochar decomposition (see also Supplementary Fig. S11 ). Th e net of their potentials for climate-change mitigation, the mitigation avoided GHG emissions due to biochar were calculated as the diff erence between impact of biochar is about one-fourth larger, on an average, than that the CO -equivalent emissions from biochar production and those that would have obtained if the same biomass were combusted for energy. Regional occurred as the biomass decomposed by other means had it not been converted to biochar. All emissions (actual or avoided) were calculated with time dependency. deviations from this average are large because of diff erences in soil Wherever possible, conservative assumptions were used to provide a high degree of fertility and available biomass. Our model predicts that the relative confi dence that our results represent a conservative estimate of the avoided GHG climate-mitigation benefi t of biochar compared with bioenergy is emissions achievable in each scenario. A detailed account of both the model and greatest in regions in which poor soils growing biomass crops can the three scenarios is given in Supplementary Methods . benefi t most from biochar additions. In contrast, biomass combustion Sustainability criteria . Biochar can be produced sustainably or unsustainably. leads to a greater climate-mitigation impact in regions with fertile Our criteria for sustainable biochar production require that biomass procured from soils where coal combustion can be eff ectively off set by biomass agricultural and silvicultural residues be extracted at a rate and in a manner that energy production. Th e global climate-mitigation potential achiev- does not cause soil erosion or soil degradation; crop residues currently in use as able from the use of terrestrial biomass may thus be maximized by animal fodder not be used as biochar feedstock; minimal carbon debt be incurred a mixed strategy favouring bioenergy in those regions with fertile from land-use change or use of feedstocks with a long life expectancy; no new lands be converted into biomass production and no agricultural land be taken out soils where coal emissions can be off set, and biochar elsewhere. of food production; no biomass wastes that have a high probability of contamina- Nevertheless, we have included biochar production in fertile, coal- tion, which would be detrimental to agricultural soils, be used; and biomass crop intensive regions in our scenarios because other potential benefi ts of production be limited to production on abandoned agricultural land that has not biochar, such as its potential for more effi cient use of water and crop subsequently been converted to pasture, forest or other uses. We further require 36 – 38 that biochar be manufactured using modern technology that eliminates soot, nutrients , may favour its use even in such regions. CH and N O emissions while recovering some of the energy released during the 4 2 We emphasize that the results presented here assume that future pyrolysis process for subsequent use. biochar production follows strict sustainability criteria. Land-use changes that incur high carbon debts and biochar production using Greenhouse gases . We consider three GHGs in this analysis: CO , CH and N O 2 4 2 technologies with poorly controlled emissions lead to both large reduc- (see Supplementary Table S14 for a summary of estimated global warming poten- tials for these GHGs). Although the diff erent atmospheric lifetimes of these gases tions in avoided emissions and excessively long carbon-payback times, ensure that there is no equivalence among them in any strict sense, we nevertheless during which net emissions are increased before any net reduction is adopt the common practice of normalizing each gas to a ‘ CO -C equivalent ’ using observed. Biochar production and use, therefore, must be guided by the estimated radiative forcing produced by the emission of each gas, integrated well-founded and well-enforced sustainability protocols if its poten- over a 100-year period following emission, using Intergovernmental Panel on Cli- tial for mitigating climate change is to be realized. mate Change 100-year global warming potentials of 23 for CH and 296 for N O. 4 2 Methods Comparison with bioenergy . To compare the net avoided GHG emissions stem- Overall approach . A model (BGRAM version 1.1) to calculate the net avoided ming from biochar with those from bioenergy production, we apply the same model GHG emissions attributable to sustainable biochar production as a function of time and sustainability criteria, but assume complete combustion to liberate the maxi- was developed and applied to the three scenarios. Th is model includes the eff ects of mum possible energy, rather than slow pyrolysis, as the conversion technology. NATURE COMMUNICATIONS | 1:56 | DOI: 10.1038/ncomms1053 | www.nature.com/naturecommunications 7 © 2010 Macmillan Publishers Limited. All rights reserved. Cumulative net avoided emissions (Pg CO -C ) 2 e Renewables Gas Oil Coal ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1053 Figure 6 | Soil-fertility constraints to cropland productivity (5  resolution). Soil fertility is indicated by hue, whereas the percentage of the gridcell currently being used as cropland is indicated by colour saturation (with white indicating the absence of cropland in a grid cell). Figure 7 | Cumulative mitigation potential of biochar relative to bioenergy. The mitigation potential is reported as a function of both soil fertility and carbon intensity of the type of energy being offset (in the MSTP scenario). Points M , M and M on the upper x axis refer to the ew w b carbon intensity of the current world electricity mix, the current world primary energy mix and the baseline energy mix assumed in our scenarios, respectively. Carbon intensity values for natural gas, oil and coal are also indicated. The relative mitigation is calculated as cumulative avoided emissions for biochar minus those for bioenergy, expressed as a fraction of the avoided emissions for bioenergy (for example, a value of 0.1 indicates that the cumulative mitigation impact of biochar is 10 % greater than that of bioenergy, a value of − 0.1 indicates that it is 10 % lower and a value of zero indicates that they have the same mitigation impact). The soil-fertility classifi cations marked on the vertical axis correspond to the soil categories mapped in Figure. 6 . Panel a (Residues) includes agricultural and forestry residues, together with green waste, as biomass inputs; Panel b (Biomass crops) includes both dedicated biomass crops and agroforestry products as biomass inputs. Panel c (Manures), includes bovine, pig and poultry manure as biomass inputs. Panel d (Total) includes all sources of biomass inputs in the proportions assumed in our model. An analogous fi gure for the Alpha scenario is shown as Supplementary Figure S10 . Technology adoption rate . Th e rate at which installed biochar production capac- ity approaches its maximum is constrained by simple economic considerations. Data for estimated capital costs are shown in Supplementary Table S15 . Th ese are Soil application and fertility classifi cation . Maximum biochar application to the − 1 implemented in the model using a Gompertz curve ( Supplementary Methods) . top 0.15 m of agricultural soils was assumed to be 50 Mg C ha . It was assumed Th e model allows for a lead time of 5 years, during which little plant capacity is that only 20 % of pasture soils will receive these application rates because of commissioned. Slow-to-moderate investment for the remainder of the fi rst decade constraints from terrain, accessibility, fi re and wind. See Supplementary Methods and rapid adoption over the following three decades at a rate of capital investment and Supplementary Figure S12 . Soil-fertility classifi cations were taken from ref. 40. Th ese were combined with consistent with the 2 % of global gross domestic product that Lord Stern estimates to be required for climate-change mitigation culminate in near-maximal biochar a 5-minute resolution map of global cropland distribution to produce a global production rates aft er a total of four decades. Net avoided GHG emissions over the map of cropland, categorized by the severity of soil-fertility constraints ( Fig. 6 ). fi rst decade are negligible, because of a combination of initially slow adoption and Carbon intensity of fuel offsets . Th e baseline carbon intensity of the fuel off sets carbon-debt payback. − 1 ( M ) used here is 17.5 kg C GJ . Th e current world primary-energy mix ( M ) has a b w − 1 carbon intensity of 16.5 kg C GJ and the current world electricity-generation mix References − 1 ( M ) has a carbon intensity of 15 kg C GJ (ref. 42). See Supplementary Methods 1 . Raupach , M . R . et al. Global and regional drivers of accelerating CO emissions . ew for the derivation of these carbon intensities. PNAS 104 , 10288 – 10293 ( 2007 ). 8 NATURE COMMUNICATIONS | 1:56 | DOI: 10.1038/ncomms1053 | www.nature.com/naturecommunications © 2010 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1053 ARTICLE 2 . Solomon , S . , Plattner , G . , Knutti , R . & Friedlingstein , P . Irreversible climate 32 . Rondon , M . , Ramirez , J . & Lehmann , J . Charcoal additions reduce net change due to carbon dioxide emissions . PN AS 106 , 1704 – 1709 ( 2009 ). emissions of greenhouse gases to the atmosphere . Pr oc. 3rd USDA Sympos. 3 . Broecker , W . S . Climate change: CO arithmetic . 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Target atmospheric CO : where should humanity aim? Open Greenhouse Gas Inventories, Volume 2: Energy (eds Eggleston, H.S., Buendia, L., Atmos. Sci. J. 2 , 217 – 231 ( 2008 ). Miwa, K., Ngara, T., Tanabe, K.) ( IGES , 2006 ) . 8 . Sombroek , W . G . , Nachtergaele , F . O . & Hebel , A . Amounts, dynamics and 36 . Tryon , E . Eff ect of charcoal on certain physical, chemical, and biological sequestering of carbon in tropical and subtropical soils . Am bio 22 , 417 – 426 ( 1993 ). properties of forest soils . Ecol. Monogr. 81 – 115 ( 1948 ). 9 . Lehmann , J . , Gaunt , J . & Rondon , M . Bio-char sequestration in terrestrial 37 . Steiner , C . et al. Nitrogen retention and plant uptake on a highly weathered ecosystems — a review . Mitig. Adapt. Strat. Glob. Change 11 , 403 – 427 ( 2006 ). central Amazonian Ferralsol amended with compost and charcoal . J. Plant 10 . Glaser , B . , Haumaier , L . , Guggenberger , G . & Zech , W . Th e ‘ Terra Preta ’ Nutr. 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Ecological Assessment for Agriculture in the 21st Century ( International Institute 13 . Lehmann , J . A handful of carbon . Na ture 447 , 143 – 144 ( 2007 ). for Applied Systems Analysis & Food and Agriculture Organization of the 14 . Schmidt , M . W . I . & Noack , A . G . Black carbon in soils and sediments: United Nations, Laxenburg, Austria , 2002 ) . analysis, distribution, implications, and current challenges . Global 41 . Erb , K . A comprehensive global 5 min resolution land-use data set for the Biogeochem. Cy. 14 , 777 – 793 ( 2000 ). year 2000 consistent with national census data . J . Land Use Sci. 2 , 191 – 224 15 . Kuzyakov , Y . , Subbotina , I . , Chen , H . , Bogomolova , I . & Xu , X . Black carbon ( 2007 ). decomposition and incorporation into soil microbial biomass estimated by 42 . International Energy Agency . K ey World Energy Statistics: 2009 ( IEA , 2009 ) . C labeling . S oil Biol. Biochem. 41 , 210 – 219 ( 2009 ). 16 . Cheng , C . , Lehmann , J . , Th ies , J . E . & Burton , S . D . Stability of black carbon in soils Acknowledgments across a climatic gradient . J . Geophys. Res. 113 , doi:10.1029/2007JG000642 ( 2008 ). D.W. and F.A.S.-P. acknowledge support from the United Kingdom ’ s Natural 17 . Lehmann , D . J . & Joseph , S . Biochar for Environmental Management: Science Environment Research Council (NERC) and Economic and Social Research Council and Technology ( Earthscan Books Ltd , 2009 ) . (ESRC). J.E.A. acknowledges support from the United States ’ Department of Energy 18 . Lenton , T . M . & Vaughan , N . E . Th e radiative forcing potential of diff erent climate (USDOE) Offi ce of Science, Offi ce of Biological and Environmental Research, Climate geoengineering options . A tmos. Chem. Phys. Discuss. 9 , 2559 – 2608 ( 2009 ). and Environmental Science Division, Mitigation Science Focus Area and from the 19 . Lehmann , J . & Joseph , S . Chapter 9: biochar systems . in B iochar for USDOE Offi ce of Fossil Energy, Terrestrial Carbon Sequestration Program. Th e Pacifi c Environmental Management: Science and Technology (eds Lehmann, J., Joseph, S.) Northwest National Laboratory is operated for the USDOE by Battelle Memorial Institute ( Earthscan Books Ltd , 2009 ) . under contract DE-AC05-76RL01830. J.L. acknowledges support from the Cooperative 20 . Whitman , T . & Lehmann , J . Biochar — one way forward for soil carbon in off set State Research Service of the U.S. Department of Agriculture and from the New York mechanisms in Africa? Environ. Sci. Policy 12 , 1024 – 1027 ( 2009 ). State Energy Research and Development Authority. S.J. acknowledges support from 21 . Elliott , D . C . Historical developments in hydroprocessing bio-oils . Energy Fuels VenEarth Group LLC. 21 , 1792 – 1815 ( 2007 ). 22 . Haberl , H . et al. Quantifying and mapping the human appropriation of net primary production in earth’s terrestrial ecosystems . PNAS 104 , 12942 . Author contributions 23 . Wackernagel , M . et al. Tracking the ecological overshoot of the human D.W. and J.E.A. produced the model and wrote the paper. F.A.S.-P. supervised the economy . PN AS 99 , 9266 ( 2002 ). NERC / ESRC-funded PhD project of which D.W ’ s contribution formed a part. 24 . Stern , N . Testimony of Lord Nicholas Stern to the Committee on Energy and J.L. and S.J. provided specialist advice. All authors commented on the paper. Commerce . At < http://archives.energycommerce.house.gov/cmte_mtgs/110- eaq-hrg.062608.Stern-Testimony.pdf > ( 2008 ) . Additional information 25 . Fargione , J . , Hill , J . , Tilman , D . , Polasky , S . & Hawthorne , P . Land clearing and Supplementary Information accompanies this paper on http: / / www.nature.com/ the biofuel carbon debt . S cience 319 , 1235 – 1238 ( 2008 ). naturecommunications 26 . Searchinger , T . et al. Use of US croplands for biofuels increases greenhouse Competing fi nancial interests: D.W., J.E.A., F.A.S.-P. and J.L. declare no competing fi nancial gases through emissions from land use change . S cience 319 , 1238 – 1240 interests. S.J. is Chairman of Anthroterra, a company conducting research into the development ( 2008 ). 27 . Field , C . B . , Campbell , J . E . & Lobell , D . B . Biomass energy: the scale of the of a biochar mineral complex to replace conventional fertilizers. Th is company plans to potential resource . Trends Ecol. Evol. 23 , 65 – 72 ( 2008 ). manufacture and sell portable pyrolysers. 28 . Tilman , D . et al. Benefi cial biofuels — the food, energy, and environment Reprints and permission information is available online at http://npg.nature.com/ trilemma . S cience 325 , 270 – 271 ( 2009 ). reprintsandpermissions/ 29 . Tilman , D . , Hill , J . & Lehman , C . Carbon-negative biofuels from low-input high-diversity grassland biomass . S cience 314 , 1598 – 1600 ( 2006 ). How to cite this article: Woolf, D. et al. Sustainable biochar to mitigate global climate 30 . Gaunt , J . L . & Lehmann , J . Energy balance and emissions associated with change. Nat. Commun. 1:56 doi: 10.1038 / ncomms1053 (2010). biochar sequestration and pyrolysis bioenergy production . Environ. Sci. Technol. 42 , 4152 – 4158 ( 2008 ). Licence: Th is work is licensed under a Creative Commons Attribution-NonCommercial- 31 . Lehmann , J . et al. Australian climate-carbon cycle feedback reduced by soil NoDerivative Works 3.0 Unported License. To view a copy of this license, visit black carbon . N ature Geosci. 1 , 832 – 835 ( 2008 ). http://creativecommons.org/licenses/by-nc-nd/3.0/ NATURE COMMUNICATIONS | 1:56 | DOI: 10.1038/ncomms1053 | www.nature.com/naturecommunications 9 © 2010 Macmillan Publishers Limited. All rights reserved. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Nature Communications Springer Journals

Sustainable biochar to mitigate global climate change

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Springer Journals
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Copyright © 2010 by The Author(s)
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Science, Humanities and Social Sciences, multidisciplinary; Science, Humanities and Social Sciences, multidisciplinary; Science, multidisciplinary
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10.1038/ncomms1053
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Abstract

ARTICLE DOI: 10.1038/ncomms1053 Received 29 Oct 2009 | Accepted 14 Jul 2010 | Published 10 Aug 2010 Sustainable biochar to mitigate global climate change 1 2 1 3 4 Dominic Woolf , James E. Amonette , F. Alayne Street-Perrott , Johannes Lehmann & Stephen Joseph Production of biochar (the carbon (C)-rich solid formed by pyrolysis of biomass) and its storage in soils have been suggested as a means of abating climate change by sequestering carbon, while simultaneously providing energy and increasing crop yields. Substantial uncertainties exist, however, regarding the impact, capacity and sustainability of biochar at the global level. In this paper we estimate the maximum sustainable technical potential of biochar to mitigate climate change. Annual net emissions of carbon dioxide (CO ), methane and nitrous oxide could be reduced by a maximum of 1.8 Pg CO -C equivalent (CO -C ) per 2 2 e year (12 % of current anthropogenic CO -C emissions; 1 Pg = 1 Gt), and total net emissions 2 e over the course of a century by 130 Pg CO -C , without endangering food security, habitat or 2 e soil conservation. Biochar has a larger climate-change mitigation potential than combustion of the same sustainably procured biomass for bioenergy, except when fertile soils are amended while coal is the fuel being offset. 1 2 School of the Environment and Society, Swansea University, Singleton Park , Swansea SA2 8PP , UK . Chemical and Materials Sciences Division, Pacifi c Northwest National Laboratory , Richland , Washington 99352, USA . Department of Crop and Soil Sciences, College of Agriculture and Life Sciences, Cornell University , Ithaca , New York 14853 , USA . The School of Materials Science and Engineering, University of New South Wales , Sydney, New South Wales 2052 , Australia . Correspondence and requests for materials should be addressed to J.E.A. (email: jim.amonette@pnl.gov ) . NATURE COMMUNICATIONS | 1:56 | DOI: 10.1038/ncomms1053 | www.nature.com/naturecommunications 1 © 2010 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1053 ince 2000, anthropogenic carbon dioxide (CO ) emissions able bioenergy; it can improve agricultural productivity, particularly have risen by more than 3 % annually , putting Earth ’ s eco- in low-fertility and degraded soils where it can be especially useful Ssystems on a trajectory towards rapid climate change that is to the world ’ s poorest farmers; it reduces the losses of nutrients and both dangerous and irreversible . To change this trajectory, a timely agricultural chemicals in run-off ; it can improve the water-holding 17,18 and ambitious programme of mitigation measures is needed. Several capacity of soils; and it is producible from biomass waste . Of the studies have shown that, to stabilize global mean surface temperature, possible strategies to remove CO from the atmosphere, biochar is cumulative anthropogenic greenhouse-gas (GHG) emissions must notable, if not unique, in this regard. be kept below a maximum upper limit, thus indicating that future Biochar can be produced at scales ranging from large industrial 2 – 6 19 net anthropogenic emissions must approach zero . If humanity facilities down to the individual farm , and even at the domestic oversteps this threshold of maximum safe cumulative emissions level , making it applicable to a variety of socioeconomic situations. (a limit that may already have been exceeded ), no amount of Various pyrolysis technologies are commercially available that yield emissions reduction will return the climate to within safe bounds. diff erent proportions of biochar and bioenergy products, such as Mitigation strategies that draw down excess CO from the atmos- bio-oil and syngas. Th e gaseous bioenergy products are typically phere would then assume an importance greater than an equivalent used to generate electricity; the bio-oil may be used directly for low- reduction in emissions. grade heating applications and, potentially, as a diesel substitute Production of biochar, in combination with its storage in soils, aft er suitable treatment . Pyrolysis processes are classifi ed into two has been suggested as one possible means of reducing the atmos- major types, fast and slow, which refer to the speed at which the pheric CO concentration (refs 8 – 13 and see also Supplementary biomass is altered. Fast pyrolysis, with biomass residence times of Note for a history of the concept and etymology of the term). Bio- a few seconds at most, generates more bio-oil and less biochar than char ’ s climate-mitigation potential stems primarily from its highly slow pyrolysis, for which biomass residence times can range from 14 – 16 recalcitrant nature , which slows the rate at which photosyntheti- hours to days. cally fi xed carbon (C) is returned to the atmosphere. In addition, Th e sustainable-biochar concept is summarized in Figure 1 . CO biochar yields several potential co-benefi ts. It is a source of renew- is removed from the atmosphere by photosynthesis. Sustainably CO removed CO by returned photosynthesis IMPACT OUTPUTS APPLICATIONS INPUTS Biofuel CO emissions Bio-oil Rice Syngas Energy Process heat Other cereals Sugar cane PROCESS Avoided Avoided Avoided Manures fossil CO soil 2 biomass decay emissions Avoided CH /N O Biomass crops 4 2 Pyrolysis Soil Biochar Stored C amendment Agroforestry Oxidation, soil C, tillage, transport Felling loss Enhanced primary productivity Figure 1 | Overview of the sustainable biochar concept. The fi gure shows inputs, process, outputs, applications and impacts on global climate . Within each of these categories, the relative proportions of the components are approximated by the height / width of the coloured fi elds. CO is removed from the atmosphere by photosynthesis to yield biomass. A sustainable fraction of the total biomass produced each year, such as agricultural residues, biomass crops and agroforestry products, is converted by pyrolysis to yield bio-oil, syngas and process heat, together with a solid product, biochar, which is a recalcitrant form of carbon and suitable as a soil amendment. The bio-oil and syngas are subsequently combusted to yield energy and CO . This energy and the process heat are used to offset fossil carbon emissions, whereas the biochar stores carbon for a signifi cantly longer period than would have occurred if the original biomass had been left to decay. In addition to fossil energy offsets and carbon storage, some emissions of methane and nitrous oxide are avoided by preventing biomass decay (see Supplementary Table S5 for example) and by amending soils with biochar. Additionally, the removal of CO by photosynthesis is enhanced by biochar amendments to previously infertile soils, thereby providing a positive feedback. CO is returned to the 2 2 atmosphere directly through combustion of bio-oil and syngas, through the slow decay of biochar in soils, and through the use of machinery to transport biomass to the pyrolysis facility, to transport biochar from the same facility to its disposal site and to incorporate biochar into the soil. In contrast to bioenergy, in which all CO that is fi xed in the biomass by photosynthesis is returned to the atmosphere quickly as fossil carbon emissions are offset, biochar has the potential for even greater impact on climate through its enhancement of the productivity of infertile soils and its effects on soil GHG fl uxes. 2 NATURE COMMUNICATIONS | 1:56 | DOI: 10.1038/ncomms1053 | www.nature.com/naturecommunications © 2010 Macmillan Publishers Limited. All rights reserved. Biomass crops, Agricultural agroforestry residues Net avoided emissions NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1053 ARTICLE procured crop residues, manures, biomass crops, timber and for- production. Of primary importance is the conversion of land to estry residues, and green waste are pyrolysed by modern techno- generate feedstock. In addition to its negative eff ects on ecosystem logy to yield bio-oil, syngas, process heat and biochar. As a result conservation, land clearance to provide feedstock may also release of pyrolysis, immediate decay of these biomass inputs is avoided. carbon stored in soils and biomass, leading to unacceptably high Th e outputs of the pyrolysis process serve to provide energy, avoid carbon-payback times before any net reduction in atmospheric CO is emissions of GHGs such as methane (CH ) and nitrous oxide achieved (ref. 25, Supplementary Methods and Supplementary (N O), and amend agricultural soils and pastures. Th e bioenergy Fig. S2 ). For example, we fi nd that a land-use change carbon debt − 1 is used to off set fossil-fuel emissions, while returning about half of greater than 22 Mg C ha (an amount that would be exceeded by the C fi xed by photosynthesis to the atmosphere. In addition to the conversion of temperate grassland to annual crops ) will result in GHG emissions avoided by preventing decay of biomass inputs, a carbon-payback time that is greater than 10 years. Clearance of soil emissions of GHGs are also decreased by biochar amendment rainforests to provide land for biomass-crop production leads to to soils. Th e biochar stores carbon in a recalcitrant form that can carbon payback times in excess of 50 years. Where rainforest on increase soil water- and nutrient-holding capacities, which typi- peatland is converted to biomass-crop production, carbon-payback cally result in increased plant growth. Th is enhanced productivity times may be in the order of 325 years. We therefore assume that no is a positive feedback that further enhances the amount of CO land clearance will be used to provide biomass feedstock, nor do we removed from the atmosphere. Slow decay of biochar in soils, include conversion of agricultural land from food to biomass-crop together with tillage and transport activities, also returns a small production as a sustainable source of feedstock, both because of the amount of CO to the atmosphere. A schematic of the model used negative consequences for food security and because it may indirectly to calculate the magnitudes of these processes is shown as Sup- induce land clearance elsewhere . Some dedicated biomass-crop plementary Figure S1 . production on abandoned, degraded agricultural soil has been Even under the most zealous investment programme, biochar included in this study as this will not adversely aff ect food security 28,29 production will ultimately be limited by the rate at which biomass and can improve biodiversity . We further assume that extraction can be extracted and pyrolysed without causing harm to the bio- rates of agricultural and forestry residues are suffi ciently low to sphere or to human welfare. Globally, human activity is responsible preclude soil erosion or loss of soil function, and that no industrially for the appropriation of 16 Pg C per year from the biosphere, which treated waste biomass posing a risk of soil contamination will be used. corresponds to 24 % of potential terrestrial net primary productiv- Other constraints on biochar production methods arise because ity (NPP) . Higher rates of appropriation will increase pressure on emissions of CH , N O, soot or volatile organic compounds combined 4 2 global ecosystems, exacerbating a situation that is already unsus- with low biochar yields (for example, from traditional charcoal kilns tainable . or smouldering slash piles) may negate some or all of the carbon- Th e main aim of this study is to provide an estimate of the theo- sequestration benefi ts, cause excessive carbon-payback times or be retical upper limit, under current conditions, to the climate-change detrimental to health. Th erefore, we do not consider any biochar pro- mitigation potential of biochar when implemented in a sustain- duction systems that rely on such technologies, and restrict our analy- able manner. Th is limit, which we term the maximum sustainable sis to systems in which modern, high-yield, low-emission pyrolysis technical potential (MSTP), represents what can be achieved when technology can feasibly be used to produce high-quality biochar. the portion of the global biomass resource that can be harvested Within these constraints, we derived a biomass-availability sce- sustainably (that is, without endangering food security, habitat or nario for our estimate of MSTP, as well as two additional scenarios, soil conservation) is converted to biochar by modern high-yield, Alpha and Beta, which represent lower demands on global biomass low-emission, pyrolysis methods. Th e fraction of the MSTP that is resources ( Table 1 ). Attainment of the MSTP would require sub- actually realized will depend on a number of socioeconomic fac- stantial alteration to global biomass management, but would not tors, including the extent of government incentives and the relative endanger food security, habitat or soil conservation. Th e Alpha sce- emphasis placed on energy production relative to climate-change nario restricts biomass availability to residues and wastes available mitigation. Aside from assuming a maximum rate of capital invest- using current technology and practices, together with a moderate ment that is consistent with that estimated to be required for cli- amount of agroforestry and biomass cropping. All three scenarios mate-change mitigation , this study does not take into account any represent fairly ambitious projects, and require progressively greater economic, social or cultural barriers that might further limit the levels of political intervention to promote greater adoption of sus- adoption of biochar technology. tainable land-use practices and increase the quantity of uncontami- Our analysis shows that sustainable global implementation of nated organic wastes available for pyrolysis. We do not consider any biochar can potentially off set a maximum of 12 % of current anthro- scenarios that are not ambitious in this study, as the intention is to pogenic CO -C equivalent (CO -C ) emissions (that is, 1.8 Pg C O - investigate whether biochar could make a substantial contribution 2 2 e 2 C per year of the 15.4 Pg CO -C emitted annually), and that over to climate-change mitigation— an aspiration that certainly will not e 2 e the course of a century, the total net off set from biochar would be be accomplished by half-hearted measures. Th e range of mitigation 130 Pg CO -C . We also show that conversion of all sustainably results reported thus refers only to the scenarios considered and does 2 e obtained biomass to maximize bioenergy, rather than biochar, pro- not encompass the full range of less-eff ective outcomes correspond- duction can off set a maximum of 10 % of the current anthropogenic ing to varying levels of inaction. Th e scenarios are based on current CO -C emissions. Th e relative climate-mitigation potentials of bio- biomass availability ( Supplementary Methods and Supplementary 2 e char and bioenergy depend on the fertility of the soil amended and Tables S1 and S2 ), the composition and energy contents of diff erent the C intensity of the fuel being off set, as well as the type of biomass. types of biomass and the biochar derived from each ( Supplementary Locations at which the soil fertility is high and coal is the fuel being Tables S3 and S4 ), and the rate of adoption of biochar technology off set are best suited for bioenergy production. Th e climate-miti- ( Supplementary Fig. S3 ). How this biomass resource base changes gation potential of biochar (with combined energy production) is over the course of 100 years will depend on the potential eff ects of higher for all other situations. changing climate, atmospheric CO , sea level, land use, agricultural practices, technology, population, diet and economic development. Results Some of these factors may increase biomass availability and some Sustainable biomass-feedstock availability . To ensure that our may decrease it. A full assessment of the wide range of possible future estimates represent a sustainable approach, we use a stringent scenarios within plausible ranges of these factors remains outside the set of criteria to assess potential feedstock availability for biochar scope of this study. NATURE COMMUNICATIONS | 1:56 | DOI: 10.1038/ncomms1053 | www.nature.com/naturecommunications 3 © 2010 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1053 Table 1 | Annual globally sustainable biomass feedstock availability. Biomass available in scenario (Pg C per year) Maximum sustainable technical Alpha Beta potential Rice 0.22 0.25 0.28 Rice husks and 70 % of paddy rice straw Rice husks and 80 % of paddy rice straw Rice husks and 90 % of paddy rice not used for animal feed not used for animal feed straw not used for animal feed Other cereals 0.072 0.13 0.18 8 % of total straw and stover (assumes 14 % of total straw and stover (35 % 20 % of total straw and stover (45 % 25 % extraction rate of crop residues minus extraction rate minus animal feed) extraction rate minus animal feed) quantity used as animal feed) Sugar cane 0.09 0.11 0.13 Waste bagasse plus 25 % of fi eld trash Waste bagasse plus 50 % of fi eld trash Waste bagasse plus 75 % of fi eld trash Manures 0.10 0.14 0.19 12.5 % of cattle manure plus 50 % of pig 19 % of cattle manure plus 70 % of pig and 25 % of cattle manure plus 90 % of and poultry manure poultry manure pig and poultry manure Biomass crops 0.30 0.45 0.60 50 % of potential production of abandoned, 75 % of potential production of abandoned, 100 % of potential production of degraded cropland that is not in other use degraded cropland that is not in other use abandoned, degraded cropland that is not in other use Forestry residues 0.14 0.14 0.14 44 % of difference between reported fellings and extraction Agroforestry 0.06 0.34 0.62 17 Mha of tropical silvopasture 85 Mha of tropical grass pasture converted 170 Mha of tropical grass pasture to silvopasture converted to silvopasture Green / wood waste 0.029 0.085 0.14 75 % of low-end estimate of yard- Alpha plus mid-range estimate of yard Beta plus high-end estimate of trimmings production and wood-milling trimmings plus urban food waste, including global yard trimmings and food residues 40 % of waste sawnwood (legislation waste, including 80 % of waste required to ensure that this fraction of waste sawn wood wood is free of harmful contaminants) Total 1.01 1.64 2.27 Avoided GHG emissions . Results for the three scenarios are Of the adverse feedbacks, biochar decomposition is the largest expressed below as a range from the Alpha scenario fi rst to the (8 – 17 Pg CO -C ), followed by loss of soil organic carbon due to diver- 2 e MSTP last. Th e model predicts that maximum avoided emissions sion of biomass from soil into biochar production (6 – 10 Pg CO -C ), 2 e of 1.0 – 1.8 Pg CO -C per year are approached by mid-century and and transport (1.3 – 1.9 Pg CO -C , see Supplementary Fig. S7 ). Contri- 2 e 2 e that, aft er a century, the cumulative avoided emissions are 66 – 130 Pg butions to the overall GHG budget from tillage (0.03 – 0.044 Pg CO - CO -C ( Fig. 2 ). Half of the avoided emissions are due to the net car- C ) and reduced N-fertilizer production (0.2 – 0.3 Pg CO -C ) are neg- 2 e e 2 e bon sequestered as biochar, 30 % to replacement of fossil-fuel energy ligible (although their fi nancial costs may not be). by pyrolysis energy and 20 % to avoided emissions of CH and N O. Th e relative importance of all these factors to the GHG budget 4 2 Cumulative and annual avoided emissions for the individual gases varies considerably among feedstocks. Notably, rice residues, green CO , CH and N O are given in Supplementary Figures S4 – S6 . waste and manure achieve the highest ratios of avoided CO -C emis- 2 4 2 2 e A detailed breakdown of the sources of cumulative avoided sions per unit of biomass-carbon (1.2 – 1.1, 0.9 and 0.8 CO -C / C, 2 e GHG emissions over 100 years is given in Figure 3 . The two respectively) because of the benefi ts of avoided CH emissions. most important factors contributing to the avoided emissions from biochar are carbon stored as biochar in soil (43 – 94 Pg Sensitivity and Monte Carlo analyses . Sensitivity and Monte Carlo CO -C ) and fossil-fuel offsets from coproduction of energy analyses with respect to reasonable values of key variables were used 2 e (18 – 39 Pg CO -C ). to estimate the uncertainty of the model results; they suggest areas 2 e Of the benefi cial feedbacks, the largest is due to avoided CH in which future research is most needed and provide guidance on emissions from biomass decomposition (14 – 17 Pg CO -C ), pre- how biochar production systems might be optimized ( Fig. 4 ). 2 e dominantly arising from the diversion of rice straw from paddy Th e strongest sensitivity is to the half-life of the recalcitrant frac- fi elds (see Supplementary Table S5 for estimate of the mean CH tion of biochar (see also Supplementary Table S6 ). Net avoided GHG emission factor). Th e next largest positive feedbacks, in order emissions vary by − 22 % to + 4 % from that obtained using the base- of decreasing magnitude, arise from biochar-enhanced NPP on line assumption of 300 years. However, most of this variation occurs cropland, which contributes 9 – 16 Pg CO -C to the net avoided for half-life < 100 years, in which range we fi nd (in agreement with 2 e emissions (if these increased crop residues are converted to bio- previous work ) that sensitivity to this factor is high. Conversely, char), followed by reductions in soil N O emissions (4.0 – 6.2 Pg for a more realistic half-life of the recalcitrant fraction ( > 100 years), CO -C ), avoided N O emissions during biomass decomposition sensitivity to this factor is low because biochar can be produced 2 e 2 (1.8 – 3.3 Pg CO -C ) and enhanced CH oxidation by dry soils much more rapidly than it decays. As currently available data sug- 2 e 4 (0.44 – 0.8 Pg CO -C ). gest that the half-life of biochar ’ s recalcitrant fraction in soil is in the 2 e 4 NATURE COMMUNICATIONS | 1:56 | DOI: 10.1038/ncomms1053 | www.nature.com/naturecommunications © 2010 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1053 ARTICLE 2.0 default values assumed in this study up to the higher rate of 5 % 33,34 Annual suggested by more recent work . Th is would increase the net avoided GHG emissions by up to 8 % . Uncertainty in the response of crop yields to biochar amend- 1.5 ment results in estimated range of − 6 % to + 7 % in the impact of enhanced NPP of cropland on net avoided GHG emissions. Sensitivities to the pyrolysis energy effi ciency ( ± 5 % ), to the half- life of the biochar ’ s labile fraction ( − 4 % to + 1 % ) and to its impact 1.0 on soil CH oxidation ( ± 1 % ) are small. Th e net eff ect of covariance of the above factors was assessed using the Monte Carlo analysis ( n = 1,000, Supplementary Table S7 ). Despite limited data on the decomposition rate of biochar in soils 0.5 and the eff ects of biochar additions on soil GHG fl uxes, sensitiv- ity within realistic ranges of these parameters is small, resulting in an estimated uncertainty of ± 8 to 10 % ( ± 1 s.d.) in the cumulative 0 avoided GHG emissions for the three scenarios. Cumulative Comparison of biochar and bioenergy approaches . Th e mitiga- tion impact of the renewable energy obtained from both biochar MSTP-Biochar production and biomass combustion depends on the carbon inten- MSTP-Combustion sity (that is, the mass of carbon emitted per unit of total energy Beta-Biochar produced) of the off set energy sources . At our baseline carbon inten- Beta-Combustion − 1 sity (17.5 kg C GJ ; see Methods section), the model predicts that, on Alpha-Biochar an average, the mitigation impact of biochar is 27 – 22 % (14 – 23 Pg Alpha-Combustion CO -C ) larger than the 52 – 107 Pg CO -C predicted if the same sus- 2 e 2 e tainably procured biomass were combusted to extract the maximum amount of energy ( Fig. 2 ). Th is advantage of biochar over bioenergy is largely attributable to the benefi cial feedbacks from enhanced crop yields and soil GHG fl uxes ( Fig. 3 , Supplementary Fig. S8 ). Because the principal contribution of biomass combustion to avoided GHG emissions is the replacement of fossil fuels ( Fig. 3 ), the bioenergy approach shows a considerably higher sensitivity to 0 20406080 100 carbon intensity than does biochar ( Fig. 5 ). Th e carbon intensity of off set energy varies from near-zero for renewable and nuclear energy Time (y) − 1 35 to 26 kg C GJ for coal combustion . Mean cumulative avoided emis- sions from biochar and biomass combustion are equal in our sce- Figure 2 | Net avoided GHG emissions. The avoided emissions are − 1 attributable to sustainable biochar production or biomass combustion over narios when the carbon intensity of off set energy is 26 – 24 kg C GJ ( Fig. 5 ). In the MSTP scenario, this corresponds to an energy mix to 100 years, relative to the current use of biomass. Results are shown for the three model scenarios, with those for sustainable biochar represented which coal combustion contributes about 80 % , whereas in the Alpha scenario, the mean mitigation benefi t of biochar remains higher than by solid lines and for biomass combustion by dashed lines. The top panel shows annual avoided emissions; the bottom panel, cumulative avoided that of bioenergy, even when 100 % coal is off set. Th e cumulative avoided emissions from both strategies decrease as the carbon inten- emissions. Diamonds indicate transition period when biochar capacity of the top 15 cm of soil fi lls up and alternative disposal options are needed. sity of the off set energy mix decreases, but the rate of decrease for biomass combustion is 2.5 – 2.7 times greater than that for biochar. As expected, the cumulative avoided emissions for biomass combustion millennial range (see Supplementary Methods , Supplementary Table are essentially zero when the carbon intensity of the energy mix is also S6 and refs 8, 15, 16, 31), the contribution of its decay to the net zero. In contrast, the cumulative avoided emissions for biochar are GHG balance over centennial timescales is likely to be small. still substantial at 48 – 91 Pg CO -C . 2 e Th e next largest sensitivity is to the pyrolysis carbon yield ( − 9 % Given that much of the increased climate mitigation from bio- to + 11 % ), indicating the importance of engineering to optimize for char relative to biomass combustion stems from the benefi cial high yields of biochar rather than for energy production. Th is will feedbacks of adding biochar to soil, and that these feedbacks will be constrained, however, by the sensitivity to the labile fraction of be greatest on the least fertile soils, the relative mitigation poten- the biochar ( − 7 % to + 4 % ), which indicates the importance of opti- tials will vary regionally with soil type (see Supplementary Meth- mizing for production of recalcitrant biochar rather than for higher ods , Supplementary Fig. S9 and Supplementary Tables S8 – S11 for yields of lower-quality biochar. an account of how these feedbacks are calculated). Th e distribution Aft er carbon yield, the next largest sensitivity is to the carbon of soils of varying fertility on global cropland is shown in Figure intensity of the fuel off set by pyrolysis energy production, with net 6 . Globally, 0.31 Gha of soils with no fertility constraints are in use avoided emissions varying by − 4 % from the baseline assumption as cropland, as well as 0.29 Gha of cropland with few fertility con- when natural gas is the fuel being off set and by + 15 % when coal straints, 0.21 Gha with slight constraints, 0.32 Gha with moderate is off set. constraints, 0.18 Gha with severe constraints, 0.13 Gha with very Varying the impact of biochar amendment on soil N O emissions severe constraints and 0.09 Gha of cropland on soils categorized as from zero to the largest reported reduction (80 % ; ref. 32) produces unsuitable for crop production. Th e amount of biomass produced a sensitivity of − 4 % to + 11 % . Further variability in the impact in soils of diff erent fertilities is shown in Supplementary Table S12 . of biochar on N O emissions arises from adjusting the fraction Figure 7 shows how the climate mitigation from biochar varies rela- of biomass-N that (if left to decompose) would be converted to tive to biomass combustion when both soil fertility and the carbon N O-N, from the Intergovernmental Panel on Climate Change intensity of energy off sets are considered. Th e relative benefi t of NATURE COMMUNICATIONS | 1:56 | DOI: 10.1038/ncomms1053 | www.nature.com/naturecommunications 5 © 2010 Macmillan Publishers Limited. All rights reserved. Annual net avoided emissions (Pg CO -C ) 2 e Enhanced NPP Green/wood waste Agroforestry Forestry residues Biomass crops Manures Sugar cane Other cereals Rice ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1053 CH Biomass CH Soil N O Biomass N O Soil Fossil fuel offset 4 4 2 2 Soil organic C Transportation and tillage C in Biochar Biochar decomposition 40 200 MSTP Biochar 32 160 24 120 16 80 8 40 0 0 -8 -40 Beta 32 160 24 120 16 80 8 40 32 160 Alpha 24 120 16 80 8 40 0 0 -8 -40 Totals Figure 3 | Breakdown of cumulative avoided GHG emissions (Pg CO -C ) from sustainable biochar production. The data are for the three model 2 e scenarios over 100 years by feedstock and factor. The left side of the fi gure displays results for each of eight feedstock types and the additional biomass residues that are attributed to NPP increases from biochar amendments; the right side displays total results by scenario for both biochar (left column) and biomass combustion (right column). For each column, the total emission-avoiding and emission-generating contributions are given, respectively, by the height of the columns above and below the zero line. The net avoided emissions are calculated as the difference between these two values. Within each column, the portion of its contribution caused by each of six emission-avoiding mechanisms and three emission-generating mechanisms is shown by a different colour. These mechanisms (from top to bottom within each column) are (1) avoided CH from biomass decay, (2) increased CH oxidation by 4 4 soil biochar, (3) avoided N O from biomass decay, (4) avoided N O caused by soil biochar, (5) fossil fuel offsets from pyrolysis energy production, 2 2 (6) avoided CO emissions from carbon stored as biochar, (7) decreased carbon stored as soil organic matter caused by diversion of biomass to biochar, (8) CO emissions from transportation and tillage activities and (9) CO emissions from decomposition of biochar in soil. 2 2 producing biochar compared with biomass combustion is greatest feedstocks ( Fig. 7b ), because avoided CH emissions from the use of when biochar is added to marginal lands and the energy produced manure, green waste and rice residues occur regardless of whether by pyrolysis is used to off set natural gas, renewable or nuclear these other feedstocks are used for energy or biochar. energy. When biochar is added to the most infertile cropland to off set the current global primary energy mix ( M ), which has a car- Discussion − 1 bon intensity of 16.5 kg C GJ , the relative benefi t from biochar is as Our analysis demonstrates that sustainable biochar production much as 79 – 64 % greater than that from bioenergy ( Fig. 7 , Supple- (with addition to soils) has the technical potential to make a substan- mentary Fig. S10 ). Th is net benefi t diminishes as more coal is off set tial contribution to mitigating climate change. Maximum avoided and as biochar is added to soils with higher fertility. Nevertheless, emissions of the order of 1.8 Pg CO -C annually, and of 130 Pg 2 e with the exception of those geographical regions having both natu- CO -C over the course of a century, are possible at current levels 2 e rally high soil fertility and good prospects for off setting coal emis- of feedstock availability, while preserving biodiversity, ecosystem sions (in which bioenergy yields up to 16 – 22 % greater mitigation stability and food security. impact than biochar), biochar shows a greater climate-mitigation Th e biochar scenarios described here, with their very high levels potential than bioenergy. Th e relative benefi t of producing biochar of biomass utilization, are not compatible with simultaneous imple- compared with bioenergy is greatest when biomass crops are used as mentation of an ambitious biomass energy strategy. Th e opportunity 6 NATURE COMMUNICATIONS | 1:56 | DOI: 10.1038/ncomms1053 | www.nature.com/naturecommunications © 2010 Macmillan Publishers Limited. All rights reserved. Cumulative avoided emissions (Pg CO -C ) 2 e Biochar Combustion Cumulative avoided emissions (Pg CO -C ) 2 e NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1053 ARTICLE 100 200 300 Half-life recalcitrant C (y) 50 1000 MSTP Biochar Pyrolysis C yield (%) 40 62 Combustion -1 15 26 C Intensity of fuel offset (kg C GJ ) Decrease in soil N O emissions (%) 0 80 Beta Cropland NPP (% yield response) 50 150 30 5 Labile-C fraction (%) Alpha Global N O emission factor (%) 1.05 5 65 85 Pyrolysis energy efficiency (%) 20 50 1 25 Half-life labile C (y) 0 200 Soil CH oxidation (mg CH per m 4 4 per year) -20 -10 0 10 20 Deviation from reported estimate (%) 5 1015202530 -1 C Intensity of fuel offset (kg C GJ ) Figure 4 | Sensitivity of the model to key variables. Sensitivity is expressed as a percentage deviation from the reported value of cumulative Figure 5 | Cumulative mitigation potential (100 years) of biochar and net avoided GHG emissions over 100 years for each scenario. Top (blue), biomass combustion as a function of carbon intensity of the type of middle (yellow) and bottom (red) bars for each variable correspond to energy being offset. The black vertical dashed line labelled M on the Alpha, Beta and MSTP scenarios. Minimum and maximum values for upper x axis refers to the carbon intensity of the baseline energy mix each variable are at the ends of the bars (with additional sensitivities to − 1 assumed in this study. Grey vertical dashed lines at 15, 19 and 26 kg C GJ recalcitrant carbon half-life of 100 and 200 years shown); baseline values denote the carbon intensity of natural gas, oil and coal, respectively. The of the key variables used in this study correspond to 0 % deviation. − 1 carbon intensity of renewable forms of energy is close to 0 kg C GJ . See also Supplementary Table S7 . cost of this forgone energy resource must be taken into account in feedstock procurement, transport, pyrolysis, energy production, soil incorporation, soil GHG fl ux, soil fertility and fertilizer use (see also Supplementary Table S13 ), an economic comparison of the two strategies. However, in terms and biomass and biochar decomposition (see also Supplementary Fig. S11 ). Th e net of their potentials for climate-change mitigation, the mitigation avoided GHG emissions due to biochar were calculated as the diff erence between impact of biochar is about one-fourth larger, on an average, than that the CO -equivalent emissions from biochar production and those that would have obtained if the same biomass were combusted for energy. Regional occurred as the biomass decomposed by other means had it not been converted to biochar. All emissions (actual or avoided) were calculated with time dependency. deviations from this average are large because of diff erences in soil Wherever possible, conservative assumptions were used to provide a high degree of fertility and available biomass. Our model predicts that the relative confi dence that our results represent a conservative estimate of the avoided GHG climate-mitigation benefi t of biochar compared with bioenergy is emissions achievable in each scenario. A detailed account of both the model and greatest in regions in which poor soils growing biomass crops can the three scenarios is given in Supplementary Methods . benefi t most from biochar additions. In contrast, biomass combustion Sustainability criteria . Biochar can be produced sustainably or unsustainably. leads to a greater climate-mitigation impact in regions with fertile Our criteria for sustainable biochar production require that biomass procured from soils where coal combustion can be eff ectively off set by biomass agricultural and silvicultural residues be extracted at a rate and in a manner that energy production. Th e global climate-mitigation potential achiev- does not cause soil erosion or soil degradation; crop residues currently in use as able from the use of terrestrial biomass may thus be maximized by animal fodder not be used as biochar feedstock; minimal carbon debt be incurred a mixed strategy favouring bioenergy in those regions with fertile from land-use change or use of feedstocks with a long life expectancy; no new lands be converted into biomass production and no agricultural land be taken out soils where coal emissions can be off set, and biochar elsewhere. of food production; no biomass wastes that have a high probability of contamina- Nevertheless, we have included biochar production in fertile, coal- tion, which would be detrimental to agricultural soils, be used; and biomass crop intensive regions in our scenarios because other potential benefi ts of production be limited to production on abandoned agricultural land that has not biochar, such as its potential for more effi cient use of water and crop subsequently been converted to pasture, forest or other uses. We further require 36 – 38 that biochar be manufactured using modern technology that eliminates soot, nutrients , may favour its use even in such regions. CH and N O emissions while recovering some of the energy released during the 4 2 We emphasize that the results presented here assume that future pyrolysis process for subsequent use. biochar production follows strict sustainability criteria. Land-use changes that incur high carbon debts and biochar production using Greenhouse gases . We consider three GHGs in this analysis: CO , CH and N O 2 4 2 technologies with poorly controlled emissions lead to both large reduc- (see Supplementary Table S14 for a summary of estimated global warming poten- tials for these GHGs). Although the diff erent atmospheric lifetimes of these gases tions in avoided emissions and excessively long carbon-payback times, ensure that there is no equivalence among them in any strict sense, we nevertheless during which net emissions are increased before any net reduction is adopt the common practice of normalizing each gas to a ‘ CO -C equivalent ’ using observed. Biochar production and use, therefore, must be guided by the estimated radiative forcing produced by the emission of each gas, integrated well-founded and well-enforced sustainability protocols if its poten- over a 100-year period following emission, using Intergovernmental Panel on Cli- tial for mitigating climate change is to be realized. mate Change 100-year global warming potentials of 23 for CH and 296 for N O. 4 2 Methods Comparison with bioenergy . To compare the net avoided GHG emissions stem- Overall approach . A model (BGRAM version 1.1) to calculate the net avoided ming from biochar with those from bioenergy production, we apply the same model GHG emissions attributable to sustainable biochar production as a function of time and sustainability criteria, but assume complete combustion to liberate the maxi- was developed and applied to the three scenarios. Th is model includes the eff ects of mum possible energy, rather than slow pyrolysis, as the conversion technology. NATURE COMMUNICATIONS | 1:56 | DOI: 10.1038/ncomms1053 | www.nature.com/naturecommunications 7 © 2010 Macmillan Publishers Limited. All rights reserved. Cumulative net avoided emissions (Pg CO -C ) 2 e Renewables Gas Oil Coal ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1053 Figure 6 | Soil-fertility constraints to cropland productivity (5  resolution). Soil fertility is indicated by hue, whereas the percentage of the gridcell currently being used as cropland is indicated by colour saturation (with white indicating the absence of cropland in a grid cell). Figure 7 | Cumulative mitigation potential of biochar relative to bioenergy. The mitigation potential is reported as a function of both soil fertility and carbon intensity of the type of energy being offset (in the MSTP scenario). Points M , M and M on the upper x axis refer to the ew w b carbon intensity of the current world electricity mix, the current world primary energy mix and the baseline energy mix assumed in our scenarios, respectively. Carbon intensity values for natural gas, oil and coal are also indicated. The relative mitigation is calculated as cumulative avoided emissions for biochar minus those for bioenergy, expressed as a fraction of the avoided emissions for bioenergy (for example, a value of 0.1 indicates that the cumulative mitigation impact of biochar is 10 % greater than that of bioenergy, a value of − 0.1 indicates that it is 10 % lower and a value of zero indicates that they have the same mitigation impact). The soil-fertility classifi cations marked on the vertical axis correspond to the soil categories mapped in Figure. 6 . Panel a (Residues) includes agricultural and forestry residues, together with green waste, as biomass inputs; Panel b (Biomass crops) includes both dedicated biomass crops and agroforestry products as biomass inputs. Panel c (Manures), includes bovine, pig and poultry manure as biomass inputs. Panel d (Total) includes all sources of biomass inputs in the proportions assumed in our model. An analogous fi gure for the Alpha scenario is shown as Supplementary Figure S10 . Technology adoption rate . Th e rate at which installed biochar production capac- ity approaches its maximum is constrained by simple economic considerations. Data for estimated capital costs are shown in Supplementary Table S15 . Th ese are Soil application and fertility classifi cation . Maximum biochar application to the − 1 implemented in the model using a Gompertz curve ( Supplementary Methods) . top 0.15 m of agricultural soils was assumed to be 50 Mg C ha . It was assumed Th e model allows for a lead time of 5 years, during which little plant capacity is that only 20 % of pasture soils will receive these application rates because of commissioned. Slow-to-moderate investment for the remainder of the fi rst decade constraints from terrain, accessibility, fi re and wind. See Supplementary Methods and rapid adoption over the following three decades at a rate of capital investment and Supplementary Figure S12 . Soil-fertility classifi cations were taken from ref. 40. Th ese were combined with consistent with the 2 % of global gross domestic product that Lord Stern estimates to be required for climate-change mitigation culminate in near-maximal biochar a 5-minute resolution map of global cropland distribution to produce a global production rates aft er a total of four decades. Net avoided GHG emissions over the map of cropland, categorized by the severity of soil-fertility constraints ( Fig. 6 ). fi rst decade are negligible, because of a combination of initially slow adoption and Carbon intensity of fuel offsets . Th e baseline carbon intensity of the fuel off sets carbon-debt payback. − 1 ( M ) used here is 17.5 kg C GJ . 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Cheng , C . , Lehmann , J . , Th ies , J . E . & Burton , S . D . Stability of black carbon in soils Acknowledgments across a climatic gradient . J . Geophys. Res. 113 , doi:10.1029/2007JG000642 ( 2008 ). D.W. and F.A.S.-P. acknowledge support from the United Kingdom ’ s Natural 17 . Lehmann , D . J . & Joseph , S . Biochar for Environmental Management: Science Environment Research Council (NERC) and Economic and Social Research Council and Technology ( Earthscan Books Ltd , 2009 ) . (ESRC). J.E.A. acknowledges support from the United States ’ Department of Energy 18 . Lenton , T . M . & Vaughan , N . E . Th e radiative forcing potential of diff erent climate (USDOE) Offi ce of Science, Offi ce of Biological and Environmental Research, Climate geoengineering options . A tmos. Chem. Phys. Discuss. 9 , 2559 – 2608 ( 2009 ). and Environmental Science Division, Mitigation Science Focus Area and from the 19 . Lehmann , J . & Joseph , S . Chapter 9: biochar systems . in B iochar for USDOE Offi ce of Fossil Energy, Terrestrial Carbon Sequestration Program. Th e Pacifi c Environmental Management: Science and Technology (eds Lehmann, J., Joseph, S.) Northwest National Laboratory is operated for the USDOE by Battelle Memorial Institute ( Earthscan Books Ltd , 2009 ) . under contract DE-AC05-76RL01830. J.L. acknowledges support from the Cooperative 20 . Whitman , T . & Lehmann , J . Biochar — one way forward for soil carbon in off set State Research Service of the U.S. Department of Agriculture and from the New York mechanisms in Africa? Environ. Sci. Policy 12 , 1024 – 1027 ( 2009 ). State Energy Research and Development Authority. S.J. acknowledges support from 21 . Elliott , D . C . Historical developments in hydroprocessing bio-oils . Energy Fuels VenEarth Group LLC. 21 , 1792 – 1815 ( 2007 ). 22 . Haberl , H . et al. Quantifying and mapping the human appropriation of net primary production in earth’s terrestrial ecosystems . PNAS 104 , 12942 . Author contributions 23 . Wackernagel , M . et al. Tracking the ecological overshoot of the human D.W. and J.E.A. produced the model and wrote the paper. F.A.S.-P. supervised the economy . PN AS 99 , 9266 ( 2002 ). NERC / ESRC-funded PhD project of which D.W ’ s contribution formed a part. 24 . Stern , N . Testimony of Lord Nicholas Stern to the Committee on Energy and J.L. and S.J. provided specialist advice. All authors commented on the paper. Commerce . At < http://archives.energycommerce.house.gov/cmte_mtgs/110- eaq-hrg.062608.Stern-Testimony.pdf > ( 2008 ) . Additional information 25 . Fargione , J . , Hill , J . , Tilman , D . , Polasky , S . & Hawthorne , P . Land clearing and Supplementary Information accompanies this paper on http: / / www.nature.com/ the biofuel carbon debt . S cience 319 , 1235 – 1238 ( 2008 ). naturecommunications 26 . Searchinger , T . et al. Use of US croplands for biofuels increases greenhouse Competing fi nancial interests: D.W., J.E.A., F.A.S.-P. and J.L. declare no competing fi nancial gases through emissions from land use change . S cience 319 , 1238 – 1240 interests. S.J. is Chairman of Anthroterra, a company conducting research into the development ( 2008 ). 27 . Field , C . B . , Campbell , J . E . & Lobell , D . B . Biomass energy: the scale of the of a biochar mineral complex to replace conventional fertilizers. Th is company plans to potential resource . Trends Ecol. Evol. 23 , 65 – 72 ( 2008 ). manufacture and sell portable pyrolysers. 28 . Tilman , D . et al. Benefi cial biofuels — the food, energy, and environment Reprints and permission information is available online at http://npg.nature.com/ trilemma . S cience 325 , 270 – 271 ( 2009 ). reprintsandpermissions/ 29 . Tilman , D . , Hill , J . & Lehman , C . Carbon-negative biofuels from low-input high-diversity grassland biomass . S cience 314 , 1598 – 1600 ( 2006 ). How to cite this article: Woolf, D. et al. Sustainable biochar to mitigate global climate 30 . Gaunt , J . L . & Lehmann , J . Energy balance and emissions associated with change. Nat. Commun. 1:56 doi: 10.1038 / ncomms1053 (2010). biochar sequestration and pyrolysis bioenergy production . Environ. Sci. Technol. 42 , 4152 – 4158 ( 2008 ). Licence: Th is work is licensed under a Creative Commons Attribution-NonCommercial- 31 . Lehmann , J . et al. Australian climate-carbon cycle feedback reduced by soil NoDerivative Works 3.0 Unported License. To view a copy of this license, visit black carbon . N ature Geosci. 1 , 832 – 835 ( 2008 ). http://creativecommons.org/licenses/by-nc-nd/3.0/ NATURE COMMUNICATIONS | 1:56 | DOI: 10.1038/ncomms1053 | www.nature.com/naturecommunications 9 © 2010 Macmillan Publishers Limited. All rights reserved.

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