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The importance of initial application and reapplication of biochar in the context of soil structure improvement

The importance of initial application and reapplication of biochar in the context of soil... J. Hydrol. Hydromech., 69, 2021, 1, 87–97 ©2021. This is an open access article distributed DOI: 10.2478/johh-2020-0044 under the Creative Commons Attribution ISSN 1338-4333 NonCommercial-NoDerivatives 4.0 License The importance of initial application and reapplication of biochar in the context of soil structure improvement 1 2 2 1 3, 4 3 Martin Juriga , Elena Aydın , Ján Horák , Juraj Chlpík , Elena Y. Rizhiya , Natalya P. Buchkina , 3 1* Eugene V. Balashov , Vladimír Šimanský Department of Soil Science, Faculty of Agrobiology and Food Resources, Slovak University of Agriculture, 949 76 Nitra, Slovakia. Department of Biometeorology and Hydrology, Faculty of Horticulture and Landscape Engineering, Slovak University of Agriculture, 949 76 Nitra, Slovakia. Department of Soil Physics, Physical Chemistry and Biophysics, Agrophysical Research Institute, 14 Grazhdansky pr., 195220 St. Petersburg, Russia. Department of Geoecology, Nature Management and Environmental Safety, Faculty of Ecology, Russian State Hydrometeorological University, 79 Voronezhskaya str., 192007 St. Petersburg, Russia. Corresponding author. E-mail: vladimir.simansky@uniag.sk Abstract: It was shown that the use of biochar provides many benefits to agriculture by improving the whole complex of soil properties, including soil structure. However, the diverse range of biochar effects depends on its physicochemical properties, its application rates, soil initial properties etc. The impacts of biochar, mainly its reapplication to soils and its interaction with nitrogen in relation to water-stable aggregates (WSA) did not receive much attention to date. The aims of the study were: (1) to evaluate the effect of initial application (in spring 2014) and reapplication (in spring 2018) of –1 –1 different biochar rates (B0, B10 and B20 t ha ) as well as application of biochar with N-fertilizer (40 to 240 kg N ha depending on the requirement of the cultivated crop) on the content of WSA as one of the most important indicators of soil structure quality, (2) to assess the interrelationships between the contents of soil organic matter (SOM) and WSA. The study was conducted in 2017–2019 as part of the field experiment with biochar on Haplic Luvisol at the experimental station of SUA in Nitra, Slovakia. Results showed that initial application as well as reapplication of biochar improved soil structure. The most favorable changes in soil structure were found in N0B20B treatment (with biochar reapplication) at which a significantly higher content of water-stable macro-aggregates (WSAma) (+15%) as well as content of WSAma size fractions of > 5 mm, 5–3 mm, 3–2 mm and 2–1 mm (+72%, +65%, +57% and +64%, respectively) was observed compared to the control. An increase in SOM content, due to both, initial biochar application and its reapplication, significantly supported the stability of soil aggregates, while organic matter including humic substances composition did not. Keywords: Biochar; Soil organic matter; Water-stable aggregates; Soil structure; Haplic Luvisols. INTRODUCTION unit of pedal soil structure is a function of physical forming forces (such as: inter and intramolecular forces, electrostatic, Structure is one of the basic physical soil characteristics, and gravitational forces) between soil particles (Grosbellet et which controls a variety of physical, chemical and biological al., 2011; Hu et al., 2015) however, its stabilization is influ- processes in soils (Bronick and Lal, 2005; Czachor and enced by internal and external factors, and their interactions Lichner, 2013; Leelamanie and Karube, 2014; Shukla, 2014). (Bogunovic et al., 2020; Jozefaciuk and Czachor, 2014; Parade- Soil structure is usually defined as spatial arrangement of soil lo et al., 2013). organic substances, mineral particles and the pores between As it was mentioned above, an aggregate is the basic unit of them (Fiedler and Reissing, 1964; Rząsa and Owczarzak, soil structure. Based on their size, soil aggregates are divided 2004). Soil structure can also be described as an arrangement of into two groups: – macro-aggregates (> 250 μm) and micro- soil particles into larger formations of different shapes and sizes aggregates (< 250 μm). The aggregates resisting the action of called aggregates (Pires et al., 2017). The soil with good struc- water are called water-stable aggregates (Fulajtár, 2006). The ture has low compaction or bulk density and a large amount of higher content of water-stable aggregates, the better and more pore space. These soils have high infiltration rate, quick water stable the soil structure (Šimanský et al., 2018b). Soil organic movement through the profile, high water retention, high water matter (SOM) is one of the most important internal factors availability to roots, low crusting on soil surface, high gas affecting soil structure (Bronick and Lal, 2005; Tisdall and exchange, high nutrient availability, easier root penetration, Oades, 1982), since SOM acts as a significant binding agent for reduced surface runoff and soil erosion intensity (Shukla, soil particles (Bronick and Lal, 2005). SOM affects the water 2014). Soil structure is dynamic complex which is not very well stability of aggregates by decreasing their wettability and in- understood despite numerous advances in clay mineralogy, creasing their mechanical strength (Onweremadu et al., 2007). colloidal chemistry and other scientific areas which have since It is already well known that soil management practices influ- led to a better understanding of genesis, characterization, and ence the content and quality of SOM (Beck-Broichsitter et al., management of soil structure (Carter and Stewart, 1996). For- 2020; Leelamanie and Manawardana, 2019), which is a very mation of aggregates in the soils is one of the most important important factor of water-stable aggregates (WSA) formation problems of soil science. Formation of soil aggregates as basic (Onweremadu et al., 2007). 87 Martin Juriga, Elena Aydın, Ján Horák, Juraj Chlpík, Elena Y. Rizhiya, Natalya P. Buchkina, Eugene V. Balashov, Vladimír Šimanský SOM content and the stability of soil aggregates are two MATERIALS AND METHODS important soil attributes that are interconnected and Site characteristics interdependent. They are accounted as significant indicators of soil quality and degradation (Czachor and Lichner, 2013; Gaida The study was conducted on the experimental site of the et al., 2013; Leelamanie and Karube, 2014; Shukla, 2014). The Slovak University of Agriculture in Nitra, located approximate- breakdown of soil macro-aggregates can be followed by a ly 5 km from Nitra, Slovakia (Dolná Malanta, 48°19'00" N, release of SOM embodied in the aggregates (Šimanský, 2013). 18°09'00" E). The experimental site is located in the western A low SOM content can lead to disturbance of aggregation and part of the Žitavská Pahorkatina hills, in the catchment of Sele- soil structure (Bernardes et al., 2004; Six et al., 2004). The use nec Stream, at an altitude of 176 m above sea level. The terrain of imbalanced farming practices due to the intensification of is flat with a slight south-western gradient. The soil type is agriculture poses a serious problem deteriorating soil fertility. classified as Haplic Luvisol (IUSS, WRB, 2015) with silt The lack of organic fertilizers and removal of plant residues loamy texture (Šimanský et al., 2018a). Soil pH prior to the leads to a decrease in SOM content in soils. Regarding this experiment set up (in 2014) was 5.71 while the average soil –1 issue, using biochar as a soil ameliorant could be a suitable organic carbon content was 9.31 g kg soil. Other soil proper- alternative. Biochar is considered to be an important source of ties are given in Table 1. The area belongs to warm temperate especially stable C compounds and its application to soil has climate zone, fully humid with warm summers. The mean an- the potential to increase the content of SOM in the long run nual precipitation amount is 539 mm and the mean annual air (Gupta and Germida, 2015; Igaz et al., 2018; Kopittke et al., temperature is 9.8 °C (Čimo et al., 2012). 2019; Šimanský et al., 2018b). Biochar is a solid, carbon-rich Table 1. Soil properties prior to experiment establishment. product and is formed during the thermochemical decomposition of various types of organic materials (Shackley Soil properties Values et al., 2016). Application of biochar is reported to improve soil –1 Clay (g kg ) 249 chemical (Juriga and Šimanský, 2019; Liang et al. 2006), –1 Silt (g kg ) 599 physical (Balashov et al., 2019; Igaz et al., 2018; Toková et al., –1 Sand (g kg ) 152 2019; Vítková et al., 2017) and biological properties (Lehmann –1 Soil organic carbon (g kg ) 9.13 et al., 2011). Biochar also can increase crop yields (Kondrlová –1 CEC (mmol kg ) 142 et al., 2017), reduce GHG emissions and increase soil carbon Base saturation (%) 85 sequestration (Lehmann et al., 2006; Šimanský et al., 2018b). –3 pH (in 1 mol.dm KCl) 5.71 The use of biochar can offer many benefits to agriculture by improving the whole complex of soil properties, including soil Experiment description structure. However, the diverse range of effects of biochar on soil and The experiment with biochar in Dolná Malanta has been es- crop parameters depends on its physicochemical properties. The tablished in spring of 2014 before crop sowing. The crop rota- latter are influenced by the conditions of biochar production tion on the site was as follows: 2014 – spring barley (Hordeum (temperature and heating duration) and the origin of feedstock vulgare L.), 2015 – maize (Zea mays L.), 2016 – spring wheat (Enders et al., 2012). At the same time, the soil-climatic (Triticum aestivum L.), 2017 – maize, 2018 – spring barley and conditions significantly affect the behavior of biochar after soil 2019 – maize. The experiment originally consisted of nine amendment. It was observed that the effect of biochar in sandy treatments (plot setting from 2014 up to 2017). The number of soils was more evident than in loamy or heavy clay soils treatments increased by another six when biochar was applied (Blanco-Canqui, 2017). Biochar behavior in soil environment for the second time (at the same rates as in 2014) in spring 2018 also depends on whether biochar is combined with fertilizers, as (plot setting from 2018 up to 2019). In total, the experiment their interaction is also significant for soil structure parameters. currently consists of the following fifteen treatments: N fertilization is necessary to reduce the wide C:N ratio that 1. N0B0–without N fertilizer and biochar application, can significantly affect soil processes after biochar application –1 2. N0B10A–without N fertilizer and with 10 t ha of biochar, (Spokas et al., 2015). In addition, there is still a lack of –1 3. N0B20A–without N fertilizer and with 20 t ha of biochar, information in scientific literature on the effect of biochar –1 4. N0B10B–without N fertilizer and with 10 t ha of reapplied reapplication on the soil properties, including soil structure. biochar, Based on the above facts, we assumed that: (1) application –1 5. N0B20B–without N fertilizer and with 20 t ha of reapplied of biochar will improve soil structure, (2) soil structure im- biochar, provement will be more evident after application of a higher 6. N1B0–with the first level of N fertilizer and without biochar dose of biochar in comparison to a lower dose, (3) more stable application, soil structure will be observed in the case of biochar reapplica- –1 7. N1B10A–with the first level of N fertilizer and 10 t ha of tion than in the case of biochar initial application, (4) N- biochar, fertilization will intensify the effect of biochar on soil structure –1 8. N1B20A–with the first level of N fertilizer and 20 t ha of and (5) the improvement of the soil structure parameters will be biochar, significantly related to an increase of the SOM content. The –1 9. N1B10B–with the first level of N fertilizer and 10 t ha of aim of this study was: (1) to evaluate the effects of initial bio- reapplied biochar, char application and reapplication at different doses (0, 10 and –1 –1 10. N1B20B–with the first level of N fertilizer and 20 t ha of 20 t ha ) alone and in combination with N fertilizer (40 to 240 –1 reapplied biochar, kg N ha depending on the requirement of the cultivated crop) 11. N2B0–with the second level of N fertilizer and without on the content of water-stable aggregates, (2) to assess the biochar application, interrelationships between contents of SOM, humus and water- –1 12. N2B10A–with the second level of N fertilizer and 10 t ha stable aggregates. of biochar, 13. N2B20A–with the second level of N fertilizer and 20 t 88 The importance of initial application and reapplication of biochar in the context of soil structure improvement –1 ha of biochar, 2011). Soil organic carbon content (C ) was determined ox- org 14. N2B10B–with the second level of N fertilizer and 10 t idometrically (Dziadowiec and Gonet, 1999). Labile carbon –1 –3 ha of reapplied biochar, (C ) content was determined using 0.005 mol dm KMnO L 4 15. N2B20B–with the second level of N fertilizer and 20 t (Loginow et al., 1987). Composition of humus fractions (humic –1 ha of reapplied biochar. acids – HA and fulvic acids – FA) was determined according to The application doses of N fertilizer varied from year to year the Belchikova and Kononova procedure (Dziadowiec and depending on the cultivated crop. The first level of N fertiliza- Gonet, 1999) and the absorbance of humic substances and tion covered the usual N requirements of the cultivated crop to humic acids was measured at 465 and 650 nm to calculate the 4/6 4/6 obtain the average yield. The second level of fertilization was color quotient Q HS and Q HA (Dziadowiec and Gonet, 1999). 50% higher for maize and spring wheat (years 2015, 2017 and Statistical analysis 2019) and 100% higher for spring barley (years 2014, 2016, 2018). Nitrogen fertilizer application doses (calculated as pure N) in the years evaluated in this study were 160 and 240 kg N The content of water-stable aggregates was evaluated by the –1 –1 ha (2017), 40 and 80 kg N ha (2018), and 160 and 240 kg N statistical analysis in Statgraphics Centurion XV. I software –1 ha (2019) at the first and second fertilization level, respective- (Statpoint Technologies, Inc., USA) using one-way analysis of ly. During this period the same type of granulated N fertilizer – variance (ANOVA). LSD test with a significance level of α = LAD27 (Duslo Šaľa a.s., Šaľa, Slovakia) was used. The ferti- 0.05 was used to compare the effects of the individual doses of lizer contained 27% of total N (equal amounts in ammoniacal initial and reapplied biochar and biochar combined with N and nitrate form) and 4.1% of magnesium oxide. The biochar fertilizer. Dependencies between the water-stable aggregates (Sonnenerde, Riedlingsdorf, Austria) was produced from paper and the soil organic matter were evaluated using a simple corre- fiber sludge and grain husks (1:1). The feedstock was processed lation matrix and were expressed by a Pearson’s correlation by pyrolysis at the temperature of 550 °C for 30 minutes in a coefficient at the different probability levels. Pyreg reactor (Pyreg GmbH, Dörth, Germany). The composi- tion and properties of biochar are shown in Table 2. RESULTS Effects of the initial application of biochar and its Table 2. Basic composition and properties of applied biochar. combination with N fertilizer on water-stable aggregates Ash (%) 38.3 The contents of water-stable macro-aggregates (WSA ) and ma Total C (%) 53.1 water-stable micro-aggregates (WSA ) in investigated treat- mi Total N (%) 1.4 ments for the period of 3 years are summarized in Figure 1. –1 Ca (g kg ) 57 Due to application of biochar alone as well as biochar in –1 Mg (g kg ) 3.9 –1 combination with both levels of N fertilizer, the total content of K (g kg ) 15 –1 WSAma has increased, while the content of WSAmi has de- Na (g kg ) 0.7 –2 –1 creased. The average results obtained 3–5 years after initial Specific surface area (m g ) 21.7 application of biochar to the soil confirmed the ameliorant’s pH 8.8 favorable effect on the soil structure. The changes were signifi- Particle size range (mm) 1–5 cant in the treatments where biochar was applied without any N-fertilizer. The content of WSA was significantly higher Soil sampling ma (+9% and +14% in relation to total content of WSA ) and the ma content of WSA significantly lower (–23% and –38% in rela- The soil samples were collected in all treatments from a mi tion to total content of WSA ) in the N0B10A and N0B20A depth of 0–0.30 m. Soil sampling was carried out at monthly mi treatments, respectively, when compared to the control treat- intervals during the growing season of maize, spring barley and ment (N0B0). Thus, the effect of single biochar application at maize in 2017, 2018 and 2019, respectively. In other words, the –1 the rate of 20 t ha was more pronounced than at the rate of 10 sampling in 2017 took place 38–43 months after initial biochar –1 t ha . The effect of combined application of biochar with the application in the spring of 2014 (9 average soil samples first level N fertilizer was not significant. On the other hand, monthly, 54 soil samples in total for 2017). In 2018, it was the significant changes were observed with application of 20 t already 49–53 months after initial biochar application (9 aver- –1 ha of biochar in combination with the second level of N ferti- age soil samples monthly, 45 soil samples in total for 2018), but lizer (N2B20A): WSA content increased by 12% in relation only 1–4 months after biochar reapplication in the spring of ma to total content of WSA while WSA content decreased by 2018 (6 another average soil samples monthly, 24 soil samples ma mi 36% in relation to total content of WSA compared to the in total for 2018). In 2019, soil samples were collected 61–66 mi control at the second fertilization level (N2B0). months after initial biochar application (9 another average soil An increase in the content of the WSA larger size fraction samples monthly, 54 soil samples in total for 2019) and 12–16 ma (> 1 mm) was observed 3–5 years after the initial application of months after biochar reapplication (6 another average soil sam- biochar (Table 3). In the case of the non-fertilized treatments ples monthly, 30 soil samples in total for 2019). A total of 207 with biochar, significantly higher contents of WSAma (size soil samples were collected and subsequently analyzed for the fractions >5 mm, 5–3 mm, 3–2 mm and 2–1 mm) were ob- period 2017–2019. served in both treatments with biochar (N0B10A and N0B20A). The contents of these size fractions were respective- Soil analytical methods ly higher by 50%, 46%, 36% and 39% for N0B10A treatment and by 63%, 63%, 46% and 39% for N0B20A treatment in The following parameters were determined in the air-dried comparison to the control N0B0. At the same time, a signifi- soil samples: content of the individual size fractions of water- cantly higher content of agronomically valuable size fraction of stable macro-aggregates (WSA ) (size fractions of > 5 mm, ma WSA (3–0.5 mm) was also found. In the case of N0B10A and 5–3 mm, 3–2 mm, 2–1 mm, 1–0.5 mm, 0.5–0.25 mm and ma N0B20A treatments, the content of this size fraction of WSA < 0.25 mm) by the Bakshayev method (Hrivňáková et al., ma 89 Martin Juriga, Elena Aydın, Ján Horák, Juraj Chlpík, Elena Y. Rizhiya, Natalya P. Buchkina, Eugene V. Balashov, Vladimír Šimanský Fig. 1. Content (%) of water-stable macro-aggregates and micro-aggregates after the initial application of biochar (in 2014) treatments: (A) –1 –1 control (without biochar and N fertilizer application), (B) biochar at a rate of 10 t ha , (C) biochar at a rate of 20 t ha t ha , (D) control (at –1 –1 the first level of N fertilization), (E) biochar at a rate of 10 t ha + first level of N, (F) biochar at a rate of 20 t ha + first level of N, (G) –1 –1 control (at the second level of N fertilization), (H) biochar at a rate of 10 t ha + second level of N, (I) biochar at a rate of 20 t ha + second level of N. Table 3. Percentage representation of individual size fractions of water-stable macro-aggregates after the initial application of biochar (in 2014). WSA content (%) ma Treatments > 5 mm 5–3 mm 3–2 mm 2–1 mm 1–0.5 mm 0.5–0.25 mm 0.5–3 mm a a a a a a a N0B0 1.41±0.13 2.57±0.07 6.49±1.07 10.2±2.66 29.4±5.86 20.5±5.25 46.1±4.06 b b b b a a b N0B10A 2.81±0.57 4.75±1.05 10.2±0.76 16.7±2.79 25.9±5.83 16.9±3.77 52.7±1.58 b c b b a a b N0B20A 3.82±0.60 6.89±0.77 12.0±1.22 16.7±1.91 25.9±6.28 16.4±7.04 54.6±1.16 a a a a a a a N1B0 2.73±1.50 4.14±2.82 8.61±3.96 14.9±8.66 28.6±4.63 17.3±5.72 52.1±11.56 a a a a a a a N1B10A 3.15±2.33 4.86±2.46 9.25±3.53 12.6±6.25 30.4±7.53 16.8±4.24 52.2±8.07 a a a a a a a N1B20A 2.24±1.20 4.36±2.66 10.9±6.43 17.4±8.84 28.3±6.54 16.6±5.61 56.6±11.38 a a a a a b a N2B0 2.09±0.48 2.76±0.89 6.57±2.03 12.4±3.16 28.5±4.34 19.9±2.41 47.5±3.10 a ab a ab a ab a N2B10A 1.83±0.38 3.90±0.65 7.62±1.09 13.3±2.76 29.9±6.79 17.8±0.91 50.9±3.22 b b b b a a b N2B20A 3.48±0.54 5.70±1.16 11.9±2.70 19.4±2.86 27.7±7.85 14.1±3.41 59.0±2.77 The description of the treatment acronyms is given in the Materials and Methods Section. The different letters (a, b, c) between the rows express statistical significance according to the LSD test. WSA – water-stable macro-aggregates. ma was, respectively, 13% and 16% higher compared to the control content of any WSAma size fractions. However, at the second N N0B0 (Table 3). A significant difference in WSAma content was fertilization level, significant changes in the content of almost also observed between the two biochar treatments: the content all the WSAma size fractions (except the fraction of 1–0.5 mm) of the aggregate size fraction 5–3 mm in N0B20A treatment were observed at the higher rate of biochar application. In this was significantly higher (+31%) in comparison to N0B10A case, the trend in WSAma content was similar to the biochar treatment. The initial biochar application in combination with higher rate application without fertilizer. For the N2B20A the first level of N fertilizer did not significantly affect the treatment, a significantly higher proportion of WSA was ma 90 The importance of initial application and reapplication of biochar in the context of soil structure improvement determined for size fractions of > 5 mm, 5–3 mm, 3–2 mm and The mean values observed during the first two years (2018– 2–1 mm (+40%, +52%, +45% and +36%, respectively), when 2019) after biochar reapplication also showed positive changes compared to the control at the second N fertilization level. At in the content of individual WSA size fractions (Table 4). For ma the same time, a proportion of WSA of 0.5–0.25 mm was this parameter, a higher efficiency of biochar reapplication at a ma –1 –1 29% lower. The content of agronomically valuable aggregate rate of 20 t ha rather than 10 t ha was also confirmed. After size fraction (3–0.5 mm) in the N2B20A treatment was also reapplication of biochar at a lower rate (N0B10B), the WSAma higher (by 20%) than in the N control. In addition, a signifi- content of the three largest size fractions of > 5 mm, 5–3 mm cantly higher WSAma content was observed in N2B20A treat- and 3–2 mm increased significantly by 60%, 60% and 41%, ment for size fractions > 5 mm (+ 47%), 3–2 mm (+ 36%) and respectively. Biochar reapplication at the higher rate (N0B20B) 3–0.5 mm (+ 14%) when compared to N2B10A treatment. resulted in a significant increase of the WSAma content in the four largest size fractions of >5 mm, 5–3 mm, 3–2 mm and 2–1 Effect of biochar and biochar in combination with N mm by 72%, 65%, 57% and 64%, respectively. At the same fertilizer reapplication on the contents of water-stable time the content of WSA of the size fraction 0.5–0.25 mm ma aggregates decreased by 25% when compared to the control. Moreover, in the case of N0B20B treatment, a significantly higher content Soil structure has also significantly improved after reapplica- (+17%) of agronomically valuable WSA size fraction (3–0.5 ma tion of biochar or biochar in combination with N fertilizer mm) was also observed when compared to the control. In addi- (Fig. 2). After reapplication of biochar alone, a significant tion, the significant differences were also observed between the increase in a WSAma content was observed in the two treat- two biochar only treatments. The N0B20B treatment showed a ments with biochar (9% for N0B10B treatment and 15% for significantly higher content of WSAma in 5–3 mm, 3–2 mm and N0B20B treatment – in relation to total content of WSAma) 2–1 mm size fractions (+13%, +28% and +28%, respectively) when compared to the control. As the WSAma content in- when compared to the N0B10B treatment. No significant effect creased, the WSAmi content decreased significantly: by 24% in of biochar reapplication in combination with the first level of N N0B10B treatment and by 41% in N0B20B treatment in rela- fertilizer on the content of any WSA size fractions was found. ma tion to total content of WSA . Similar to the initial application In contrast, biochar reapplication in combination with the sec- mi of biochar, reapplication of biochar alone was more favorable at ond level of N fertilizer, resulted in a significantly higher –1 –1 a rate of 20 t ha rather than of 10 t ha . The obtained results WSA content of 5–3 mm size fraction (+45% for N2B10B ma confirm no significant effect from reapplication of biochar in treatment) and 3–2 mm size fraction (+44% for N2B20B treat- combination with the first level of N fertilizer. However, when ment) compared to the N-control. At the same time, the content biochar was reapplied together with the second level of N ferti- of WSA size fraction 0.5–0.25 mm significantly decreased (– ma lizer, higher WSAma contents (+12% and +11% in relation to 26% for N2B20B treatment). In addition, the content of the total content of WSAma) and lower WSAmi contents (–35% and agronomically valuable WSAma size fraction (3–0.5 mm) in- –30% in relation to total content of WSAmi) were found in creased significantly in both treatments: 19% and 22% for N2B10B and N2B20B treatments, respectively. N2B10B and + N2B20B treatments, respectively. Fig. 2. Content (%) of water-stable macro-aggregates and micro-aggregates in the treatments with reapplied biochar (initial 2014 and reap- –1 plied in 2018) in individual treatments: (A) control (without biochar and N fertilizer application), (B) reapplied biochar at a rate of 10 t ha , –1 –1 (C) reapplied biochar at a rate of 20 t ha t ha , (D) control (at the first level of N fertilization), (E) reapplied biochar at a rate of 10 t ha + –1 first level of N, (F) reapplied biochar at a rate of 20 t ha + first level of N, (G) control (at the second level of N fertilization), (H) reapplied –1 –1 biochar at a rate of 10 t ha + second level of N, (I) reapplied biochar at a rate of 20 t ha + second level of N. 91 Martin Juriga, Elena Aydın, Ján Horák, Juraj Chlpík, Elena Y. Rizhiya, Natalya P. Buchkina, Eugene V. Balashov, Vladimír Šimanský Table 4. Percentage representation of individual size fractions of water-stable macro-aggregates in the treatments with reapplied biochar (in 2018). WSAma content (%) Treatments > 5 mm 5–3 mm 3–2 mm 2–1 mm 1–0.5 mm 0.5–0.25 mm 0.5–3 mm a a a a a b a N0B0 1.64±1.10 2.37±0.70 5.66±1.87 8.79±1.43 29.3±6.75 21.9±2.65 43.7±5.29 b b b a a ab ab N0B10B 4.07±1.87 5.97±0.66 9.55±1.32 9.90±2.73 29.8±5.18 17.7±3.78 49.2±4.49 b c c b a a b N0B20B 5.80±1.68 6.86±0.85 13.3±1.38 13.8±1.46 25.8±3.49 16.5±2.04 52.9±5.48 a a a a a a a N1B0 3.24±1.62 4.55±1.26 8.45±2.95 16.5±4.02 28.8±2.61 18.1±5.92 53.7±10.6 a a a a a a a N1B10B 3.22±1.67 4.88±2.30 8.90±2.85 15.2±7.98 29.5±2.52 18.2±3.79 53.6±9.05 a a a a b a a N1B20B 2.81±1.19 3.57±0.92 17.4±1.48 14.2±6.17 35.9±1.07 17.5±3.04 57.5±6.34 a a a a a b a N2B0 2.47±1.67 2.47±0.69 5.38±1.30 10.5±7.11 29.6±3.67 21.1±1.87 45.5±5.81 a b ab a a ab b N2B10B 2.73±1.16 4.48±0.68 9.07±2.10 15.9±8.30 31.5±8.16 17.7±3.72 56.5±7.38 a ab b a a a b N2B20B 2.20±1.04 3.63±0.70 9.67±1.76 15.4±8.72 33.5±5.38 15.6±3.08 58.6±8.62 The description of the treatment acronyms is given in the Materials and Methods Section. The different letters (a, b, c) between the rows express statistical significance according to the LSD test. WSAma – water-stable macro-aggregates. Table 5. Correlation relationships between parameters of soil organic matter, humus and soil structure parameters for the treatments with initial biochar application (in 2014). Individual size fractions of WSA (mm) ma Parameter WSAma WSAmi > 5 5–3 3–2 2–1 1–0.5 0.5–0.25 0.5–3 Corg 0.268* –0.268* 0.436*** 0.277* n. s. n. s. n. s. n. s. n. s. CL n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. CHS –0.329** 0.329** –0.346** n. s. n. s. n. s. n. s. n. s. n. s. C –0.342** 0.342** –0.375** n. s. n. s. n. s. n. s. n. s. n. s. HA C n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. FA C :C n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. HA FA Q n. s. n. s. n. s. n. s. n. s. n. s. n. s. –0.262* n. s. HS QHA n. s. n. s. n. s. n. s. n. s. n. s. n. s. –0.271* n. s. * = P ≤ 0.05, ** = P ≤ 0.01, *** = P ≤ 0.001, n. s. – nonsignificant. C –total organic carbon, C –labile carbon, C –carbon of humic substances, C –carbon of humic acids, C –carbon of fulvic acids, org L HS HA FA CHA:CFA–ratio between carbon of humic acids and carbon of fulvic acids, QHS–color quotient of humic substances, QHA–color quotient of humic acids, WSAma–water-stable macro-aggregates, WSAmi–water-stable micro-aggregates. Evaluation of relationships between parameters of soil quotients of humic substances (QHS) and humic acids (QHA), organic matter and water-stable aggregates affected by which are the expression of the humic substances and humic initial biochar application acids stability, were in negative correlation with the content of the smallest WSAma size fraction (0.5–0.25 mm). Correlations between contents of soil organic matter (SOM) and water-stable aggregates depending on the initial biochar Evaluation of relationships between parameters of soil application are shown in Table 5. The results of our study organic matter and water-stable aggregates affected by showed that for the period 2017–2019 the soil organic carbon biochar reapplication (C ) content had a positive effect on the content of WSA . At org ma the same time, the content of C significantly affected the According to our results, the C significantly contributed to org org content of the two largest WSA size fractions (> 5 mm and a higher content of WSA and to a content of agronomically ma ma 5–3 mm) but did not affect the content of the smaller water- valuable WSA size fraction after biochar reapplication into ma stable aggregates. the soil (Table 6). A significant effect of Corg on the content of No significant correlations were observed between soil the three largest WSAma size fractions (> 5 mm, 5–3 mm and structure and the soil labile carbon content (CL). The obtained 3–2 mm) was also demonstrated, while it had a significant results also showed that the extractable carbon of humic sub- negative effect on the content of the smallest WSAma size frac- stances (CHS) and humic acids (CHA) had a significant negative tion. CL content after biochar reapplication significantly con- correlation with WSAma content as well as with the content of tributed to the formation of WSAma of 1–0.5 mm. CHS and CHA the largest WSA size fraction (> 5 mm). In addition, the color contents negatively correlated with the content of WSA size ma ma 92 The importance of initial application and reapplication of biochar in the context of soil structure improvement Table 6. Correlation relationships between parameters of soil organic matter, humus and soil structure parameters in the treatments with reapplied biochar (initial 2014 and reapplied in 2018). Individual size fractions of WSAma (mm) Parameter WSAma WSAmi > 5 5–3 3–2 2–1 1–0.5 0.5–0.25 0.5–3 Corg 0.342** –0.342** 0.326** 0.313* 0.447*** n. s. n. s. –0.303* 0.287* C n. s. n. s. n. s. n. s. n. s. n. s. 0.262* n. s. n. s. C –0.299* 0.299* –0.328** –0.326** –0.380** n. s. n. s. 0.265* n. s. HS C –0.330** 0.330** –0.319* –0.359** –0.453*** n. s. n. s. 0.268* n. s. HA C n. s. n. s. –0.276* n. s. n. s. n. s. n. s. n. s. n. s. FA CHA:CFA n. s. n. s. n. s. n. s. –0.328** n. s. n. s. n. s. n. s. QHS n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. QHA n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. * = P ≤ 0.05, ** = P ≤ 0.01, *** = P ≤ 0.001, n. s. – nonsignificant. C –total organic carbon, C –labile carbon, C –carbon of humic substances, C –carbon of humic acids, C –carbon of fulvic acids, org L HS HA FA CHA:CFA–ration between carbon of humic acids to carbon of fulvic acids, QHS–color quotient of humic substances, QHA–color quotient of humic acids, WSA –water-stable macro-aggregates, WSA –water-stable micro-aggregates. ma mi fractions of > 5 mm, 5–3 mm and 3–2 mm and positively corre- and at a high temperature usually has a lower content of nutri- lated with the content of 0.5–0.25 mm size fraction of WSA . ents and aliphatic C compounds. In addition, it has a larger ma In addition, the CHA:CFA ratio negatively correlated with the specific surface area and sorption capacity (Igaz et al., 2018). content of 3–2 mm size fraction of WSAma. These properties of biochar can have a negative effect on ag- gregation, especially in sandy soils, which are poor in SOM and DISCUSSION nutrients, and where the microbial activity leading to aggrega- Effects of initial application of biochar and biochar with tion can be suppressed. On the other hand, Demisie et al. (2014) N-fertilizer on soil structure observed an increased stability of soil aggregates due to an increase of SOM and microbial activity after biochar (made of Up to date, several authors have described an improvement wood at a temperature of 600 °C) application into the clay soil. in soil structural status due to application of biochar (Gupta and In our case, biochar was made from cereal husks and paper Germida, 2015; Juriga et al., 2018; Lu et al., 2014; Obia et al., fiber sludge in a ratio of 1:1 and at a pyrolysis temperature of 2016; Sing and Cowie, 2014; Šimanský et al., 2016). For soil 550 °C. Biochars are generally characterized by a broad C:N structure evaluation, WSA content is one of the most suitable ratio, which may contribute to the lack of N for plants as well ma and popular parameters as it expresses the representation of as soil microorganisms after its depletion from the soil. There- individual size fractions of macro-aggregates and their re- fore, it appears important to apply N-rich fertilizers, which sistance to water forces (Scott, 2000). According to Šimanský contribute to a narrower C:N ratio and promote plant growth and Bajčan (2014), content of WSAma size fraction of 3–0.5 and microbial activity, together with biochar (Spokas et al., mm is the most valuable for agronomic practice, because at the 2015). For this reason, we also included into the experiment the optimal content of these aggregates in a soil, favorable physical treatments where biochar application was combined with min- conditions, suitable for most of the cultivated crops, are being eral N fertilization at two different application levels. Mineral formed. The content of these aggregates was significantly high- fertilizers are an important source of nutrients for plants as well er in N0B10A (+13%) and N0B20A (+16%) treatments when as for soil microorganisms. Through their application, the pro- compared to the control. In overall, biochar application at a rate duction of root and above-ground plant biomass can be in- –1 of 20 t ha proved to be more effective than at a rate of 10 t creased, which contributes to a higher SOM content and soil –1 ha . This finding corresponds to the statement of Blanco- structure improvement. However, an increase in microbial Canqui (2017), according to which the amount of applied bio- activity may increase the degree of SOM mineralization, which char is a key factor in the extent of its effects on soil structure. in turn may worsen the soil structure conditions (Dong et al., –1 –1 In our case, rates of 10 t ha and 20 t ha of biochar were used, 2014; Geisseler and Scow, 2014). Since biochar is a source of however from the available literature it is clear that much high- mainly stable aromatic C compounds, it can reduce the decom- er rates are also often used in other experiments. For example, position of organic C by stabilizing its labile fractions and thus Dong et al. (2016) tested the efficiency of biochar application at promote aggregation (Elzobair et al., 2017; Jien et al., 2015). –1 –1 –1 rates of 30 t ha , 60 t ha and 90 t ha . The results of their These statements are in agreement with our findings, as signifi- three-year study showed that the content of WSAma increased cantly better soil structure was formed in the case of biochar –1 significantly when compared to the control only in case of application at the rate of 20 t ha combined with the second –1 biochar application at a rate of 60 t ha (+45%). Their study level of N fertilizer when compared to N fertilization alone. was performed on a sandy soil and, as pointed out by Blanco- Canqui (2017), the effect of biochar on soil structure improve- Effects of reapplication of biochar and biochar with ment to a large extent also depends on soil texture. Although N fertilizer on water-stable aggregates more pronounced effects are usually observed on sandy soils, as reported by Gul et al. (2015), the properties of biochar, depend- Biochar applied to the soil undergoes gradual changes, ing on the pyrolysis temperature and the type of feedstock, are which also cause a change in its effects on the soil properties. crucial. Thus, biochar made from the material rich in cellulose Some of the biochar effects on soils and crops can be weakened 93 Martin Juriga, Elena Aydın, Ján Horák, Juraj Chlpík, Elena Y. Rizhiya, Natalya P. Buchkina, Eugene V. Balashov, Vladimír Šimanský or even lost during the process of biochar aging. For this rea- treatment N2B20B it was +44% for 3–2 mm size fraction of son, it is rational to consider repeated application of biochar to WSA . However, at the first level of N fertilization, no signifi- ma soils at certain time intervals (Blanco-Canqui, 2017; Ren et al., cant difference was found between the treatments with a com- 2018). Regarding soil structure, different effects can be ob- bination of biochar and N fertilizer and the treatment with N served shortly after the application of biochar. Due to its pre- fertilizer alone. dominantly aromatic structure, biochars are characterized by a relatively low content of aliphatic C compounds, which are Relationship between soil organic matter and water-stable released into the soil shortly after the incorporation of biochar aggregates influenced by initial biochar application and are considered an important element for the formation and stability of soil aggregates (Moreno-Barriga et al., 2017; The effects of initial biochar application and biochar with N Mukherjee and Lal, 2013). On the other hand, as Blanco- fertilizer on the parameters of SOM during the period of 2017– Canqui, (2017) claimed, the effects on the soil structure change 2019 have been partially published (Juriga et al., 2018). The become apparent only after a certain time after biochar applica- formation of soil aggregates is generally dependent on the tion. In our case, biochar reapplication significantly improved presence of cementing agents combining the soil particles into the soil structure parameters already during the first two years. micro-aggregates and micro-aggregates into macro-aggregates This is proven by the significantly higher WSA content in (Bronick and Lal, 2005; Tisdall and Oades, 1982). As already ma both biochar alone treatments: N0B10B and N0B20B (+9% and mentioned, SOM is a key component in the soil aggregation +15%, respectively) as well as by the improvement of other process. For this reason, the addition of organic matter to the evaluated soil structure parameters when compared to the con- soil is considered to be an effective tool to improve soil struc- trol. The use of mineral N fertilizers often leads to a decrease in ture, especially in soils with low SOM content (Bronick and soil pH, which can result in worse soil structure (Haynes and Lal, 2005; Demisie et al., 2014). Biochar has the potential to Naidu, 1998). However, Bethlenfalvay et al. (1999) pointed out increase Corg content in soil in the long run (Gupta and Germi- that N fertilization can significantly improve the structural da, 2015; Juriga et al., 2018; Lu et al., 2014), which corre- conditions of soils (in alkaline soils) also through the acidifying sponds to the findings of Juriga et al. (2018), who observed a effect. They found that lowering pH due to N fertilization pro- significant increase in C content due to initial application of org moted the activity of soil fungi, thereby increased the formation biochar. The results of our study showed that the increased and stability of soil aggregates. On the other hand, Fungo et al. content of C significantly contributed to the formation of org (2017) tested the effect of urea alone and urea in combination larger size fractions of WSA (> 5 and 5–3 mm) and to the ma with biochar on soil structure. They observed that fertilization improvement of the parameters describing stability of water- with urea alone did not have a significant effect on the WSA stable macro-aggregates. C content is an important indicator of ma L content, but in combination with biochar, 15% higher WSAma soil quality, which can also significantly contribute to the for- content was found. A significant improvement of soil structure mation and stability of soil aggregates (Šimanský, 2013), which due to combined reapplication of biochar and N fertilizer when has been proven so far in many studies (e. g. Juriga et al., 2018; compared to N fertilizer alone was also observed in our study at Polláková et al., 2018; Šimanský et al., 2016). However, in our the second level of N fertilization. In our case, LAD 27 was case, soil structure was not significantly affected by the CL used as an N fertilizer and the fertilizer’s physiological acidic content. Juriga et al. (2018) reported a statistically significant effect was eliminated by added dolomite. Overall, alkaline decrease in the extractability of CHS, CHA and CFA due to initial biochar tends to increase soil pH, especially of acid soils, which application of biochar. The humic substances are also consid- was also our case (Horák, 2015; Šimanský et al., 2018a). At the ered to be an important cementing agent involved in the for- same time, after biochar reapplication, we found a significant mation and stability of soil aggregates (Bronick and Lal, 2005; increase in the content of larger WSA (size fraction of > 5–1 Jindo et al., 2016; Spaccini et al., 2002). As reported by ma mm). The beneficial effect of biochar reapplication on the Mierzwa-Hersztek et al. (2018), biochar added to the soil could individual WSA size fractions was also reported by Liu et al. result in a decrease of the humic acid carbon and fulvic acid ma (2014) for sandy soils. However, these authors observed a carbon in relation to the total organic carbon content, what in significant increase in the content of larger and smaller WSAma our case could be the reason of observed significant negative size fractions of > 2 mm (+33%), 2–0.5 mm (+28%) and 0.5– correlations between CHS, CHA contents and WSAma content 0.25 mm (+24%) when compared to the control. According to after the initial application of biochar (Table 5). Wang et al. (2018) long-term mineral N fertilization tends to increase the proportion of especially larger WSAma size frac- Relationships between soil organic matter and water-stable tions (> 2 mm). For example, Wang et al. (2014) found that N aggregates influenced by biochar reapplication fertilization significantly increased the proportion of macro- aggregates of larger size fractions (> 2 mm) by +7% when In soils where SOM is the main substance affecting the soil compared to the control. However, the share of smaller macro- structure, a role of C in the formation of individual aggregate org aggregates did not significantly increase. The authors explained size fractions tends to increase with aggregate size (Lugato et that this fertilizing effect was a result of an increase in root al., 2010; Wang et al., 2017). This statement is in agreement biomass and fungal mycelium in these aggregate size fractions, with our findings, as the C content contributed to the for- org what is consistent with the statement of Bronick and Lal (2005) mation of the three largest WSA size fractions (> 5 mm, 5–3 ma that plant roots and fungal mycelium are important factors mm and 3–2 mm). According to Demisie et al. (2014), biochar influencing the formation of larger macro-aggregates in particu- reapplication can significantly increase the CL content in the lar. Our results further showed that the effect of combined soil, but this effect tends to be short-term. In addition, biochar –1 –1 reapplication of 10 t ha and 20 t ha of biochar together with particles can bind CL from the soil, which can reduce its content the second level of N fertilizer was significantly stronger than in the soil and thus reduce its participation in aggregation. The the effect of N fertilizer alone. In the N2B10B treatment a CL content after biochar reapplication significantly contributed significantly higher WSA content of 5–3 mm size fraction to the formation of WSA of 1–0.5 mm but did not increase ma ma was found (+45%) in comparison with the N-control and in the the stability of water-stable macro-aggregates. Humic acids can 94 The importance of initial application and reapplication of biochar in the context of soil structure improvement react with clay minerals to form organic-mineral colloids, 1/0747/20 and VEGA 1/0064/19, by the SLOVAK which are the foundation for soil aggregates. At the same time, RESEARCH AND DEVELOPMENT AGENDY, grant number humic acids can be absorbed by metal oxides, changing their APVV-15-0160 and CULTURAL AND EDUCATIONAL electric charge and thus increasing their participation in soil GRANT AGENCY, grant number KEGA 019SPU-4/2020. aggregation (Bronick and Lal, 2005). Due to its high surface This publication is also the result of „Scientific support of activity, biochar can bind humic acids on its surface, which climate change adaptation in agriculture and mitigation of soil increases its reactivity and thus its contribution to the formation degradation” (ITMS2014+ 313011W580) project implementa- of soil structure (Liang et al., 2015). However, in our case we tion supported by the Integrated Infrastructure Operational found significant negative correlations between soil structure Programme funded by the ERDF. Drs Balashov E.V., Buchkina and the humic substances after biochar reapplication (Table 6), N.P. and Rizhiya E.Y. were working according to the scientific which could be explained in a decrease of the humic acid car- plan of the Agrophysical Research Institute. bon and fulvic acid carbon in relation to the total organic car- bon content after biochar reapplication as it was published in Conflicts of interest. The authors declare no conflict of interest. this experiment by Juriga et al. (2018). Zhao et al. (2017) also pointed out that such results may be related to the properties of REFERENCES the biochar used. They found that biochar produced at lower pyrolysis temperatures (300 °C and 400 °C) had a higher con- Balashov, E., Mukhina, I., Rizhiya, E., 2019. 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Total Environ., 624, 1131–1139. ochar and biochar with nitrogen can improve the parameters Zhao, S., Tan, N., Li, Z., Yang, Y., Zhang, X., Liu, D., Zhang, of soil organic matter and soil structure? Biologia, 71, A., Wang, X., 2017. Varying pyrolysis temperature impacts 989–995. application effects of biochar on soil labile organic carbon Šimanský, V., Horák, J., Igaz, D., Balashov, E., Jonczak, J., and humic fractions. Appl. Soil Ecol., 116, 399–409. 2018a. Biochar and biochar with N fertilizer as a potential tool for improving soil sorption of nutrients. J. Soils Sedi- Received 9 October 2020 ments, 18, 1432–1440. Accepted 7 November 2020 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Hydrology and Hydromechanics de Gruyter

The importance of initial application and reapplication of biochar in the context of soil structure improvement

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Abstract

J. Hydrol. Hydromech., 69, 2021, 1, 87–97 ©2021. This is an open access article distributed DOI: 10.2478/johh-2020-0044 under the Creative Commons Attribution ISSN 1338-4333 NonCommercial-NoDerivatives 4.0 License The importance of initial application and reapplication of biochar in the context of soil structure improvement 1 2 2 1 3, 4 3 Martin Juriga , Elena Aydın , Ján Horák , Juraj Chlpík , Elena Y. Rizhiya , Natalya P. Buchkina , 3 1* Eugene V. Balashov , Vladimír Šimanský Department of Soil Science, Faculty of Agrobiology and Food Resources, Slovak University of Agriculture, 949 76 Nitra, Slovakia. Department of Biometeorology and Hydrology, Faculty of Horticulture and Landscape Engineering, Slovak University of Agriculture, 949 76 Nitra, Slovakia. Department of Soil Physics, Physical Chemistry and Biophysics, Agrophysical Research Institute, 14 Grazhdansky pr., 195220 St. Petersburg, Russia. Department of Geoecology, Nature Management and Environmental Safety, Faculty of Ecology, Russian State Hydrometeorological University, 79 Voronezhskaya str., 192007 St. Petersburg, Russia. Corresponding author. E-mail: vladimir.simansky@uniag.sk Abstract: It was shown that the use of biochar provides many benefits to agriculture by improving the whole complex of soil properties, including soil structure. However, the diverse range of biochar effects depends on its physicochemical properties, its application rates, soil initial properties etc. The impacts of biochar, mainly its reapplication to soils and its interaction with nitrogen in relation to water-stable aggregates (WSA) did not receive much attention to date. The aims of the study were: (1) to evaluate the effect of initial application (in spring 2014) and reapplication (in spring 2018) of –1 –1 different biochar rates (B0, B10 and B20 t ha ) as well as application of biochar with N-fertilizer (40 to 240 kg N ha depending on the requirement of the cultivated crop) on the content of WSA as one of the most important indicators of soil structure quality, (2) to assess the interrelationships between the contents of soil organic matter (SOM) and WSA. The study was conducted in 2017–2019 as part of the field experiment with biochar on Haplic Luvisol at the experimental station of SUA in Nitra, Slovakia. Results showed that initial application as well as reapplication of biochar improved soil structure. The most favorable changes in soil structure were found in N0B20B treatment (with biochar reapplication) at which a significantly higher content of water-stable macro-aggregates (WSAma) (+15%) as well as content of WSAma size fractions of > 5 mm, 5–3 mm, 3–2 mm and 2–1 mm (+72%, +65%, +57% and +64%, respectively) was observed compared to the control. An increase in SOM content, due to both, initial biochar application and its reapplication, significantly supported the stability of soil aggregates, while organic matter including humic substances composition did not. Keywords: Biochar; Soil organic matter; Water-stable aggregates; Soil structure; Haplic Luvisols. INTRODUCTION unit of pedal soil structure is a function of physical forming forces (such as: inter and intramolecular forces, electrostatic, Structure is one of the basic physical soil characteristics, and gravitational forces) between soil particles (Grosbellet et which controls a variety of physical, chemical and biological al., 2011; Hu et al., 2015) however, its stabilization is influ- processes in soils (Bronick and Lal, 2005; Czachor and enced by internal and external factors, and their interactions Lichner, 2013; Leelamanie and Karube, 2014; Shukla, 2014). (Bogunovic et al., 2020; Jozefaciuk and Czachor, 2014; Parade- Soil structure is usually defined as spatial arrangement of soil lo et al., 2013). organic substances, mineral particles and the pores between As it was mentioned above, an aggregate is the basic unit of them (Fiedler and Reissing, 1964; Rząsa and Owczarzak, soil structure. Based on their size, soil aggregates are divided 2004). Soil structure can also be described as an arrangement of into two groups: – macro-aggregates (> 250 μm) and micro- soil particles into larger formations of different shapes and sizes aggregates (< 250 μm). The aggregates resisting the action of called aggregates (Pires et al., 2017). The soil with good struc- water are called water-stable aggregates (Fulajtár, 2006). The ture has low compaction or bulk density and a large amount of higher content of water-stable aggregates, the better and more pore space. These soils have high infiltration rate, quick water stable the soil structure (Šimanský et al., 2018b). Soil organic movement through the profile, high water retention, high water matter (SOM) is one of the most important internal factors availability to roots, low crusting on soil surface, high gas affecting soil structure (Bronick and Lal, 2005; Tisdall and exchange, high nutrient availability, easier root penetration, Oades, 1982), since SOM acts as a significant binding agent for reduced surface runoff and soil erosion intensity (Shukla, soil particles (Bronick and Lal, 2005). SOM affects the water 2014). Soil structure is dynamic complex which is not very well stability of aggregates by decreasing their wettability and in- understood despite numerous advances in clay mineralogy, creasing their mechanical strength (Onweremadu et al., 2007). colloidal chemistry and other scientific areas which have since It is already well known that soil management practices influ- led to a better understanding of genesis, characterization, and ence the content and quality of SOM (Beck-Broichsitter et al., management of soil structure (Carter and Stewart, 1996). For- 2020; Leelamanie and Manawardana, 2019), which is a very mation of aggregates in the soils is one of the most important important factor of water-stable aggregates (WSA) formation problems of soil science. Formation of soil aggregates as basic (Onweremadu et al., 2007). 87 Martin Juriga, Elena Aydın, Ján Horák, Juraj Chlpík, Elena Y. Rizhiya, Natalya P. Buchkina, Eugene V. Balashov, Vladimír Šimanský SOM content and the stability of soil aggregates are two MATERIALS AND METHODS important soil attributes that are interconnected and Site characteristics interdependent. They are accounted as significant indicators of soil quality and degradation (Czachor and Lichner, 2013; Gaida The study was conducted on the experimental site of the et al., 2013; Leelamanie and Karube, 2014; Shukla, 2014). The Slovak University of Agriculture in Nitra, located approximate- breakdown of soil macro-aggregates can be followed by a ly 5 km from Nitra, Slovakia (Dolná Malanta, 48°19'00" N, release of SOM embodied in the aggregates (Šimanský, 2013). 18°09'00" E). The experimental site is located in the western A low SOM content can lead to disturbance of aggregation and part of the Žitavská Pahorkatina hills, in the catchment of Sele- soil structure (Bernardes et al., 2004; Six et al., 2004). The use nec Stream, at an altitude of 176 m above sea level. The terrain of imbalanced farming practices due to the intensification of is flat with a slight south-western gradient. The soil type is agriculture poses a serious problem deteriorating soil fertility. classified as Haplic Luvisol (IUSS, WRB, 2015) with silt The lack of organic fertilizers and removal of plant residues loamy texture (Šimanský et al., 2018a). Soil pH prior to the leads to a decrease in SOM content in soils. Regarding this experiment set up (in 2014) was 5.71 while the average soil –1 issue, using biochar as a soil ameliorant could be a suitable organic carbon content was 9.31 g kg soil. Other soil proper- alternative. Biochar is considered to be an important source of ties are given in Table 1. The area belongs to warm temperate especially stable C compounds and its application to soil has climate zone, fully humid with warm summers. The mean an- the potential to increase the content of SOM in the long run nual precipitation amount is 539 mm and the mean annual air (Gupta and Germida, 2015; Igaz et al., 2018; Kopittke et al., temperature is 9.8 °C (Čimo et al., 2012). 2019; Šimanský et al., 2018b). Biochar is a solid, carbon-rich Table 1. Soil properties prior to experiment establishment. product and is formed during the thermochemical decomposition of various types of organic materials (Shackley Soil properties Values et al., 2016). Application of biochar is reported to improve soil –1 Clay (g kg ) 249 chemical (Juriga and Šimanský, 2019; Liang et al. 2006), –1 Silt (g kg ) 599 physical (Balashov et al., 2019; Igaz et al., 2018; Toková et al., –1 Sand (g kg ) 152 2019; Vítková et al., 2017) and biological properties (Lehmann –1 Soil organic carbon (g kg ) 9.13 et al., 2011). Biochar also can increase crop yields (Kondrlová –1 CEC (mmol kg ) 142 et al., 2017), reduce GHG emissions and increase soil carbon Base saturation (%) 85 sequestration (Lehmann et al., 2006; Šimanský et al., 2018b). –3 pH (in 1 mol.dm KCl) 5.71 The use of biochar can offer many benefits to agriculture by improving the whole complex of soil properties, including soil Experiment description structure. However, the diverse range of effects of biochar on soil and The experiment with biochar in Dolná Malanta has been es- crop parameters depends on its physicochemical properties. The tablished in spring of 2014 before crop sowing. The crop rota- latter are influenced by the conditions of biochar production tion on the site was as follows: 2014 – spring barley (Hordeum (temperature and heating duration) and the origin of feedstock vulgare L.), 2015 – maize (Zea mays L.), 2016 – spring wheat (Enders et al., 2012). At the same time, the soil-climatic (Triticum aestivum L.), 2017 – maize, 2018 – spring barley and conditions significantly affect the behavior of biochar after soil 2019 – maize. The experiment originally consisted of nine amendment. It was observed that the effect of biochar in sandy treatments (plot setting from 2014 up to 2017). The number of soils was more evident than in loamy or heavy clay soils treatments increased by another six when biochar was applied (Blanco-Canqui, 2017). Biochar behavior in soil environment for the second time (at the same rates as in 2014) in spring 2018 also depends on whether biochar is combined with fertilizers, as (plot setting from 2018 up to 2019). In total, the experiment their interaction is also significant for soil structure parameters. currently consists of the following fifteen treatments: N fertilization is necessary to reduce the wide C:N ratio that 1. N0B0–without N fertilizer and biochar application, can significantly affect soil processes after biochar application –1 2. N0B10A–without N fertilizer and with 10 t ha of biochar, (Spokas et al., 2015). In addition, there is still a lack of –1 3. N0B20A–without N fertilizer and with 20 t ha of biochar, information in scientific literature on the effect of biochar –1 4. N0B10B–without N fertilizer and with 10 t ha of reapplied reapplication on the soil properties, including soil structure. biochar, Based on the above facts, we assumed that: (1) application –1 5. N0B20B–without N fertilizer and with 20 t ha of reapplied of biochar will improve soil structure, (2) soil structure im- biochar, provement will be more evident after application of a higher 6. N1B0–with the first level of N fertilizer and without biochar dose of biochar in comparison to a lower dose, (3) more stable application, soil structure will be observed in the case of biochar reapplica- –1 7. N1B10A–with the first level of N fertilizer and 10 t ha of tion than in the case of biochar initial application, (4) N- biochar, fertilization will intensify the effect of biochar on soil structure –1 8. N1B20A–with the first level of N fertilizer and 20 t ha of and (5) the improvement of the soil structure parameters will be biochar, significantly related to an increase of the SOM content. The –1 9. N1B10B–with the first level of N fertilizer and 10 t ha of aim of this study was: (1) to evaluate the effects of initial bio- reapplied biochar, char application and reapplication at different doses (0, 10 and –1 –1 10. N1B20B–with the first level of N fertilizer and 20 t ha of 20 t ha ) alone and in combination with N fertilizer (40 to 240 –1 reapplied biochar, kg N ha depending on the requirement of the cultivated crop) 11. N2B0–with the second level of N fertilizer and without on the content of water-stable aggregates, (2) to assess the biochar application, interrelationships between contents of SOM, humus and water- –1 12. N2B10A–with the second level of N fertilizer and 10 t ha stable aggregates. of biochar, 13. N2B20A–with the second level of N fertilizer and 20 t 88 The importance of initial application and reapplication of biochar in the context of soil structure improvement –1 ha of biochar, 2011). Soil organic carbon content (C ) was determined ox- org 14. N2B10B–with the second level of N fertilizer and 10 t idometrically (Dziadowiec and Gonet, 1999). Labile carbon –1 –3 ha of reapplied biochar, (C ) content was determined using 0.005 mol dm KMnO L 4 15. N2B20B–with the second level of N fertilizer and 20 t (Loginow et al., 1987). Composition of humus fractions (humic –1 ha of reapplied biochar. acids – HA and fulvic acids – FA) was determined according to The application doses of N fertilizer varied from year to year the Belchikova and Kononova procedure (Dziadowiec and depending on the cultivated crop. The first level of N fertiliza- Gonet, 1999) and the absorbance of humic substances and tion covered the usual N requirements of the cultivated crop to humic acids was measured at 465 and 650 nm to calculate the 4/6 4/6 obtain the average yield. The second level of fertilization was color quotient Q HS and Q HA (Dziadowiec and Gonet, 1999). 50% higher for maize and spring wheat (years 2015, 2017 and Statistical analysis 2019) and 100% higher for spring barley (years 2014, 2016, 2018). Nitrogen fertilizer application doses (calculated as pure N) in the years evaluated in this study were 160 and 240 kg N The content of water-stable aggregates was evaluated by the –1 –1 ha (2017), 40 and 80 kg N ha (2018), and 160 and 240 kg N statistical analysis in Statgraphics Centurion XV. I software –1 ha (2019) at the first and second fertilization level, respective- (Statpoint Technologies, Inc., USA) using one-way analysis of ly. During this period the same type of granulated N fertilizer – variance (ANOVA). LSD test with a significance level of α = LAD27 (Duslo Šaľa a.s., Šaľa, Slovakia) was used. The ferti- 0.05 was used to compare the effects of the individual doses of lizer contained 27% of total N (equal amounts in ammoniacal initial and reapplied biochar and biochar combined with N and nitrate form) and 4.1% of magnesium oxide. The biochar fertilizer. Dependencies between the water-stable aggregates (Sonnenerde, Riedlingsdorf, Austria) was produced from paper and the soil organic matter were evaluated using a simple corre- fiber sludge and grain husks (1:1). The feedstock was processed lation matrix and were expressed by a Pearson’s correlation by pyrolysis at the temperature of 550 °C for 30 minutes in a coefficient at the different probability levels. Pyreg reactor (Pyreg GmbH, Dörth, Germany). The composi- tion and properties of biochar are shown in Table 2. RESULTS Effects of the initial application of biochar and its Table 2. Basic composition and properties of applied biochar. combination with N fertilizer on water-stable aggregates Ash (%) 38.3 The contents of water-stable macro-aggregates (WSA ) and ma Total C (%) 53.1 water-stable micro-aggregates (WSA ) in investigated treat- mi Total N (%) 1.4 ments for the period of 3 years are summarized in Figure 1. –1 Ca (g kg ) 57 Due to application of biochar alone as well as biochar in –1 Mg (g kg ) 3.9 –1 combination with both levels of N fertilizer, the total content of K (g kg ) 15 –1 WSAma has increased, while the content of WSAmi has de- Na (g kg ) 0.7 –2 –1 creased. The average results obtained 3–5 years after initial Specific surface area (m g ) 21.7 application of biochar to the soil confirmed the ameliorant’s pH 8.8 favorable effect on the soil structure. The changes were signifi- Particle size range (mm) 1–5 cant in the treatments where biochar was applied without any N-fertilizer. The content of WSA was significantly higher Soil sampling ma (+9% and +14% in relation to total content of WSA ) and the ma content of WSA significantly lower (–23% and –38% in rela- The soil samples were collected in all treatments from a mi tion to total content of WSA ) in the N0B10A and N0B20A depth of 0–0.30 m. Soil sampling was carried out at monthly mi treatments, respectively, when compared to the control treat- intervals during the growing season of maize, spring barley and ment (N0B0). Thus, the effect of single biochar application at maize in 2017, 2018 and 2019, respectively. In other words, the –1 the rate of 20 t ha was more pronounced than at the rate of 10 sampling in 2017 took place 38–43 months after initial biochar –1 t ha . The effect of combined application of biochar with the application in the spring of 2014 (9 average soil samples first level N fertilizer was not significant. On the other hand, monthly, 54 soil samples in total for 2017). In 2018, it was the significant changes were observed with application of 20 t already 49–53 months after initial biochar application (9 aver- –1 ha of biochar in combination with the second level of N ferti- age soil samples monthly, 45 soil samples in total for 2018), but lizer (N2B20A): WSA content increased by 12% in relation only 1–4 months after biochar reapplication in the spring of ma to total content of WSA while WSA content decreased by 2018 (6 another average soil samples monthly, 24 soil samples ma mi 36% in relation to total content of WSA compared to the in total for 2018). In 2019, soil samples were collected 61–66 mi control at the second fertilization level (N2B0). months after initial biochar application (9 another average soil An increase in the content of the WSA larger size fraction samples monthly, 54 soil samples in total for 2019) and 12–16 ma (> 1 mm) was observed 3–5 years after the initial application of months after biochar reapplication (6 another average soil sam- biochar (Table 3). In the case of the non-fertilized treatments ples monthly, 30 soil samples in total for 2019). A total of 207 with biochar, significantly higher contents of WSAma (size soil samples were collected and subsequently analyzed for the fractions >5 mm, 5–3 mm, 3–2 mm and 2–1 mm) were ob- period 2017–2019. served in both treatments with biochar (N0B10A and N0B20A). The contents of these size fractions were respective- Soil analytical methods ly higher by 50%, 46%, 36% and 39% for N0B10A treatment and by 63%, 63%, 46% and 39% for N0B20A treatment in The following parameters were determined in the air-dried comparison to the control N0B0. At the same time, a signifi- soil samples: content of the individual size fractions of water- cantly higher content of agronomically valuable size fraction of stable macro-aggregates (WSA ) (size fractions of > 5 mm, ma WSA (3–0.5 mm) was also found. In the case of N0B10A and 5–3 mm, 3–2 mm, 2–1 mm, 1–0.5 mm, 0.5–0.25 mm and ma N0B20A treatments, the content of this size fraction of WSA < 0.25 mm) by the Bakshayev method (Hrivňáková et al., ma 89 Martin Juriga, Elena Aydın, Ján Horák, Juraj Chlpík, Elena Y. Rizhiya, Natalya P. Buchkina, Eugene V. Balashov, Vladimír Šimanský Fig. 1. Content (%) of water-stable macro-aggregates and micro-aggregates after the initial application of biochar (in 2014) treatments: (A) –1 –1 control (without biochar and N fertilizer application), (B) biochar at a rate of 10 t ha , (C) biochar at a rate of 20 t ha t ha , (D) control (at –1 –1 the first level of N fertilization), (E) biochar at a rate of 10 t ha + first level of N, (F) biochar at a rate of 20 t ha + first level of N, (G) –1 –1 control (at the second level of N fertilization), (H) biochar at a rate of 10 t ha + second level of N, (I) biochar at a rate of 20 t ha + second level of N. Table 3. Percentage representation of individual size fractions of water-stable macro-aggregates after the initial application of biochar (in 2014). WSA content (%) ma Treatments > 5 mm 5–3 mm 3–2 mm 2–1 mm 1–0.5 mm 0.5–0.25 mm 0.5–3 mm a a a a a a a N0B0 1.41±0.13 2.57±0.07 6.49±1.07 10.2±2.66 29.4±5.86 20.5±5.25 46.1±4.06 b b b b a a b N0B10A 2.81±0.57 4.75±1.05 10.2±0.76 16.7±2.79 25.9±5.83 16.9±3.77 52.7±1.58 b c b b a a b N0B20A 3.82±0.60 6.89±0.77 12.0±1.22 16.7±1.91 25.9±6.28 16.4±7.04 54.6±1.16 a a a a a a a N1B0 2.73±1.50 4.14±2.82 8.61±3.96 14.9±8.66 28.6±4.63 17.3±5.72 52.1±11.56 a a a a a a a N1B10A 3.15±2.33 4.86±2.46 9.25±3.53 12.6±6.25 30.4±7.53 16.8±4.24 52.2±8.07 a a a a a a a N1B20A 2.24±1.20 4.36±2.66 10.9±6.43 17.4±8.84 28.3±6.54 16.6±5.61 56.6±11.38 a a a a a b a N2B0 2.09±0.48 2.76±0.89 6.57±2.03 12.4±3.16 28.5±4.34 19.9±2.41 47.5±3.10 a ab a ab a ab a N2B10A 1.83±0.38 3.90±0.65 7.62±1.09 13.3±2.76 29.9±6.79 17.8±0.91 50.9±3.22 b b b b a a b N2B20A 3.48±0.54 5.70±1.16 11.9±2.70 19.4±2.86 27.7±7.85 14.1±3.41 59.0±2.77 The description of the treatment acronyms is given in the Materials and Methods Section. The different letters (a, b, c) between the rows express statistical significance according to the LSD test. WSA – water-stable macro-aggregates. ma was, respectively, 13% and 16% higher compared to the control content of any WSAma size fractions. However, at the second N N0B0 (Table 3). A significant difference in WSAma content was fertilization level, significant changes in the content of almost also observed between the two biochar treatments: the content all the WSAma size fractions (except the fraction of 1–0.5 mm) of the aggregate size fraction 5–3 mm in N0B20A treatment were observed at the higher rate of biochar application. In this was significantly higher (+31%) in comparison to N0B10A case, the trend in WSAma content was similar to the biochar treatment. The initial biochar application in combination with higher rate application without fertilizer. For the N2B20A the first level of N fertilizer did not significantly affect the treatment, a significantly higher proportion of WSA was ma 90 The importance of initial application and reapplication of biochar in the context of soil structure improvement determined for size fractions of > 5 mm, 5–3 mm, 3–2 mm and The mean values observed during the first two years (2018– 2–1 mm (+40%, +52%, +45% and +36%, respectively), when 2019) after biochar reapplication also showed positive changes compared to the control at the second N fertilization level. At in the content of individual WSA size fractions (Table 4). For ma the same time, a proportion of WSA of 0.5–0.25 mm was this parameter, a higher efficiency of biochar reapplication at a ma –1 –1 29% lower. The content of agronomically valuable aggregate rate of 20 t ha rather than 10 t ha was also confirmed. After size fraction (3–0.5 mm) in the N2B20A treatment was also reapplication of biochar at a lower rate (N0B10B), the WSAma higher (by 20%) than in the N control. In addition, a signifi- content of the three largest size fractions of > 5 mm, 5–3 mm cantly higher WSAma content was observed in N2B20A treat- and 3–2 mm increased significantly by 60%, 60% and 41%, ment for size fractions > 5 mm (+ 47%), 3–2 mm (+ 36%) and respectively. Biochar reapplication at the higher rate (N0B20B) 3–0.5 mm (+ 14%) when compared to N2B10A treatment. resulted in a significant increase of the WSAma content in the four largest size fractions of >5 mm, 5–3 mm, 3–2 mm and 2–1 Effect of biochar and biochar in combination with N mm by 72%, 65%, 57% and 64%, respectively. At the same fertilizer reapplication on the contents of water-stable time the content of WSA of the size fraction 0.5–0.25 mm ma aggregates decreased by 25% when compared to the control. Moreover, in the case of N0B20B treatment, a significantly higher content Soil structure has also significantly improved after reapplica- (+17%) of agronomically valuable WSA size fraction (3–0.5 ma tion of biochar or biochar in combination with N fertilizer mm) was also observed when compared to the control. In addi- (Fig. 2). After reapplication of biochar alone, a significant tion, the significant differences were also observed between the increase in a WSAma content was observed in the two treat- two biochar only treatments. The N0B20B treatment showed a ments with biochar (9% for N0B10B treatment and 15% for significantly higher content of WSAma in 5–3 mm, 3–2 mm and N0B20B treatment – in relation to total content of WSAma) 2–1 mm size fractions (+13%, +28% and +28%, respectively) when compared to the control. As the WSAma content in- when compared to the N0B10B treatment. No significant effect creased, the WSAmi content decreased significantly: by 24% in of biochar reapplication in combination with the first level of N N0B10B treatment and by 41% in N0B20B treatment in rela- fertilizer on the content of any WSA size fractions was found. ma tion to total content of WSA . Similar to the initial application In contrast, biochar reapplication in combination with the sec- mi of biochar, reapplication of biochar alone was more favorable at ond level of N fertilizer, resulted in a significantly higher –1 –1 a rate of 20 t ha rather than of 10 t ha . The obtained results WSA content of 5–3 mm size fraction (+45% for N2B10B ma confirm no significant effect from reapplication of biochar in treatment) and 3–2 mm size fraction (+44% for N2B20B treat- combination with the first level of N fertilizer. However, when ment) compared to the N-control. At the same time, the content biochar was reapplied together with the second level of N ferti- of WSA size fraction 0.5–0.25 mm significantly decreased (– ma lizer, higher WSAma contents (+12% and +11% in relation to 26% for N2B20B treatment). In addition, the content of the total content of WSAma) and lower WSAmi contents (–35% and agronomically valuable WSAma size fraction (3–0.5 mm) in- –30% in relation to total content of WSAmi) were found in creased significantly in both treatments: 19% and 22% for N2B10B and N2B20B treatments, respectively. N2B10B and + N2B20B treatments, respectively. Fig. 2. Content (%) of water-stable macro-aggregates and micro-aggregates in the treatments with reapplied biochar (initial 2014 and reap- –1 plied in 2018) in individual treatments: (A) control (without biochar and N fertilizer application), (B) reapplied biochar at a rate of 10 t ha , –1 –1 (C) reapplied biochar at a rate of 20 t ha t ha , (D) control (at the first level of N fertilization), (E) reapplied biochar at a rate of 10 t ha + –1 first level of N, (F) reapplied biochar at a rate of 20 t ha + first level of N, (G) control (at the second level of N fertilization), (H) reapplied –1 –1 biochar at a rate of 10 t ha + second level of N, (I) reapplied biochar at a rate of 20 t ha + second level of N. 91 Martin Juriga, Elena Aydın, Ján Horák, Juraj Chlpík, Elena Y. Rizhiya, Natalya P. Buchkina, Eugene V. Balashov, Vladimír Šimanský Table 4. Percentage representation of individual size fractions of water-stable macro-aggregates in the treatments with reapplied biochar (in 2018). WSAma content (%) Treatments > 5 mm 5–3 mm 3–2 mm 2–1 mm 1–0.5 mm 0.5–0.25 mm 0.5–3 mm a a a a a b a N0B0 1.64±1.10 2.37±0.70 5.66±1.87 8.79±1.43 29.3±6.75 21.9±2.65 43.7±5.29 b b b a a ab ab N0B10B 4.07±1.87 5.97±0.66 9.55±1.32 9.90±2.73 29.8±5.18 17.7±3.78 49.2±4.49 b c c b a a b N0B20B 5.80±1.68 6.86±0.85 13.3±1.38 13.8±1.46 25.8±3.49 16.5±2.04 52.9±5.48 a a a a a a a N1B0 3.24±1.62 4.55±1.26 8.45±2.95 16.5±4.02 28.8±2.61 18.1±5.92 53.7±10.6 a a a a a a a N1B10B 3.22±1.67 4.88±2.30 8.90±2.85 15.2±7.98 29.5±2.52 18.2±3.79 53.6±9.05 a a a a b a a N1B20B 2.81±1.19 3.57±0.92 17.4±1.48 14.2±6.17 35.9±1.07 17.5±3.04 57.5±6.34 a a a a a b a N2B0 2.47±1.67 2.47±0.69 5.38±1.30 10.5±7.11 29.6±3.67 21.1±1.87 45.5±5.81 a b ab a a ab b N2B10B 2.73±1.16 4.48±0.68 9.07±2.10 15.9±8.30 31.5±8.16 17.7±3.72 56.5±7.38 a ab b a a a b N2B20B 2.20±1.04 3.63±0.70 9.67±1.76 15.4±8.72 33.5±5.38 15.6±3.08 58.6±8.62 The description of the treatment acronyms is given in the Materials and Methods Section. The different letters (a, b, c) between the rows express statistical significance according to the LSD test. WSAma – water-stable macro-aggregates. Table 5. Correlation relationships between parameters of soil organic matter, humus and soil structure parameters for the treatments with initial biochar application (in 2014). Individual size fractions of WSA (mm) ma Parameter WSAma WSAmi > 5 5–3 3–2 2–1 1–0.5 0.5–0.25 0.5–3 Corg 0.268* –0.268* 0.436*** 0.277* n. s. n. s. n. s. n. s. n. s. CL n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. CHS –0.329** 0.329** –0.346** n. s. n. s. n. s. n. s. n. s. n. s. C –0.342** 0.342** –0.375** n. s. n. s. n. s. n. s. n. s. n. s. HA C n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. FA C :C n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. HA FA Q n. s. n. s. n. s. n. s. n. s. n. s. n. s. –0.262* n. s. HS QHA n. s. n. s. n. s. n. s. n. s. n. s. n. s. –0.271* n. s. * = P ≤ 0.05, ** = P ≤ 0.01, *** = P ≤ 0.001, n. s. – nonsignificant. C –total organic carbon, C –labile carbon, C –carbon of humic substances, C –carbon of humic acids, C –carbon of fulvic acids, org L HS HA FA CHA:CFA–ratio between carbon of humic acids and carbon of fulvic acids, QHS–color quotient of humic substances, QHA–color quotient of humic acids, WSAma–water-stable macro-aggregates, WSAmi–water-stable micro-aggregates. Evaluation of relationships between parameters of soil quotients of humic substances (QHS) and humic acids (QHA), organic matter and water-stable aggregates affected by which are the expression of the humic substances and humic initial biochar application acids stability, were in negative correlation with the content of the smallest WSAma size fraction (0.5–0.25 mm). Correlations between contents of soil organic matter (SOM) and water-stable aggregates depending on the initial biochar Evaluation of relationships between parameters of soil application are shown in Table 5. The results of our study organic matter and water-stable aggregates affected by showed that for the period 2017–2019 the soil organic carbon biochar reapplication (C ) content had a positive effect on the content of WSA . At org ma the same time, the content of C significantly affected the According to our results, the C significantly contributed to org org content of the two largest WSA size fractions (> 5 mm and a higher content of WSA and to a content of agronomically ma ma 5–3 mm) but did not affect the content of the smaller water- valuable WSA size fraction after biochar reapplication into ma stable aggregates. the soil (Table 6). A significant effect of Corg on the content of No significant correlations were observed between soil the three largest WSAma size fractions (> 5 mm, 5–3 mm and structure and the soil labile carbon content (CL). The obtained 3–2 mm) was also demonstrated, while it had a significant results also showed that the extractable carbon of humic sub- negative effect on the content of the smallest WSAma size frac- stances (CHS) and humic acids (CHA) had a significant negative tion. CL content after biochar reapplication significantly con- correlation with WSAma content as well as with the content of tributed to the formation of WSAma of 1–0.5 mm. CHS and CHA the largest WSA size fraction (> 5 mm). In addition, the color contents negatively correlated with the content of WSA size ma ma 92 The importance of initial application and reapplication of biochar in the context of soil structure improvement Table 6. Correlation relationships between parameters of soil organic matter, humus and soil structure parameters in the treatments with reapplied biochar (initial 2014 and reapplied in 2018). Individual size fractions of WSAma (mm) Parameter WSAma WSAmi > 5 5–3 3–2 2–1 1–0.5 0.5–0.25 0.5–3 Corg 0.342** –0.342** 0.326** 0.313* 0.447*** n. s. n. s. –0.303* 0.287* C n. s. n. s. n. s. n. s. n. s. n. s. 0.262* n. s. n. s. C –0.299* 0.299* –0.328** –0.326** –0.380** n. s. n. s. 0.265* n. s. HS C –0.330** 0.330** –0.319* –0.359** –0.453*** n. s. n. s. 0.268* n. s. HA C n. s. n. s. –0.276* n. s. n. s. n. s. n. s. n. s. n. s. FA CHA:CFA n. s. n. s. n. s. n. s. –0.328** n. s. n. s. n. s. n. s. QHS n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. QHA n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. n. s. * = P ≤ 0.05, ** = P ≤ 0.01, *** = P ≤ 0.001, n. s. – nonsignificant. C –total organic carbon, C –labile carbon, C –carbon of humic substances, C –carbon of humic acids, C –carbon of fulvic acids, org L HS HA FA CHA:CFA–ration between carbon of humic acids to carbon of fulvic acids, QHS–color quotient of humic substances, QHA–color quotient of humic acids, WSA –water-stable macro-aggregates, WSA –water-stable micro-aggregates. ma mi fractions of > 5 mm, 5–3 mm and 3–2 mm and positively corre- and at a high temperature usually has a lower content of nutri- lated with the content of 0.5–0.25 mm size fraction of WSA . ents and aliphatic C compounds. In addition, it has a larger ma In addition, the CHA:CFA ratio negatively correlated with the specific surface area and sorption capacity (Igaz et al., 2018). content of 3–2 mm size fraction of WSAma. These properties of biochar can have a negative effect on ag- gregation, especially in sandy soils, which are poor in SOM and DISCUSSION nutrients, and where the microbial activity leading to aggrega- Effects of initial application of biochar and biochar with tion can be suppressed. On the other hand, Demisie et al. (2014) N-fertilizer on soil structure observed an increased stability of soil aggregates due to an increase of SOM and microbial activity after biochar (made of Up to date, several authors have described an improvement wood at a temperature of 600 °C) application into the clay soil. in soil structural status due to application of biochar (Gupta and In our case, biochar was made from cereal husks and paper Germida, 2015; Juriga et al., 2018; Lu et al., 2014; Obia et al., fiber sludge in a ratio of 1:1 and at a pyrolysis temperature of 2016; Sing and Cowie, 2014; Šimanský et al., 2016). For soil 550 °C. Biochars are generally characterized by a broad C:N structure evaluation, WSA content is one of the most suitable ratio, which may contribute to the lack of N for plants as well ma and popular parameters as it expresses the representation of as soil microorganisms after its depletion from the soil. There- individual size fractions of macro-aggregates and their re- fore, it appears important to apply N-rich fertilizers, which sistance to water forces (Scott, 2000). According to Šimanský contribute to a narrower C:N ratio and promote plant growth and Bajčan (2014), content of WSAma size fraction of 3–0.5 and microbial activity, together with biochar (Spokas et al., mm is the most valuable for agronomic practice, because at the 2015). For this reason, we also included into the experiment the optimal content of these aggregates in a soil, favorable physical treatments where biochar application was combined with min- conditions, suitable for most of the cultivated crops, are being eral N fertilization at two different application levels. Mineral formed. The content of these aggregates was significantly high- fertilizers are an important source of nutrients for plants as well er in N0B10A (+13%) and N0B20A (+16%) treatments when as for soil microorganisms. Through their application, the pro- compared to the control. In overall, biochar application at a rate duction of root and above-ground plant biomass can be in- –1 of 20 t ha proved to be more effective than at a rate of 10 t creased, which contributes to a higher SOM content and soil –1 ha . This finding corresponds to the statement of Blanco- structure improvement. However, an increase in microbial Canqui (2017), according to which the amount of applied bio- activity may increase the degree of SOM mineralization, which char is a key factor in the extent of its effects on soil structure. in turn may worsen the soil structure conditions (Dong et al., –1 –1 In our case, rates of 10 t ha and 20 t ha of biochar were used, 2014; Geisseler and Scow, 2014). Since biochar is a source of however from the available literature it is clear that much high- mainly stable aromatic C compounds, it can reduce the decom- er rates are also often used in other experiments. For example, position of organic C by stabilizing its labile fractions and thus Dong et al. (2016) tested the efficiency of biochar application at promote aggregation (Elzobair et al., 2017; Jien et al., 2015). –1 –1 –1 rates of 30 t ha , 60 t ha and 90 t ha . The results of their These statements are in agreement with our findings, as signifi- three-year study showed that the content of WSAma increased cantly better soil structure was formed in the case of biochar –1 significantly when compared to the control only in case of application at the rate of 20 t ha combined with the second –1 biochar application at a rate of 60 t ha (+45%). Their study level of N fertilizer when compared to N fertilization alone. was performed on a sandy soil and, as pointed out by Blanco- Canqui (2017), the effect of biochar on soil structure improve- Effects of reapplication of biochar and biochar with ment to a large extent also depends on soil texture. Although N fertilizer on water-stable aggregates more pronounced effects are usually observed on sandy soils, as reported by Gul et al. (2015), the properties of biochar, depend- Biochar applied to the soil undergoes gradual changes, ing on the pyrolysis temperature and the type of feedstock, are which also cause a change in its effects on the soil properties. crucial. Thus, biochar made from the material rich in cellulose Some of the biochar effects on soils and crops can be weakened 93 Martin Juriga, Elena Aydın, Ján Horák, Juraj Chlpík, Elena Y. Rizhiya, Natalya P. Buchkina, Eugene V. Balashov, Vladimír Šimanský or even lost during the process of biochar aging. For this rea- treatment N2B20B it was +44% for 3–2 mm size fraction of son, it is rational to consider repeated application of biochar to WSA . However, at the first level of N fertilization, no signifi- ma soils at certain time intervals (Blanco-Canqui, 2017; Ren et al., cant difference was found between the treatments with a com- 2018). Regarding soil structure, different effects can be ob- bination of biochar and N fertilizer and the treatment with N served shortly after the application of biochar. Due to its pre- fertilizer alone. dominantly aromatic structure, biochars are characterized by a relatively low content of aliphatic C compounds, which are Relationship between soil organic matter and water-stable released into the soil shortly after the incorporation of biochar aggregates influenced by initial biochar application and are considered an important element for the formation and stability of soil aggregates (Moreno-Barriga et al., 2017; The effects of initial biochar application and biochar with N Mukherjee and Lal, 2013). On the other hand, as Blanco- fertilizer on the parameters of SOM during the period of 2017– Canqui, (2017) claimed, the effects on the soil structure change 2019 have been partially published (Juriga et al., 2018). The become apparent only after a certain time after biochar applica- formation of soil aggregates is generally dependent on the tion. In our case, biochar reapplication significantly improved presence of cementing agents combining the soil particles into the soil structure parameters already during the first two years. micro-aggregates and micro-aggregates into macro-aggregates This is proven by the significantly higher WSA content in (Bronick and Lal, 2005; Tisdall and Oades, 1982). As already ma both biochar alone treatments: N0B10B and N0B20B (+9% and mentioned, SOM is a key component in the soil aggregation +15%, respectively) as well as by the improvement of other process. For this reason, the addition of organic matter to the evaluated soil structure parameters when compared to the con- soil is considered to be an effective tool to improve soil struc- trol. The use of mineral N fertilizers often leads to a decrease in ture, especially in soils with low SOM content (Bronick and soil pH, which can result in worse soil structure (Haynes and Lal, 2005; Demisie et al., 2014). Biochar has the potential to Naidu, 1998). However, Bethlenfalvay et al. (1999) pointed out increase Corg content in soil in the long run (Gupta and Germi- that N fertilization can significantly improve the structural da, 2015; Juriga et al., 2018; Lu et al., 2014), which corre- conditions of soils (in alkaline soils) also through the acidifying sponds to the findings of Juriga et al. (2018), who observed a effect. They found that lowering pH due to N fertilization pro- significant increase in C content due to initial application of org moted the activity of soil fungi, thereby increased the formation biochar. The results of our study showed that the increased and stability of soil aggregates. On the other hand, Fungo et al. content of C significantly contributed to the formation of org (2017) tested the effect of urea alone and urea in combination larger size fractions of WSA (> 5 and 5–3 mm) and to the ma with biochar on soil structure. They observed that fertilization improvement of the parameters describing stability of water- with urea alone did not have a significant effect on the WSA stable macro-aggregates. C content is an important indicator of ma L content, but in combination with biochar, 15% higher WSAma soil quality, which can also significantly contribute to the for- content was found. A significant improvement of soil structure mation and stability of soil aggregates (Šimanský, 2013), which due to combined reapplication of biochar and N fertilizer when has been proven so far in many studies (e. g. Juriga et al., 2018; compared to N fertilizer alone was also observed in our study at Polláková et al., 2018; Šimanský et al., 2016). However, in our the second level of N fertilization. In our case, LAD 27 was case, soil structure was not significantly affected by the CL used as an N fertilizer and the fertilizer’s physiological acidic content. Juriga et al. (2018) reported a statistically significant effect was eliminated by added dolomite. Overall, alkaline decrease in the extractability of CHS, CHA and CFA due to initial biochar tends to increase soil pH, especially of acid soils, which application of biochar. The humic substances are also consid- was also our case (Horák, 2015; Šimanský et al., 2018a). At the ered to be an important cementing agent involved in the for- same time, after biochar reapplication, we found a significant mation and stability of soil aggregates (Bronick and Lal, 2005; increase in the content of larger WSA (size fraction of > 5–1 Jindo et al., 2016; Spaccini et al., 2002). As reported by ma mm). The beneficial effect of biochar reapplication on the Mierzwa-Hersztek et al. (2018), biochar added to the soil could individual WSA size fractions was also reported by Liu et al. result in a decrease of the humic acid carbon and fulvic acid ma (2014) for sandy soils. However, these authors observed a carbon in relation to the total organic carbon content, what in significant increase in the content of larger and smaller WSAma our case could be the reason of observed significant negative size fractions of > 2 mm (+33%), 2–0.5 mm (+28%) and 0.5– correlations between CHS, CHA contents and WSAma content 0.25 mm (+24%) when compared to the control. According to after the initial application of biochar (Table 5). Wang et al. (2018) long-term mineral N fertilization tends to increase the proportion of especially larger WSAma size frac- Relationships between soil organic matter and water-stable tions (> 2 mm). For example, Wang et al. (2014) found that N aggregates influenced by biochar reapplication fertilization significantly increased the proportion of macro- aggregates of larger size fractions (> 2 mm) by +7% when In soils where SOM is the main substance affecting the soil compared to the control. However, the share of smaller macro- structure, a role of C in the formation of individual aggregate org aggregates did not significantly increase. The authors explained size fractions tends to increase with aggregate size (Lugato et that this fertilizing effect was a result of an increase in root al., 2010; Wang et al., 2017). This statement is in agreement biomass and fungal mycelium in these aggregate size fractions, with our findings, as the C content contributed to the for- org what is consistent with the statement of Bronick and Lal (2005) mation of the three largest WSA size fractions (> 5 mm, 5–3 ma that plant roots and fungal mycelium are important factors mm and 3–2 mm). According to Demisie et al. (2014), biochar influencing the formation of larger macro-aggregates in particu- reapplication can significantly increase the CL content in the lar. Our results further showed that the effect of combined soil, but this effect tends to be short-term. In addition, biochar –1 –1 reapplication of 10 t ha and 20 t ha of biochar together with particles can bind CL from the soil, which can reduce its content the second level of N fertilizer was significantly stronger than in the soil and thus reduce its participation in aggregation. The the effect of N fertilizer alone. In the N2B10B treatment a CL content after biochar reapplication significantly contributed significantly higher WSA content of 5–3 mm size fraction to the formation of WSA of 1–0.5 mm but did not increase ma ma was found (+45%) in comparison with the N-control and in the the stability of water-stable macro-aggregates. Humic acids can 94 The importance of initial application and reapplication of biochar in the context of soil structure improvement react with clay minerals to form organic-mineral colloids, 1/0747/20 and VEGA 1/0064/19, by the SLOVAK which are the foundation for soil aggregates. At the same time, RESEARCH AND DEVELOPMENT AGENDY, grant number humic acids can be absorbed by metal oxides, changing their APVV-15-0160 and CULTURAL AND EDUCATIONAL electric charge and thus increasing their participation in soil GRANT AGENCY, grant number KEGA 019SPU-4/2020. aggregation (Bronick and Lal, 2005). Due to its high surface This publication is also the result of „Scientific support of activity, biochar can bind humic acids on its surface, which climate change adaptation in agriculture and mitigation of soil increases its reactivity and thus its contribution to the formation degradation” (ITMS2014+ 313011W580) project implementa- of soil structure (Liang et al., 2015). However, in our case we tion supported by the Integrated Infrastructure Operational found significant negative correlations between soil structure Programme funded by the ERDF. Drs Balashov E.V., Buchkina and the humic substances after biochar reapplication (Table 6), N.P. and Rizhiya E.Y. were working according to the scientific which could be explained in a decrease of the humic acid car- plan of the Agrophysical Research Institute. bon and fulvic acid carbon in relation to the total organic car- bon content after biochar reapplication as it was published in Conflicts of interest. The authors declare no conflict of interest. this experiment by Juriga et al. (2018). Zhao et al. (2017) also pointed out that such results may be related to the properties of REFERENCES the biochar used. They found that biochar produced at lower pyrolysis temperatures (300 °C and 400 °C) had a higher con- Balashov, E., Mukhina, I., Rizhiya, E., 2019. 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Journal

Journal of Hydrology and Hydromechanicsde Gruyter

Published: Mar 1, 2021

Keywords: Biochar; Soil organic matter; Water-stable aggregates; Soil structure; Haplic Luvisols

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