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Short‐term corn yield response associated with nitrogen dynamics from fall‐seeded cover crops under no‐till dryland conditions

Short‐term corn yield response associated with nitrogen dynamics from fall‐seeded cover crops... AbbreviationsANMapparent nitrogen mineralizedGDDgrowing degree daysNUEnitrogen use efficiencyPANplant available nitrogenINTRODUCTIONNitrogen (N) is commonly the limiting factor in corn (Zea mays L.) production. Ideally, N fertilization should account for the difference between soil available N and N required for optimum crop yield, but complex soil N dynamics make predicting annual N application amounts difficult (Tao et al., 2018; White et al., 2021). Benefits of improving our ability to apply the right amount of N based on understanding of soil N dynamics include reducing the risk of nitrate contamination of surface and ground water, reducing hypoxia in freshwater and marine systems, and increasing profit for producers (Isbell et al., 2021; Malakoff, 1998; Meisinger et al., 1991). Crop management practices and their interactions with environmental factors produce annual variability in soil N dynamics and challenge the ability to determine the most efficient in‐season N crop requirements.One such crop management practice is cover cropping; there is increased interest among producers to include cover crops in no‐till production systems, particularly with the ability to chemically terminate cover crops without harming the cash crop. Cover crops can increase cash crop yields, sequester soil carbon (C), scavenge fall soil N to reduce leaching, fix atmospheric N, reduce soil erosion, and control weed species (Abdalla et al., 2019; Bowman et al., 2000; Thorup‐Kristensen et al., 2003). Although there are multiple benefits of cover crops, it remains difficult to predict how cover crops affect N dynamics in‐season and to estimate available N for the following cash crop. Efficient N management depends on the timing of crop residue decomposition, whether the mineralized N will be lost by leaching or temporally immobilized, and whether temporally immobilized N is remineralized in synchrony with cash crop demands (Huntington et al., 1985; Isbell et al., 2021; Nevins et al., 2020; Wagger, 1989). Grass‐type cover crops can significantly reduce N leaching and are recognized as effective scavengers of residual N but may provide limited N for the immediately following cash crop (Ditsch & Alley, 1991; McCracken et al., 1994; Shipley et al., 1992; Wagger, 1989). Legume cover crops can supply significant amounts of N for the subsequent cash crop and may minimize N leaching during winter and spring (Coombs et al., 2017; Ebelhar et al., 1984; Rosecrance et al., 2000; Wagger, 1989).In the face of increasing costs for fertilizer N and awareness of water quality issues, producers are looking for regionally relevant information on cover crops influence on fertilizer requirements for the following cash crop, commonly corn, in the U.S. Midwest. Major factors that control annual N mineralization patterns are the amount and composition of plant residues, soil temperature, precipitation patterns, and soil characteristics (Kätterer et al., 1998; Kruse et al., 2004; White et al., 2021). Thus, regional climate and annual weather conditions, soil type, and field management are inherent considerations for estimations of N available for corn crops following cover crops.Using representative legume and grass cover crops under no‐till, dryland conditions in the Mollisol soils of the U.S. Northern Plains, our objectives were to (a) determine the effect of cover crops on corn yields, and (b) estimate the amount of N that can be supplied to corn by mineralization of the cover crop residues. Common methods of examining soil N mineralization measure potentially mineralizable N using aerobic and anaerobic incubations in the laboratory which can have limited translation to field situations (Geisseler et al., 2019; Keeney, 1982; Vigil & Kissel, 1991). We used an in‐situ (field incubation) N mineralization method because N transformations are strongly affected by site‐specific conditions and local weather patterns (Geisseler et al., 2019; Khanna & Raison, 2013; Kolberg et al., 1997). We compared our estimates of potential contribution of fall seeded cover crops to in‐season available N to corn with those commonly calculated by using cover crop biomass and N content and residual soil N. We hypothesized that the estimates of in‐season plant available N could be used to predict corn yield responses to the cover crop treatments.Core IdeasNitrogen mineralization during corn growth stages V6‐R3 was the best predictor of crop yield.Fall seeded legume cover crops improved corn yield under nitrogen‐limited conditions.Legume cover crops resulted in higher in‐season nitrogen mineralization.MATERIALS AND METHODSField site and experimental designA field experiment was conducted over two consecutive growing seasons in two adjacent locations. The location of the two‐site‐years (Site Year 1, Fall 2006–2007; Site Year 2, Fall 2007–2008) were both located at the USDA–ARS North Central Agricultural Research Laboratory (NCARL; 44°20′ N, 96°47′ W) on Kranzburg (fine‐silty, mixed, superactive, frigid Calcic Hapludolls) and Brookings silty clay loams (fine‐silty, mixed, superactive, frigid Pachic Hapludolls) near Brookings, SD. The field studies were initiated in a previously established (2001) no‐till, winter wheat (Triticum aestivum L.)–corn–soybean (Glycine max) rotation with cover crops seeded following winter wheat harvest. Initial soil samples (0–15, 15–30, and 30–60 cm) were collected by compositing six cores (32‐mm diameter) at each location following winter wheat harvest and prior to cover crop planting; initial soil test characteristics and soil types are reported in Table 1.1TABLEInitial soil characteristics following wheat harvest, prior to cover crop planting at each experimental siteLocationDepthTotal NaTotal CaOMbNO3cPdpHeECecmg kg−1mg kg−1S m−1Site Year 10–151.822.03610.152.07.20.08315–301.518.6310.921.07.60.069 30–600.815.1150.54.07.80.057Site Year 20–151.922.63924.963.07.30.09815–301.518.3325.127.07.30.116 30–600.616.6162.24.07.70.151aTotal nitrogen (N) and organic carbon (C): CN Dry combustion analyzer.bOrganic matter (OM): Estimation by loss of ignition.cSoil nitrate‐N (NO3–N): flow injection analysis of 2 M KCl extracts.dAvailable phosphorus (P): Mehlich‐P3 method.epH and EC: 1:1 (v/v) soil/water mix.The experimental design was a randomized complete block design with four replications with an individual plot size of 6.1 by 15.2 m2. The four cover crop treatments were no cover crop, white sweet clover (Melilotus officinalis), winter cereal rye (Secale cereale), and hairy vetch (Vicia Villosa Roth.). In the year preceding cover crops, winter wheat ‘Briggs SD‐02’ (South Dakota State University Foundation seed, Brookings, SD) was planted at a seeding rate of 344 seed m−2 with starter fertilizer of 16 kg N ha−1, 18 kg phosphorus (P) ha−1, and 12 kg potassium (K) ha−1. Ammonium nitrate was applied on the soil surface at a rate of 57 kg N ha−1 at the Feekes 5 growth stage. Following winter wheat harvest and before planting cover crops, Roundup Weathermax was applied at a rate of 2.3 L ha−1 to the entire experimental site, to terminate volunteer wheat and the presences of late season weeds. Cover crops were planted into winter wheat stubble using a JD1590 NT drill with a row spacing of 19‐cm at a rate of 22.4 kg ha−1 for sweet clover (depth of 1.27 cm), 115 kg ha−1 of cereal rye (depth of 3.81 cm) and 33.5 kg ha−1 of vetch (depth of 3.81 cm). Cover crops were allowed to grow throughout the fall, winter, and the following spring until chemically killed with Roundup Weathermax (4.6 L ha−1) and Dicamba (2.3 L ha−1) prior to corn planting.After cover crop termination, corn hybrid ‘Dekalb 44–92’ (Dekalb Genetics Co.; 94‐d maturity) treated with Trifloxystrobin (Novartis Crop Protection) was planted at a seeding rate of 61,750 seeds ha−1 at 51‐cm row spacing with a Kinze 3400 planter. Starter fertilizer was applied at planting at a rate of 17 kg N ha−1 and 48 kg P ha−1 as ammonium phosphate and 57 kg N ha−1 applied in‐season at corn growth stage V6 as ammonium nitrate. Fertilizer was reduced below that traditionally required through fall soil samples, to allow for the examination of cover crop treatments on N dynamics and crop performance rather than an artificially created environment with large amounts of N fertilizer. Detailed agronomic information for the two site‐years were reported in Table 2.2TABLEAgronomic management information for planting, fertilization, harvesting, and cover crop termination and biomass samplingManagements2006–20072007–2008Site Year 1Site Year 2Wheat harvest10 Aug. 20062 Aug. 2007Herbicide burndown17 Aug. 200613 Aug. 2007Cover crop planting22 Aug. 200615 Aug. 2007Cover crop sampling/burndown14 May 200729 May 2008Corn planting18 May 200729 May 2008Nitrogen applicationa20 June 200730 June 2008Corn harvest29 Oct. 20075 Nov. 2008aNitrogen (N) application side‐dress at 57 kg N ha−1 with ammonium nitrate at corn growth stage of V6.Plant and grain analysisCover crop biomass samples were hand harvested by collecting two randomly selected areas (0.2 by 0.5 m2) in each plot once in the spring prior to herbicide application and corn planting. Biomass samples were dried in a forced air oven at 60 °C to a constant weight. Biomass samples were ground to pass a 2‐mm screen using a Wiley mill (Thomas Scientific) then further milled to pass through a <1‐mm screen utilizing an Udy Cyclone mill (Udy Corporation). Corn was harvested with a plot combine (MF8‐XP, Kincaid Equipment Manufacturing) and a Harvest Master (HM400 Legacy) yield monitor was used to determine grain weight, test weight, and grain moisture. Yields were adjusted to 155 g kg−1 moisture content. During the harvesting process, grain samples were collected from each experimental plot, and subsamples were ground to pass through a <1‐mm screen using a Knifetec 1095 sample mill (FOSS A/S). Cover crop biomass and corn grain samples were analyzed for total N (TN) and total carbon (TC) utilizing a dry combustion analyzer (Leco TruSpec analyzer, Leco Corporation). Total N uptake for cover crop biomass and corn grain was calculated by multiplying biomass or grain (kg ha−1) by TN concentration (kg N kg−1) and expressed as kg N ha−1.In‐situ N mineralizationIn‐situ net N mineralization was measured using intact cores deployed with ion exchange resins (1:1 mixture of anion and cation exchange resins; DiStefano & Gholz, 1986; Kolberg et al., 1997) three consecutive time periods of the corn growing season. The three time periods of measurements were: (a) corn planting until sidedress fertilization at corn V6 growth stage; (b) after corn fertilization (V6) until corn R3 reproductive stage; and (d) corn R3 reproductive stage until R6 physiological maturity stage. Two sampling locations were established in each plot within a nonwheel track inter‐row resulting in two sets of measurements per plot for the beginning and ending of each time period. Existing residues were left in place. At the beginning of each incubation period, four 3.2‐cm‐diameter, 10‐cm‐deep soil cores were collected from each sampling location and composited for determination of initial soil inorganic N content. At each sampling location, a pair of in‐situ cores was temporally removed between corn rows by driving an aluminum cylinder (10‐cm depth, 4.7‐cm diameter) with a slide hammer into the ground and subsequently removing the cylinder with locking pliers. The lower 10‐mm of soil in each aluminum cylinder was excavated and replaced with a nylon mesh bag containing 15 g of 50:50 mixture of anion and cation exchange resins (Sybron Ionac ASB‐1, C‐249, Sybron Chemicals) to capture inorganic N transported by water draining from the intact core. The intact core with resins was reinstalled into the hole using a rubber mallet as necessary. Prior to use, resin bags were presoaked overnight in 0.5 M NaHCO3, rinsed three times with deionized water, and dewatered by gentle compression. These procedures were repeated for each period at each of the two sampling locations within each plot. Detailed information regarding the incubation duration of the in‐situ soil cores and accumulative growing degree days (GDD) for these incubation durations were reported in Table 3. Growing degree days (Equation 1) were calculated to normalize the incubation periods based on the soil temperature (°C) at top 5‐cm using the formula:1Growingdegreedays(GDD)=Tmax+Tmin2−Tbase\begin{equation}{\rm{Growing\, degree\, days\, (GDD)}} = \left( {\frac{{{{\rm{T}}}_{{\rm{max}}} + {{\rm{T}}}_{{\rm{min}}}}}{2}} \right) - {{\rm{T}}}_{{\rm{base}}}\end{equation}where Tmax and Tmin are daily maximum and minimum soil temperatures, respectively, and Tbase is the base temperature (10 °C; Aspiauzu & Shaw, 1972; McMaster & Wilhelm, 1997). When the average daily temperature was less than Tbase, it was set to equal Tbase. Accumulated growing degree days (AccGDD) were determined by summing the GDD from January until the cover crops were terminated (Table 2).3TABLEThe starting and ending date, total number of days, and accumulative growing degree days (AccGDD) for deployment of in‐situ intact soil cores for nitrogen mineralization measurementsSampling periodSite Year 1Site Year 2Planting‐V6: Period 1Start31 May 20075 June 2008End20 June 200730 June 2008Total days2025AccGDDa356381V6–R3: Period 2Start29 June 20077 July 2008End26 July 200731 July 2008Total days2724AccGDD584547R3–R6: Period 3Start26 July 200731 July 2008End30 Aug. 20071 Oct. 2008Total days3562AccGDD699920aAccGDD were calculated where the average air temperature minus 10 in °C using method of (Aspiauzu & Shaw, 1972) during each incubation period at all locations.At the end of each incubation period, pairs of in‐situ soil cores and their resin bags were removed from the ground at each sampling location, placed in separate clean plastic bags, and kept in a cooler while in the field. Soil samples, including composite cores collected at the beginning of each incubation period, were stored at 4 °C and extracted within 1 wk. Soils were sieved (4.75 mm) at field moisture, thoroughly mixed, and 2 g of soil was extracted with 20 ml of 2 M KCl. Soil extracts were filtered through pre‐washed #42 Whatman filter paper and inorganic NO3− and NH4+ determined along with standards, blanks, and spiked controls by flow injection analysis on a Lachat Instruments autoanalyzer with the QuickChem sodium salicylate method 12‐107‐06‐2‐A (Hofer, 2001) and QuickChem 12‐107‐04‐1‐B with Cd reduction (Knepel, 2003). Extracts from each pair of cores were pooled for analytical determinations. Pairs of resin bags were extracted five times to recover >85% of inorganic N in the resin bags. Data (mass N per mass extracted wet soil) were normalized per gram dry soil using gravimetric soil moisture measurements performed on soil sample splits. Total C and N were determined on dried, sieved (2 mm) soil subsamples by dry combustion (LECO CN 2000 analyzer, Leco Corp.; Nelson & Sommers, 1996). Masses of C, N, NO3–N, and NH4–N per dry gram soil were expressed per volume soil using bulk density measurements (Blake & Hartge, 1982) from independent cores collected from each sampling location within the field plots. To convert to units commonly used in soil fertility, the mass of N per soil volume (mg N cm−3) was transformed to kg N ha−1 within a 15‐cm depth by multiplying by 1,500 m3 ha−1. Net N mineralization (Nmint) for each of the two sampling locations within each plot was calculated for each incubation period (t) as follows (Equation 2):2Nmintkgha−1=Ncoret+Nresint−Nsoilt\begin{equation}{\rm{Nmin}}_{\rm{t}}\left( {{\rm{kg\, ha}}^{ - {\rm{1}}}} \right){\rm{ = Ncor}}{{\rm{e}}}_{\rm{t}} + {\rm{Nresi}}{{\rm{n}}}_{\rm{t}} - {\rm{ Nsoil}}_{\rm{t}}\end{equation}where Ncoret is inorganic N in the soil core at the end of periodt, Nresint is inorganic N in the resin bag at the end of periodt, and Nsoilt is inorganic N in the soil core collected at the beginning of periodt (Raison et al., 1987). Net N mineralization (kg N ha−1) per period was divided by the number of days in each incubation period to express the average net N mineralization per day (kg N ha d−1) occurring during each period. For comparison between years, the accumulative net N mineralized (kg N ha−1) over a typical 100‐d corn growing season was calculated by multiplying net N mint (kg N ha d−1) for each period by 33.33 d and summing together the contributions of the three periods.Alternative estimates for plant available N from cover cropsWe evaluated alternative estimates for plant available N from cover crops utilizing previously published calculations that we subsequently used in regression analyses to predict corn yields. Plant available N (PAN) for the following crop was calculated using aboveground cover crop biomass, cover crop N content, and a factor based on decomposition over a 10‐wk period (0.5) as described by Sullivan and Andrews (2012). This calculation is accessible to producers and relies on empirical determination of decomposition factors under standardized laboratory conditions.3PANSulkgha−1=BiomassccNcontentcc0.5\begin{equation} {\mathrm{PAN}}_{\mathrm{Sul}}\left(\mathrm{kg} \ {\mathrm{ha}}^{-1}\right)=\left({\mathrm{Biomass}}_{\mathrm{cc}}\right)\left(\mathrm{N}{\mathrm{content}}_{\mathrm{cc}}\right)0.5 \end{equation}Vigil and Kissel (1991) used cover crop C and N content from eight previously published studies to fit regression equations that predict PAN resulting from decomposition of cover crop residues. We used the best fit equation with cover crop N content from all studies to calculate PANVig for our cover crop treatments.4PANVigkgha−1=−53.44+16.9810Ncontentcc12\begin{equation}{\rm{PAN}}_{{\rm{Vig}}}\left( {{\rm{kg\, ha}}^{{\rm{-1}}}} \right){\rm{ = }} -\!53.44 + 16.98{\left[10\left({\rm{N\, content}}_{{\rm{cc}}} \right) \right]}^{\frac{1}{2}}\end{equation}These estimates (Equations 3 and 4) are based upon cover crop biomass and/or N content and can be approximated prior to the following cash crop growing season. Another approach requires in‐season measurements to estimate N mineralization, the apparent N mineralized (ANM), which uses soil nitrate samples collected prior to corn planting (Nsoilplanting) and samples collected following harvest (Nsoilharvest), corn N uptake (Nuptake), and in‐season fertilizer N application (Nfert; Schepers & Meisinger, 1994; Zubillaga et al., 2021). Apparent N mineralization is calculated as follows (Equation 5):5ANMkgha−1=Nsoilharvest+Nuptake−Nsoilplanting+Nfert\begin{eqnarray}{\rm{ANM }}\left( {{\rm{kg\, ha}}^{{\rm{-1}}}} \right) &=& \left( {{\rm{Nsoil}}_{{\rm{harvest}}} + {\rm{Nuptake}}} \right){\rm{ }}\nonumber\\ && - {\rm{ }}\left( {{\rm{Nsoil}}_{{\rm{planting}}} + {\rm{Nfert}}} \right)\end{eqnarray}Nitrogen use efficiencyNitrogen use efficiency (NUE) as defined by Moll et al. (1982) as production per unit of N available from the soil (fertilizer N, plus initial soil N); we expanded this equation to include 100‐d N mineralization as part of available N from the soil. The efficiency of corn yield per unit N input from fertilizer, N mineralized over 100 d from cover crop, and soil inorganic N at planting was determined by calculating NUE.6NUE=CorngrainyieldNfert+Nmin100d+Nsoilplanting100\begin{equation}{\rm{NUE}} = \left( {\frac{{{\rm{Corn\, grain\, yield}}}}{{{\rm{Nfert}} + {\rm{Nmin}}_{{\rm{100d}}} + {\rm{Nsoil}}_{{\rm{planting}}}}}} \right)100\end{equation}where corn yield in each plot is divided by the sum of accumulative net mineralization (100‐d N mineralization) and total N fertilization applied at planting and sidedress during the corn growing season plus the soil N at planting.Data analysisA univariate distribution for each variable was determined; outliers determined using studentized residuals >2.5 were removed prior to data analysis. Statistical analysis of variance was conducted using PROC GLM with SAS (SAS Institute, 2017) to evaluate spring cover crop biomass, biomass N uptake, cover crop C:N ratio, corn yield, grain N uptake, soil inorganic, N mineralization for each incubation period, accumulative (100‐d) N mineralization, PAN, ANM, and NUE with cover treatment as a fixed factor and replication as a random effect. Analysis was preformed to test the effect of site year, treatment main effect, and the interaction between site year and main effect. Due to the significantly different (p < .0001) site year, the main effects of the fixed treatment factors were tested by site year. The mean separation was analyzed by using PDIFF option within the LSMEANS statement when F‐test indicated that significant differences existed (p < .10). Due to the inherent temporal and spatial variability in soil properties and limited replication, an α = .10 level was used (Moebius‐Clune et al., 2008). Regression analysis of variance was performed using the PROC REG in SAS (SAS Institute, 2017). Linear models were evaluated that included both site‐years and cover crop to predict corn yield, with predictor variables of in‐situ 100‐d N mineralization, ANM, PAN, and period 2 N mineralization daily rate.RESULTS AND DISCUSSIONInitial site characteristicsInitial soil characteristics for the experimental sites are reported for three sampling depths (0–15, 15–30, and 30–60 cm; Table 1). The adjacent sites had similar initial soil characteristics for total N, total C, organic matter content (OM), available (P, pH, and electrical conductivity (EC) with average of 1.85 g N kg−1, 22.3 g C kg−1, 37.5 g OM kg−1, 57.5 mg P kg−1, 7.25 and 0.091 S m−1 respectively at the top 15‐cm soil. Initial soil nitrate (NO3) concentrations were different between the two locations with Site Year 2 having higher residual soil NO3 at all depths compared with Site Year 1, probably because of weather conditions in Year 1 which were highly conducive to N mineralization processes.Cover crop establishment and growthDuring the two growing seasons, timing of precipitation was a key factor in cover crop establishment and growth (Figure 1a, b). Above‐average precipitation in August of both years (144 and 164 mm, Site Year 1 and 2, respectively, compared with 78 mm 30‐yr average) at cover crop planting helped seed germinate. Legume cover crop biomass (Table 4) at Site Year 1 was limited by a shorter growing season and less precipitation in October (5‐mm; 30‐yr averages: 52‐mm), which strongly affected sweet clover fall growth in addition to clover's inability to overwinter. The potential cover crop growing season at Site Year 1 was almost a month shorter than the Site Year 2 because cover crops were planted 7‐d later and terminated 14‐d earlier at Site Year 1 compared with Site Year 2. Although Site Year 1 short growing season inhibited cover crop biomass production, overall precipitation was lower for Site Year 2. Total precipitation during the cover crop growing season (Sept.–May) was 417 and 281 mm for Site Year 1 and Site Year 2 locations, respectively, compared with a 30‐yr average of 347 mm. The overall increased precipitation as Site Year 1 seemed to favor growth of the rye cover crop in contrast to the legumes that were limited by the 1‐mo‐long drought in October of that year (Figure 1). In both site‐years, the rye cover crop had significantly higher biomass with a larger C:N ratio compared with sweet clover and hairy vetch (Table 4). Rye biomass production was substantially higher at Site Year 1 compared with Site Year 2, whereas legume cover crop biomass was higher at Site Year 2 compared with Site Year 1. Vetch outproduced sweet clover at Site Year 1, whereas the reverse was true at Site Year 2.1FIGUREMonthly precipitation (a) and averaged soil temperature (b) at top 5‐cm soil and 30‐yr averages (1981–2010) in Brookings, SD in 2006–20084TABLECover crop end of season biomass dry matter, nitrogen (N) uptake and carbon:nitrogen (C:N) ratioLocationCover cropsBiomassN uptakeC:N ratiokg ha−1 Site Year 1Rye2,378 a39.6 a26.8 aSweet cloverNDaNDND Vetch377 b14.7 b10.9 bSite Year 2Rye1,249 a17.3 c31.5 aSweet clover984 b44.9 a9.5 b Vetch657 c31.3 b9.2 bNote. Means within a column followed the same letter by sites are not significantly different at the .10 probability level.aND, nondetectable amounts of above‐ground biomass.In‐situ N mineralizationAlthough overall N availability to the following cash crop is important, the timing of N availability within the growing season factors into crop response. Mean mineralization rates for Period 2 (corn growth stage V6–R3) were significantly higher than Periods 1 or 3 at both locations (Figure 2). The high N mineralization observed in Period 2 during corn growth stages V6–R3 matches reported peaks of N demand by a growing corn crop (Abendroth et al., 2011; Schepers & Meisinger, 1994). Importantly, N mineralization peaks following both rye and legume cover crops were synchronized with corn demand. Measurements of meaningful changes in plant available N during the season and documentation of practices that produce synchrony between N supply and demand are critical to future efforts to improve N use efficiencies (Congreves et al., 2021). Average N mineralization across all treatments during Period 2 was 3.02 and 1.52 kg N ha d−1 at Site Year 1 and Site Year 2, respectively. The vetch treatment had significantly higher N mineralization than rye during the first and third periods at both sites and numerically higher mineralization during the second period at both sites. Sweet clover had significantly higher N mineralization than rye and no cover crop during all three periods at Site Year 2.2FIGUREDaily rate of N mineralized for incubation period using in‐situ soil intact cores in Site Year 1 (a) and Site Year 2 (b). Within a period and site‐year, bars sharing a common lowercase letter are not significantly different at the .10 probability level. Values below the incubation period indicate average means in each period by site‐year. Within a site‐year, means sharing a common uppercase letter are not significantly different at the .05 probability level. No detectable amounts of aboveground biomass were found for sweet clover in Site Year 1The accumulative net N mineralized (kg N ha−1) over a typical 100‐d corn growing season was higher following successful legume cover crops than nonlegume cover crop (Figure 3). The differences in N mineralized following legumes or rye are likely results of the differing C:N ratios (Table 4). Researchers have commonly found that larger C:N ratios (>25:1) found in grass cover crops like rye can favor N immobilization over mineralization over the near‐term (Aulakh et al., 1991; Shipley et al., 1992; Sullivan & Andrews, 2012). Although little N (27 kg N ha−1) was mineralized in the no cover crop treatment at Site Year 2, about 157 kg N ha−1 was mineralized at Site Year 1 which exceeded N mineralized following rye and indicated N immobilization by the high biomass rye in Site Year 1. The history of these two adjacent sites were identical, so the most plausible explanation was that high temperatures and precipitation amounts (Figure 1) during Site Year 1 were favorable to N mineralization activities from either the soil or cover crop residues. Notably, mineralization of soil organic N in the no cover treatment without any residue sources resulted in a significant net decrease in soil inorganic N following corn uptake at Site Year 1(Figure 4). In contrast, legume treatments generally had higher residual soil inorganic N following the corn growing season than the rye and no cover crop treatments. The high N content of legume residues reduces competition for available N by microorganisms and increases decomposition and N mineralization (Janzen & Kucey, 1988; Vigil & Kissel, 1991). The elevated N mineralization activities following legumes provided quantities of plant available N approximately equivalent to the annual N fertilizer recommendations in both site‐years and increased residual soil inorganic N. This finding is consistent with research in the eastern United States reporting that legume cover crops can fix most of the N required for maximum corn yield in no‐till systems (Decker et al., 1994; Ebelhar et al., 1984; Mitchell & Tell, 1977; Wagger, 1989).3FIGUREThe accumulative net nitrogen (N) mineralization over 100‐d corn using in‐situ soil intact cores for N mineralization measurements. Within a site‐year, bars sharing a common letter are not significantly different at the .10 probability level. No detectable amounts of aboveground biomass were found for sweet clover in Site Year 14FIGUREChange in inorganic soil nitrogen (N) from the start of Period 1 (planting) until the end of Period 3 (growth stage R6)Comparative estimates for plant available N from cover cropsWe compared our in‐situ N mineralization measurements with other approaches to estimating PAN supplied by cover crops to succeeding cash crop. These estimates are based upon easily collected data including cover crop biomass and N content and soil inorganic N. Estimates of PAN based on cover crop biomass and/or biomass N (Sullivan & Andrews, 2012; Vigil & Kissel, 1991) result in substantially lower estimated N made available to the corn crop (Table 5) than our mineralized N measurements as shown in Figure 3. Estimates for PAN across both site‐years from rye ranged from 2 to 20 and 7 to 65 kg N ha−1 for legumes. These types of estimates inherently ignore mineralization of soil organic N, the stimulation of soil N mineralization activities by cover crops via direct (plant exudates, residues) and indirect (e.g., soil structure) mechanisms, and the contribution of soil N fixed by legumes but not assimilated into legume biomass. Estimates of ANM based on soil N, fertilizer N, and N uptake (Schepers & Meisinger, 1994; Zubillaga et al., 2021) were also low, ranging from 16 to 57 kg N ha−1 across all treatments and site‐years, but consistent with expectations based on cover crop biomass and C:N ratios. Low estimates of PAN from cover crops using easily measured quantities would encourage over‐application of fertilizer N by producers. Probably the most realistic estimate for N provided by the cover crops to the following corn was calculated by subtracting the 100‐d accumulative net N mineralized in the no‐cover treatment from the cover crop treatments (Table 5). This calculation resulted in a net immobilization of 55 kg ha−1 for rye and a net mineralization of 31 kg ha−1 provided by vetch at Site Year 1, and 33, 135, and 94 kg ha−1 provided by rye, sweet clover, and vetch covers, respectively, at Site Year 2.5TABLEEstimates of plant available nitrogen (PAN), 100‐d accumulative in‐situ nitrogen (N) mineralization and apparent N mineralization (ANM), to corn following cover crop growthTreatmentPANVigaPANSulbIn‐situ NcANMdIn‐situ N normekg ha−1Site Year 1No cover crop156.6 a30.4 bRye1.5 b19.8 b100.9 a49.7 a−54.9 aSweet cloverf–––––Vetch54.5 a7.3 b202.5 a53.7 a31.3 aSite Year 2No cover crop26.8 c16.2 cRye9.8 b8.7 b59.8 bc22.7 bc33.1 bSweet clover61.9 a22.4 a172.4 a57.4 a134.9 abVetch64.8 a15.7 ab127.7 ab33.5 b94.1 aNote. Means within a column followed the same letter by sites are not significantly different at the .10 probability level.aVigil & Kissel, 1991.bSullivan & Andrews, 2012.c100‐d accumulative N mineralization, Kolberg et al., 1997.dSchepers & Meisinger, 1994.eIn‐situ N normalized by subtracting no cover crop treatment from cover crop treatment for each block of the study.fNondetectable amounts of above‐ground biomass for sweet clover at Site Year 1.Corn grain yield and N uptakeCumulative precipitation in May through June during corn planting was lower for Site Year 1 (123 mm) compared with the 30‐yr (1981–2010) averages (185 mm), whereas Site Year 2 had above average precipitation (228 cm; (NOAA, 2019), but both were adequate for crop establishment. Total precipitation during the growing season at Site Year 1 and Site Year 2 was 410 and 381 mm, respectively, during the corn growing season (May–Oct.), compared with 30‐yr averages of 478 mm resulting in a total water deficit of 14 (Site Year 1) and 20% (Site Year 2) that could have suppressed corn yield overall.Legume cover crop treatments had numerically higher corn grain yields at Site Year 1 (vetch; 8,496 kg ha−1) and Site Year 2 (sweet clover; 6,464 kg ha−1) compared with the no cover crop treatment (Table 6). Increased corn yields following legume cover crops have been observed at other more eastern locations in North America (Coombs et al., 2017; Decker et al., 1994; Wagger, 1989) The legume treatments produced the highest amount of mineralized N over the corn growing season, and perhaps more importantly had the highest N mineralization rates during Period 2 (corn growth stage V6–R3) when corn N demand is highest. The rye cover crop treatment had the lowest corn grain yields for both site‐years, 7,112 and 4,702 kg ha−1 for Site Years 1 and 2, respectively, but neither was significantly different from the no cover crop treatment. Past research has found corn yield decreased after rye cover crops due to N immobilization, soil water depletion, or allelopathy associated with cover crop biomass decomposition (Eckert, 1988; Moore et al., 2014; Wagger, 1989). At Site Year 1, vetch produced significantly higher N uptake by the corn compared with rye, but neither cover crop treatment was significantly different from the no cover crop treatment. At Site Year 2, N uptake by corn was significantly higher after sweet clover compared with the other treatments. In that year, corn grain N content was significantly higher following the high biomass sweet clover cover crop compared with the other treatments.6TABLECorn yield, nitrogen (N) uptake, and total N collected at harvest 2007 (Site Year 1) and 2008 (Site Year 2)LocationTreatmentYieldN uptakeTotal Nkg ha−1g kg−1Site Year 1No cover crop7,954 ab87.4 ab10.5 aRye7,112 b75.9 b10.8 aSweet clovera––– Vetch8,496 a90.5 a10.7 aSite Year 2No cover crop4,759 b50.0 b10.5 bRye4,702 b48.9 b10.4 bSweet clover6,464 a81.7 a12.7 a Vetch5,358 ab60.8 b11.5 abNote. Means within a column followed the same letter by sites are not significantly different at the .10 probability level.aNondetectable amounts of above‐ground biomass for sweet clover at Site Year 2.When evaluating the relationship between the different seasonal N mineralization estimates and corn yield, we found that the 100‐d accumulative in‐situ net N mineralization measurement best predicted grain yield (R2 = .66) compared with ANM (R2 = .44) and the two PAN estimates (R2 < .03; Figure 5). The two PAN estimates which utilize only cover crop biomass and N were not good predictors of corn yield and may promote over‐fertilization by underestimating N provided by cover crops. Because corn yield is dependent on the right amount of N available to the crop at the right time, we regressed our in‐situ N mineralization rates for period 2 (corn growth stage V6–R3) against corn yield (Figure 6). Rates of in‐situ N mineralization during corn growth stage V6–R3 provided a superior prediction (R2 = .83) of corn yields compared with estimates of total N provided by cover crops over the entire growing season. In‐situ net N mineralization has been linked to net primary production of trees (Khanna & Raison, 2013), but we did not encounter any similar reports for prediction of corn yields. In our study, the relationship between in‐situ net N mineralization and corn yields may be facilitated by the lean N‐fertility regime. Measurements of in‐situ N mineralization over a single time period (corn growth stage V6–R3) would be advantageous in terms of time and money and could be performed in regional cover crop field trials conducted by university extension professionals. A transition from a single timepoint measure of PAN to measurements throughout the growing season will enable adoption of management practices that improve NUE and minimize negative effects of over‐application (Congreves et al., 2021).5FIGURERelationship between estimates of in‐situ 100‐d N mineralization (a); apparent N mineralization (ANM; b); plant‐available N Vigil & Kissel (1991) (PANVig; c); and plant‐available N, Sullivan & Andrews (2012) (PANSul; d) for both site‐years. Panels (c) and (d) include no cover crop control6FIGURERelationship between incubation Period 2 (V6–R3) N mineralization rate and corn yield for all treatments across both site‐yearsNitrogen use efficiencyNitrogen use efficiency is defined as the ratio of crop production to the input of N and has traditionally been utilized to evaluate different N fertilizer application methods. Within the context of this experiment, we utilized NUE to evaluate the response of corn to in‐season available N (N fertilizer application and contributions of N mineralization of soil organic N and cover crop residues) according to Equation 6 (Moll et al., 1982). All treatments received the same amount of fertilizer and there were minimal differences between treatments in starting concentrations of soil inorganic N. Because the no cover crop and rye treatment had no or little N mineralized from residues during the season, the corn utilized N applied in fertilizer or mineralized from the soil resulting in a high apparent NUE (Figure 7) and lower residual soil inorganic N (Figure 3). In contrast, large amounts of N were mineralized from legume residues and that N was evidently taken up by corn (Figure 6) but resulted in a low NUE and higher residual soil inorganic N (Figure 3). Despite the equal amounts of fertilizer applied, treatments with high NUE (no cover, rye) had lower corn yields than those with low NUE (legumes). In this circumstance, a low NUE indicates a sufficient supply of N from the soil to meet crop demand which is desirable and allows fertilizer application to be reduced (Congreves et al., 2021). Thus, low NUE associated with legume treatments indicated that less or no fertilizer can be applied to corn following legume cover crops while maintaining yield as suggested by Rosecrance et al. (2000) and Teasdale et al. (2008). In succeeding years, the cover crop treatments will supply additional PAN from continued mineralization of persisting cover crop residues and soil organic matter enriched in N by prior N mineralization of plant residues (Chim et al., 2022).7FIGURENitrogen use efficiency (NUE) in corn following cover crops. Within a site‐year, bars sharing a common letter are not significantly different at the .10 probability level. No detectable amounts of aboveground biomass were found for sweet clover at Site Year 1CONCLUSIONSPredicting the supply of N by cover crops to a following corn crop is difficult because of annual variations in weather; however, we found that in‐situ measurements of N mineralization provided credible estimates for this value over two growing seasons with varied weather patterns. Legume cover crops generally produced higher seasonal amounts of mineralized N compared with rye or no cover crops and resulted in numerically higher corn yields. The cover crop treatments did not significantly decrease corn yields in either site‐year, despite N immobilization by a large rye cover crop in one of the years. Seasonal (100‐d) in‐situ N mineralization measurements better predicted yields across all treatments compared with other approaches that use cover crop biomass and N content. Regardless of treatment, the highest rates of N mineralization were observed during periods of high N demand by corn (V6–R3) and N mineralization during this period was a superior predictor of corn yield. The lower apparent NUE incorporating in‐season N mineralization indicated that less fertilizer N can be applied in the growing season following legume cover crops, while maintaining crop yield.ACKNOWLEDGMENTSThe authors thank Kurt Dagel and Max Pravecek for excellent technical assistance. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.AUTHOR CONTRIBUTIONSBee Khim Chim: Conceptualization; Formal analysis; Writing – original draft. Shannon L. Osborne: Conceptualization; Formal analysis; Methodology; Resources; Supervision; Writing – review & editing. R. 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Short‐term corn yield response associated with nitrogen dynamics from fall‐seeded cover crops under no‐till dryland conditions

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Abstract

AbbreviationsANMapparent nitrogen mineralizedGDDgrowing degree daysNUEnitrogen use efficiencyPANplant available nitrogenINTRODUCTIONNitrogen (N) is commonly the limiting factor in corn (Zea mays L.) production. Ideally, N fertilization should account for the difference between soil available N and N required for optimum crop yield, but complex soil N dynamics make predicting annual N application amounts difficult (Tao et al., 2018; White et al., 2021). Benefits of improving our ability to apply the right amount of N based on understanding of soil N dynamics include reducing the risk of nitrate contamination of surface and ground water, reducing hypoxia in freshwater and marine systems, and increasing profit for producers (Isbell et al., 2021; Malakoff, 1998; Meisinger et al., 1991). Crop management practices and their interactions with environmental factors produce annual variability in soil N dynamics and challenge the ability to determine the most efficient in‐season N crop requirements.One such crop management practice is cover cropping; there is increased interest among producers to include cover crops in no‐till production systems, particularly with the ability to chemically terminate cover crops without harming the cash crop. Cover crops can increase cash crop yields, sequester soil carbon (C), scavenge fall soil N to reduce leaching, fix atmospheric N, reduce soil erosion, and control weed species (Abdalla et al., 2019; Bowman et al., 2000; Thorup‐Kristensen et al., 2003). Although there are multiple benefits of cover crops, it remains difficult to predict how cover crops affect N dynamics in‐season and to estimate available N for the following cash crop. Efficient N management depends on the timing of crop residue decomposition, whether the mineralized N will be lost by leaching or temporally immobilized, and whether temporally immobilized N is remineralized in synchrony with cash crop demands (Huntington et al., 1985; Isbell et al., 2021; Nevins et al., 2020; Wagger, 1989). Grass‐type cover crops can significantly reduce N leaching and are recognized as effective scavengers of residual N but may provide limited N for the immediately following cash crop (Ditsch & Alley, 1991; McCracken et al., 1994; Shipley et al., 1992; Wagger, 1989). Legume cover crops can supply significant amounts of N for the subsequent cash crop and may minimize N leaching during winter and spring (Coombs et al., 2017; Ebelhar et al., 1984; Rosecrance et al., 2000; Wagger, 1989).In the face of increasing costs for fertilizer N and awareness of water quality issues, producers are looking for regionally relevant information on cover crops influence on fertilizer requirements for the following cash crop, commonly corn, in the U.S. Midwest. Major factors that control annual N mineralization patterns are the amount and composition of plant residues, soil temperature, precipitation patterns, and soil characteristics (Kätterer et al., 1998; Kruse et al., 2004; White et al., 2021). Thus, regional climate and annual weather conditions, soil type, and field management are inherent considerations for estimations of N available for corn crops following cover crops.Using representative legume and grass cover crops under no‐till, dryland conditions in the Mollisol soils of the U.S. Northern Plains, our objectives were to (a) determine the effect of cover crops on corn yields, and (b) estimate the amount of N that can be supplied to corn by mineralization of the cover crop residues. Common methods of examining soil N mineralization measure potentially mineralizable N using aerobic and anaerobic incubations in the laboratory which can have limited translation to field situations (Geisseler et al., 2019; Keeney, 1982; Vigil & Kissel, 1991). We used an in‐situ (field incubation) N mineralization method because N transformations are strongly affected by site‐specific conditions and local weather patterns (Geisseler et al., 2019; Khanna & Raison, 2013; Kolberg et al., 1997). We compared our estimates of potential contribution of fall seeded cover crops to in‐season available N to corn with those commonly calculated by using cover crop biomass and N content and residual soil N. We hypothesized that the estimates of in‐season plant available N could be used to predict corn yield responses to the cover crop treatments.Core IdeasNitrogen mineralization during corn growth stages V6‐R3 was the best predictor of crop yield.Fall seeded legume cover crops improved corn yield under nitrogen‐limited conditions.Legume cover crops resulted in higher in‐season nitrogen mineralization.MATERIALS AND METHODSField site and experimental designA field experiment was conducted over two consecutive growing seasons in two adjacent locations. The location of the two‐site‐years (Site Year 1, Fall 2006–2007; Site Year 2, Fall 2007–2008) were both located at the USDA–ARS North Central Agricultural Research Laboratory (NCARL; 44°20′ N, 96°47′ W) on Kranzburg (fine‐silty, mixed, superactive, frigid Calcic Hapludolls) and Brookings silty clay loams (fine‐silty, mixed, superactive, frigid Pachic Hapludolls) near Brookings, SD. The field studies were initiated in a previously established (2001) no‐till, winter wheat (Triticum aestivum L.)–corn–soybean (Glycine max) rotation with cover crops seeded following winter wheat harvest. Initial soil samples (0–15, 15–30, and 30–60 cm) were collected by compositing six cores (32‐mm diameter) at each location following winter wheat harvest and prior to cover crop planting; initial soil test characteristics and soil types are reported in Table 1.1TABLEInitial soil characteristics following wheat harvest, prior to cover crop planting at each experimental siteLocationDepthTotal NaTotal CaOMbNO3cPdpHeECecmg kg−1mg kg−1S m−1Site Year 10–151.822.03610.152.07.20.08315–301.518.6310.921.07.60.069 30–600.815.1150.54.07.80.057Site Year 20–151.922.63924.963.07.30.09815–301.518.3325.127.07.30.116 30–600.616.6162.24.07.70.151aTotal nitrogen (N) and organic carbon (C): CN Dry combustion analyzer.bOrganic matter (OM): Estimation by loss of ignition.cSoil nitrate‐N (NO3–N): flow injection analysis of 2 M KCl extracts.dAvailable phosphorus (P): Mehlich‐P3 method.epH and EC: 1:1 (v/v) soil/water mix.The experimental design was a randomized complete block design with four replications with an individual plot size of 6.1 by 15.2 m2. The four cover crop treatments were no cover crop, white sweet clover (Melilotus officinalis), winter cereal rye (Secale cereale), and hairy vetch (Vicia Villosa Roth.). In the year preceding cover crops, winter wheat ‘Briggs SD‐02’ (South Dakota State University Foundation seed, Brookings, SD) was planted at a seeding rate of 344 seed m−2 with starter fertilizer of 16 kg N ha−1, 18 kg phosphorus (P) ha−1, and 12 kg potassium (K) ha−1. Ammonium nitrate was applied on the soil surface at a rate of 57 kg N ha−1 at the Feekes 5 growth stage. Following winter wheat harvest and before planting cover crops, Roundup Weathermax was applied at a rate of 2.3 L ha−1 to the entire experimental site, to terminate volunteer wheat and the presences of late season weeds. Cover crops were planted into winter wheat stubble using a JD1590 NT drill with a row spacing of 19‐cm at a rate of 22.4 kg ha−1 for sweet clover (depth of 1.27 cm), 115 kg ha−1 of cereal rye (depth of 3.81 cm) and 33.5 kg ha−1 of vetch (depth of 3.81 cm). Cover crops were allowed to grow throughout the fall, winter, and the following spring until chemically killed with Roundup Weathermax (4.6 L ha−1) and Dicamba (2.3 L ha−1) prior to corn planting.After cover crop termination, corn hybrid ‘Dekalb 44–92’ (Dekalb Genetics Co.; 94‐d maturity) treated with Trifloxystrobin (Novartis Crop Protection) was planted at a seeding rate of 61,750 seeds ha−1 at 51‐cm row spacing with a Kinze 3400 planter. Starter fertilizer was applied at planting at a rate of 17 kg N ha−1 and 48 kg P ha−1 as ammonium phosphate and 57 kg N ha−1 applied in‐season at corn growth stage V6 as ammonium nitrate. Fertilizer was reduced below that traditionally required through fall soil samples, to allow for the examination of cover crop treatments on N dynamics and crop performance rather than an artificially created environment with large amounts of N fertilizer. Detailed agronomic information for the two site‐years were reported in Table 2.2TABLEAgronomic management information for planting, fertilization, harvesting, and cover crop termination and biomass samplingManagements2006–20072007–2008Site Year 1Site Year 2Wheat harvest10 Aug. 20062 Aug. 2007Herbicide burndown17 Aug. 200613 Aug. 2007Cover crop planting22 Aug. 200615 Aug. 2007Cover crop sampling/burndown14 May 200729 May 2008Corn planting18 May 200729 May 2008Nitrogen applicationa20 June 200730 June 2008Corn harvest29 Oct. 20075 Nov. 2008aNitrogen (N) application side‐dress at 57 kg N ha−1 with ammonium nitrate at corn growth stage of V6.Plant and grain analysisCover crop biomass samples were hand harvested by collecting two randomly selected areas (0.2 by 0.5 m2) in each plot once in the spring prior to herbicide application and corn planting. Biomass samples were dried in a forced air oven at 60 °C to a constant weight. Biomass samples were ground to pass a 2‐mm screen using a Wiley mill (Thomas Scientific) then further milled to pass through a <1‐mm screen utilizing an Udy Cyclone mill (Udy Corporation). Corn was harvested with a plot combine (MF8‐XP, Kincaid Equipment Manufacturing) and a Harvest Master (HM400 Legacy) yield monitor was used to determine grain weight, test weight, and grain moisture. Yields were adjusted to 155 g kg−1 moisture content. During the harvesting process, grain samples were collected from each experimental plot, and subsamples were ground to pass through a <1‐mm screen using a Knifetec 1095 sample mill (FOSS A/S). Cover crop biomass and corn grain samples were analyzed for total N (TN) and total carbon (TC) utilizing a dry combustion analyzer (Leco TruSpec analyzer, Leco Corporation). Total N uptake for cover crop biomass and corn grain was calculated by multiplying biomass or grain (kg ha−1) by TN concentration (kg N kg−1) and expressed as kg N ha−1.In‐situ N mineralizationIn‐situ net N mineralization was measured using intact cores deployed with ion exchange resins (1:1 mixture of anion and cation exchange resins; DiStefano & Gholz, 1986; Kolberg et al., 1997) three consecutive time periods of the corn growing season. The three time periods of measurements were: (a) corn planting until sidedress fertilization at corn V6 growth stage; (b) after corn fertilization (V6) until corn R3 reproductive stage; and (d) corn R3 reproductive stage until R6 physiological maturity stage. Two sampling locations were established in each plot within a nonwheel track inter‐row resulting in two sets of measurements per plot for the beginning and ending of each time period. Existing residues were left in place. At the beginning of each incubation period, four 3.2‐cm‐diameter, 10‐cm‐deep soil cores were collected from each sampling location and composited for determination of initial soil inorganic N content. At each sampling location, a pair of in‐situ cores was temporally removed between corn rows by driving an aluminum cylinder (10‐cm depth, 4.7‐cm diameter) with a slide hammer into the ground and subsequently removing the cylinder with locking pliers. The lower 10‐mm of soil in each aluminum cylinder was excavated and replaced with a nylon mesh bag containing 15 g of 50:50 mixture of anion and cation exchange resins (Sybron Ionac ASB‐1, C‐249, Sybron Chemicals) to capture inorganic N transported by water draining from the intact core. The intact core with resins was reinstalled into the hole using a rubber mallet as necessary. Prior to use, resin bags were presoaked overnight in 0.5 M NaHCO3, rinsed three times with deionized water, and dewatered by gentle compression. These procedures were repeated for each period at each of the two sampling locations within each plot. Detailed information regarding the incubation duration of the in‐situ soil cores and accumulative growing degree days (GDD) for these incubation durations were reported in Table 3. Growing degree days (Equation 1) were calculated to normalize the incubation periods based on the soil temperature (°C) at top 5‐cm using the formula:1Growingdegreedays(GDD)=Tmax+Tmin2−Tbase\begin{equation}{\rm{Growing\, degree\, days\, (GDD)}} = \left( {\frac{{{{\rm{T}}}_{{\rm{max}}} + {{\rm{T}}}_{{\rm{min}}}}}{2}} \right) - {{\rm{T}}}_{{\rm{base}}}\end{equation}where Tmax and Tmin are daily maximum and minimum soil temperatures, respectively, and Tbase is the base temperature (10 °C; Aspiauzu & Shaw, 1972; McMaster & Wilhelm, 1997). When the average daily temperature was less than Tbase, it was set to equal Tbase. Accumulated growing degree days (AccGDD) were determined by summing the GDD from January until the cover crops were terminated (Table 2).3TABLEThe starting and ending date, total number of days, and accumulative growing degree days (AccGDD) for deployment of in‐situ intact soil cores for nitrogen mineralization measurementsSampling periodSite Year 1Site Year 2Planting‐V6: Period 1Start31 May 20075 June 2008End20 June 200730 June 2008Total days2025AccGDDa356381V6–R3: Period 2Start29 June 20077 July 2008End26 July 200731 July 2008Total days2724AccGDD584547R3–R6: Period 3Start26 July 200731 July 2008End30 Aug. 20071 Oct. 2008Total days3562AccGDD699920aAccGDD were calculated where the average air temperature minus 10 in °C using method of (Aspiauzu & Shaw, 1972) during each incubation period at all locations.At the end of each incubation period, pairs of in‐situ soil cores and their resin bags were removed from the ground at each sampling location, placed in separate clean plastic bags, and kept in a cooler while in the field. Soil samples, including composite cores collected at the beginning of each incubation period, were stored at 4 °C and extracted within 1 wk. Soils were sieved (4.75 mm) at field moisture, thoroughly mixed, and 2 g of soil was extracted with 20 ml of 2 M KCl. Soil extracts were filtered through pre‐washed #42 Whatman filter paper and inorganic NO3− and NH4+ determined along with standards, blanks, and spiked controls by flow injection analysis on a Lachat Instruments autoanalyzer with the QuickChem sodium salicylate method 12‐107‐06‐2‐A (Hofer, 2001) and QuickChem 12‐107‐04‐1‐B with Cd reduction (Knepel, 2003). Extracts from each pair of cores were pooled for analytical determinations. Pairs of resin bags were extracted five times to recover >85% of inorganic N in the resin bags. Data (mass N per mass extracted wet soil) were normalized per gram dry soil using gravimetric soil moisture measurements performed on soil sample splits. Total C and N were determined on dried, sieved (2 mm) soil subsamples by dry combustion (LECO CN 2000 analyzer, Leco Corp.; Nelson & Sommers, 1996). Masses of C, N, NO3–N, and NH4–N per dry gram soil were expressed per volume soil using bulk density measurements (Blake & Hartge, 1982) from independent cores collected from each sampling location within the field plots. To convert to units commonly used in soil fertility, the mass of N per soil volume (mg N cm−3) was transformed to kg N ha−1 within a 15‐cm depth by multiplying by 1,500 m3 ha−1. Net N mineralization (Nmint) for each of the two sampling locations within each plot was calculated for each incubation period (t) as follows (Equation 2):2Nmintkgha−1=Ncoret+Nresint−Nsoilt\begin{equation}{\rm{Nmin}}_{\rm{t}}\left( {{\rm{kg\, ha}}^{ - {\rm{1}}}} \right){\rm{ = Ncor}}{{\rm{e}}}_{\rm{t}} + {\rm{Nresi}}{{\rm{n}}}_{\rm{t}} - {\rm{ Nsoil}}_{\rm{t}}\end{equation}where Ncoret is inorganic N in the soil core at the end of periodt, Nresint is inorganic N in the resin bag at the end of periodt, and Nsoilt is inorganic N in the soil core collected at the beginning of periodt (Raison et al., 1987). Net N mineralization (kg N ha−1) per period was divided by the number of days in each incubation period to express the average net N mineralization per day (kg N ha d−1) occurring during each period. For comparison between years, the accumulative net N mineralized (kg N ha−1) over a typical 100‐d corn growing season was calculated by multiplying net N mint (kg N ha d−1) for each period by 33.33 d and summing together the contributions of the three periods.Alternative estimates for plant available N from cover cropsWe evaluated alternative estimates for plant available N from cover crops utilizing previously published calculations that we subsequently used in regression analyses to predict corn yields. Plant available N (PAN) for the following crop was calculated using aboveground cover crop biomass, cover crop N content, and a factor based on decomposition over a 10‐wk period (0.5) as described by Sullivan and Andrews (2012). This calculation is accessible to producers and relies on empirical determination of decomposition factors under standardized laboratory conditions.3PANSulkgha−1=BiomassccNcontentcc0.5\begin{equation} {\mathrm{PAN}}_{\mathrm{Sul}}\left(\mathrm{kg} \ {\mathrm{ha}}^{-1}\right)=\left({\mathrm{Biomass}}_{\mathrm{cc}}\right)\left(\mathrm{N}{\mathrm{content}}_{\mathrm{cc}}\right)0.5 \end{equation}Vigil and Kissel (1991) used cover crop C and N content from eight previously published studies to fit regression equations that predict PAN resulting from decomposition of cover crop residues. We used the best fit equation with cover crop N content from all studies to calculate PANVig for our cover crop treatments.4PANVigkgha−1=−53.44+16.9810Ncontentcc12\begin{equation}{\rm{PAN}}_{{\rm{Vig}}}\left( {{\rm{kg\, ha}}^{{\rm{-1}}}} \right){\rm{ = }} -\!53.44 + 16.98{\left[10\left({\rm{N\, content}}_{{\rm{cc}}} \right) \right]}^{\frac{1}{2}}\end{equation}These estimates (Equations 3 and 4) are based upon cover crop biomass and/or N content and can be approximated prior to the following cash crop growing season. Another approach requires in‐season measurements to estimate N mineralization, the apparent N mineralized (ANM), which uses soil nitrate samples collected prior to corn planting (Nsoilplanting) and samples collected following harvest (Nsoilharvest), corn N uptake (Nuptake), and in‐season fertilizer N application (Nfert; Schepers & Meisinger, 1994; Zubillaga et al., 2021). Apparent N mineralization is calculated as follows (Equation 5):5ANMkgha−1=Nsoilharvest+Nuptake−Nsoilplanting+Nfert\begin{eqnarray}{\rm{ANM }}\left( {{\rm{kg\, ha}}^{{\rm{-1}}}} \right) &=& \left( {{\rm{Nsoil}}_{{\rm{harvest}}} + {\rm{Nuptake}}} \right){\rm{ }}\nonumber\\ && - {\rm{ }}\left( {{\rm{Nsoil}}_{{\rm{planting}}} + {\rm{Nfert}}} \right)\end{eqnarray}Nitrogen use efficiencyNitrogen use efficiency (NUE) as defined by Moll et al. (1982) as production per unit of N available from the soil (fertilizer N, plus initial soil N); we expanded this equation to include 100‐d N mineralization as part of available N from the soil. The efficiency of corn yield per unit N input from fertilizer, N mineralized over 100 d from cover crop, and soil inorganic N at planting was determined by calculating NUE.6NUE=CorngrainyieldNfert+Nmin100d+Nsoilplanting100\begin{equation}{\rm{NUE}} = \left( {\frac{{{\rm{Corn\, grain\, yield}}}}{{{\rm{Nfert}} + {\rm{Nmin}}_{{\rm{100d}}} + {\rm{Nsoil}}_{{\rm{planting}}}}}} \right)100\end{equation}where corn yield in each plot is divided by the sum of accumulative net mineralization (100‐d N mineralization) and total N fertilization applied at planting and sidedress during the corn growing season plus the soil N at planting.Data analysisA univariate distribution for each variable was determined; outliers determined using studentized residuals >2.5 were removed prior to data analysis. Statistical analysis of variance was conducted using PROC GLM with SAS (SAS Institute, 2017) to evaluate spring cover crop biomass, biomass N uptake, cover crop C:N ratio, corn yield, grain N uptake, soil inorganic, N mineralization for each incubation period, accumulative (100‐d) N mineralization, PAN, ANM, and NUE with cover treatment as a fixed factor and replication as a random effect. Analysis was preformed to test the effect of site year, treatment main effect, and the interaction between site year and main effect. Due to the significantly different (p < .0001) site year, the main effects of the fixed treatment factors were tested by site year. The mean separation was analyzed by using PDIFF option within the LSMEANS statement when F‐test indicated that significant differences existed (p < .10). Due to the inherent temporal and spatial variability in soil properties and limited replication, an α = .10 level was used (Moebius‐Clune et al., 2008). Regression analysis of variance was performed using the PROC REG in SAS (SAS Institute, 2017). Linear models were evaluated that included both site‐years and cover crop to predict corn yield, with predictor variables of in‐situ 100‐d N mineralization, ANM, PAN, and period 2 N mineralization daily rate.RESULTS AND DISCUSSIONInitial site characteristicsInitial soil characteristics for the experimental sites are reported for three sampling depths (0–15, 15–30, and 30–60 cm; Table 1). The adjacent sites had similar initial soil characteristics for total N, total C, organic matter content (OM), available (P, pH, and electrical conductivity (EC) with average of 1.85 g N kg−1, 22.3 g C kg−1, 37.5 g OM kg−1, 57.5 mg P kg−1, 7.25 and 0.091 S m−1 respectively at the top 15‐cm soil. Initial soil nitrate (NO3) concentrations were different between the two locations with Site Year 2 having higher residual soil NO3 at all depths compared with Site Year 1, probably because of weather conditions in Year 1 which were highly conducive to N mineralization processes.Cover crop establishment and growthDuring the two growing seasons, timing of precipitation was a key factor in cover crop establishment and growth (Figure 1a, b). Above‐average precipitation in August of both years (144 and 164 mm, Site Year 1 and 2, respectively, compared with 78 mm 30‐yr average) at cover crop planting helped seed germinate. Legume cover crop biomass (Table 4) at Site Year 1 was limited by a shorter growing season and less precipitation in October (5‐mm; 30‐yr averages: 52‐mm), which strongly affected sweet clover fall growth in addition to clover's inability to overwinter. The potential cover crop growing season at Site Year 1 was almost a month shorter than the Site Year 2 because cover crops were planted 7‐d later and terminated 14‐d earlier at Site Year 1 compared with Site Year 2. Although Site Year 1 short growing season inhibited cover crop biomass production, overall precipitation was lower for Site Year 2. Total precipitation during the cover crop growing season (Sept.–May) was 417 and 281 mm for Site Year 1 and Site Year 2 locations, respectively, compared with a 30‐yr average of 347 mm. The overall increased precipitation as Site Year 1 seemed to favor growth of the rye cover crop in contrast to the legumes that were limited by the 1‐mo‐long drought in October of that year (Figure 1). In both site‐years, the rye cover crop had significantly higher biomass with a larger C:N ratio compared with sweet clover and hairy vetch (Table 4). Rye biomass production was substantially higher at Site Year 1 compared with Site Year 2, whereas legume cover crop biomass was higher at Site Year 2 compared with Site Year 1. Vetch outproduced sweet clover at Site Year 1, whereas the reverse was true at Site Year 2.1FIGUREMonthly precipitation (a) and averaged soil temperature (b) at top 5‐cm soil and 30‐yr averages (1981–2010) in Brookings, SD in 2006–20084TABLECover crop end of season biomass dry matter, nitrogen (N) uptake and carbon:nitrogen (C:N) ratioLocationCover cropsBiomassN uptakeC:N ratiokg ha−1 Site Year 1Rye2,378 a39.6 a26.8 aSweet cloverNDaNDND Vetch377 b14.7 b10.9 bSite Year 2Rye1,249 a17.3 c31.5 aSweet clover984 b44.9 a9.5 b Vetch657 c31.3 b9.2 bNote. Means within a column followed the same letter by sites are not significantly different at the .10 probability level.aND, nondetectable amounts of above‐ground biomass.In‐situ N mineralizationAlthough overall N availability to the following cash crop is important, the timing of N availability within the growing season factors into crop response. Mean mineralization rates for Period 2 (corn growth stage V6–R3) were significantly higher than Periods 1 or 3 at both locations (Figure 2). The high N mineralization observed in Period 2 during corn growth stages V6–R3 matches reported peaks of N demand by a growing corn crop (Abendroth et al., 2011; Schepers & Meisinger, 1994). Importantly, N mineralization peaks following both rye and legume cover crops were synchronized with corn demand. Measurements of meaningful changes in plant available N during the season and documentation of practices that produce synchrony between N supply and demand are critical to future efforts to improve N use efficiencies (Congreves et al., 2021). Average N mineralization across all treatments during Period 2 was 3.02 and 1.52 kg N ha d−1 at Site Year 1 and Site Year 2, respectively. The vetch treatment had significantly higher N mineralization than rye during the first and third periods at both sites and numerically higher mineralization during the second period at both sites. Sweet clover had significantly higher N mineralization than rye and no cover crop during all three periods at Site Year 2.2FIGUREDaily rate of N mineralized for incubation period using in‐situ soil intact cores in Site Year 1 (a) and Site Year 2 (b). Within a period and site‐year, bars sharing a common lowercase letter are not significantly different at the .10 probability level. Values below the incubation period indicate average means in each period by site‐year. Within a site‐year, means sharing a common uppercase letter are not significantly different at the .05 probability level. No detectable amounts of aboveground biomass were found for sweet clover in Site Year 1The accumulative net N mineralized (kg N ha−1) over a typical 100‐d corn growing season was higher following successful legume cover crops than nonlegume cover crop (Figure 3). The differences in N mineralized following legumes or rye are likely results of the differing C:N ratios (Table 4). Researchers have commonly found that larger C:N ratios (>25:1) found in grass cover crops like rye can favor N immobilization over mineralization over the near‐term (Aulakh et al., 1991; Shipley et al., 1992; Sullivan & Andrews, 2012). Although little N (27 kg N ha−1) was mineralized in the no cover crop treatment at Site Year 2, about 157 kg N ha−1 was mineralized at Site Year 1 which exceeded N mineralized following rye and indicated N immobilization by the high biomass rye in Site Year 1. The history of these two adjacent sites were identical, so the most plausible explanation was that high temperatures and precipitation amounts (Figure 1) during Site Year 1 were favorable to N mineralization activities from either the soil or cover crop residues. Notably, mineralization of soil organic N in the no cover treatment without any residue sources resulted in a significant net decrease in soil inorganic N following corn uptake at Site Year 1(Figure 4). In contrast, legume treatments generally had higher residual soil inorganic N following the corn growing season than the rye and no cover crop treatments. The high N content of legume residues reduces competition for available N by microorganisms and increases decomposition and N mineralization (Janzen & Kucey, 1988; Vigil & Kissel, 1991). The elevated N mineralization activities following legumes provided quantities of plant available N approximately equivalent to the annual N fertilizer recommendations in both site‐years and increased residual soil inorganic N. This finding is consistent with research in the eastern United States reporting that legume cover crops can fix most of the N required for maximum corn yield in no‐till systems (Decker et al., 1994; Ebelhar et al., 1984; Mitchell & Tell, 1977; Wagger, 1989).3FIGUREThe accumulative net nitrogen (N) mineralization over 100‐d corn using in‐situ soil intact cores for N mineralization measurements. Within a site‐year, bars sharing a common letter are not significantly different at the .10 probability level. No detectable amounts of aboveground biomass were found for sweet clover in Site Year 14FIGUREChange in inorganic soil nitrogen (N) from the start of Period 1 (planting) until the end of Period 3 (growth stage R6)Comparative estimates for plant available N from cover cropsWe compared our in‐situ N mineralization measurements with other approaches to estimating PAN supplied by cover crops to succeeding cash crop. These estimates are based upon easily collected data including cover crop biomass and N content and soil inorganic N. Estimates of PAN based on cover crop biomass and/or biomass N (Sullivan & Andrews, 2012; Vigil & Kissel, 1991) result in substantially lower estimated N made available to the corn crop (Table 5) than our mineralized N measurements as shown in Figure 3. Estimates for PAN across both site‐years from rye ranged from 2 to 20 and 7 to 65 kg N ha−1 for legumes. These types of estimates inherently ignore mineralization of soil organic N, the stimulation of soil N mineralization activities by cover crops via direct (plant exudates, residues) and indirect (e.g., soil structure) mechanisms, and the contribution of soil N fixed by legumes but not assimilated into legume biomass. Estimates of ANM based on soil N, fertilizer N, and N uptake (Schepers & Meisinger, 1994; Zubillaga et al., 2021) were also low, ranging from 16 to 57 kg N ha−1 across all treatments and site‐years, but consistent with expectations based on cover crop biomass and C:N ratios. Low estimates of PAN from cover crops using easily measured quantities would encourage over‐application of fertilizer N by producers. Probably the most realistic estimate for N provided by the cover crops to the following corn was calculated by subtracting the 100‐d accumulative net N mineralized in the no‐cover treatment from the cover crop treatments (Table 5). This calculation resulted in a net immobilization of 55 kg ha−1 for rye and a net mineralization of 31 kg ha−1 provided by vetch at Site Year 1, and 33, 135, and 94 kg ha−1 provided by rye, sweet clover, and vetch covers, respectively, at Site Year 2.5TABLEEstimates of plant available nitrogen (PAN), 100‐d accumulative in‐situ nitrogen (N) mineralization and apparent N mineralization (ANM), to corn following cover crop growthTreatmentPANVigaPANSulbIn‐situ NcANMdIn‐situ N normekg ha−1Site Year 1No cover crop156.6 a30.4 bRye1.5 b19.8 b100.9 a49.7 a−54.9 aSweet cloverf–––––Vetch54.5 a7.3 b202.5 a53.7 a31.3 aSite Year 2No cover crop26.8 c16.2 cRye9.8 b8.7 b59.8 bc22.7 bc33.1 bSweet clover61.9 a22.4 a172.4 a57.4 a134.9 abVetch64.8 a15.7 ab127.7 ab33.5 b94.1 aNote. Means within a column followed the same letter by sites are not significantly different at the .10 probability level.aVigil & Kissel, 1991.bSullivan & Andrews, 2012.c100‐d accumulative N mineralization, Kolberg et al., 1997.dSchepers & Meisinger, 1994.eIn‐situ N normalized by subtracting no cover crop treatment from cover crop treatment for each block of the study.fNondetectable amounts of above‐ground biomass for sweet clover at Site Year 1.Corn grain yield and N uptakeCumulative precipitation in May through June during corn planting was lower for Site Year 1 (123 mm) compared with the 30‐yr (1981–2010) averages (185 mm), whereas Site Year 2 had above average precipitation (228 cm; (NOAA, 2019), but both were adequate for crop establishment. Total precipitation during the growing season at Site Year 1 and Site Year 2 was 410 and 381 mm, respectively, during the corn growing season (May–Oct.), compared with 30‐yr averages of 478 mm resulting in a total water deficit of 14 (Site Year 1) and 20% (Site Year 2) that could have suppressed corn yield overall.Legume cover crop treatments had numerically higher corn grain yields at Site Year 1 (vetch; 8,496 kg ha−1) and Site Year 2 (sweet clover; 6,464 kg ha−1) compared with the no cover crop treatment (Table 6). Increased corn yields following legume cover crops have been observed at other more eastern locations in North America (Coombs et al., 2017; Decker et al., 1994; Wagger, 1989) The legume treatments produced the highest amount of mineralized N over the corn growing season, and perhaps more importantly had the highest N mineralization rates during Period 2 (corn growth stage V6–R3) when corn N demand is highest. The rye cover crop treatment had the lowest corn grain yields for both site‐years, 7,112 and 4,702 kg ha−1 for Site Years 1 and 2, respectively, but neither was significantly different from the no cover crop treatment. Past research has found corn yield decreased after rye cover crops due to N immobilization, soil water depletion, or allelopathy associated with cover crop biomass decomposition (Eckert, 1988; Moore et al., 2014; Wagger, 1989). At Site Year 1, vetch produced significantly higher N uptake by the corn compared with rye, but neither cover crop treatment was significantly different from the no cover crop treatment. At Site Year 2, N uptake by corn was significantly higher after sweet clover compared with the other treatments. In that year, corn grain N content was significantly higher following the high biomass sweet clover cover crop compared with the other treatments.6TABLECorn yield, nitrogen (N) uptake, and total N collected at harvest 2007 (Site Year 1) and 2008 (Site Year 2)LocationTreatmentYieldN uptakeTotal Nkg ha−1g kg−1Site Year 1No cover crop7,954 ab87.4 ab10.5 aRye7,112 b75.9 b10.8 aSweet clovera––– Vetch8,496 a90.5 a10.7 aSite Year 2No cover crop4,759 b50.0 b10.5 bRye4,702 b48.9 b10.4 bSweet clover6,464 a81.7 a12.7 a Vetch5,358 ab60.8 b11.5 abNote. Means within a column followed the same letter by sites are not significantly different at the .10 probability level.aNondetectable amounts of above‐ground biomass for sweet clover at Site Year 2.When evaluating the relationship between the different seasonal N mineralization estimates and corn yield, we found that the 100‐d accumulative in‐situ net N mineralization measurement best predicted grain yield (R2 = .66) compared with ANM (R2 = .44) and the two PAN estimates (R2 < .03; Figure 5). The two PAN estimates which utilize only cover crop biomass and N were not good predictors of corn yield and may promote over‐fertilization by underestimating N provided by cover crops. Because corn yield is dependent on the right amount of N available to the crop at the right time, we regressed our in‐situ N mineralization rates for period 2 (corn growth stage V6–R3) against corn yield (Figure 6). Rates of in‐situ N mineralization during corn growth stage V6–R3 provided a superior prediction (R2 = .83) of corn yields compared with estimates of total N provided by cover crops over the entire growing season. In‐situ net N mineralization has been linked to net primary production of trees (Khanna & Raison, 2013), but we did not encounter any similar reports for prediction of corn yields. In our study, the relationship between in‐situ net N mineralization and corn yields may be facilitated by the lean N‐fertility regime. Measurements of in‐situ N mineralization over a single time period (corn growth stage V6–R3) would be advantageous in terms of time and money and could be performed in regional cover crop field trials conducted by university extension professionals. A transition from a single timepoint measure of PAN to measurements throughout the growing season will enable adoption of management practices that improve NUE and minimize negative effects of over‐application (Congreves et al., 2021).5FIGURERelationship between estimates of in‐situ 100‐d N mineralization (a); apparent N mineralization (ANM; b); plant‐available N Vigil & Kissel (1991) (PANVig; c); and plant‐available N, Sullivan & Andrews (2012) (PANSul; d) for both site‐years. Panels (c) and (d) include no cover crop control6FIGURERelationship between incubation Period 2 (V6–R3) N mineralization rate and corn yield for all treatments across both site‐yearsNitrogen use efficiencyNitrogen use efficiency is defined as the ratio of crop production to the input of N and has traditionally been utilized to evaluate different N fertilizer application methods. Within the context of this experiment, we utilized NUE to evaluate the response of corn to in‐season available N (N fertilizer application and contributions of N mineralization of soil organic N and cover crop residues) according to Equation 6 (Moll et al., 1982). All treatments received the same amount of fertilizer and there were minimal differences between treatments in starting concentrations of soil inorganic N. Because the no cover crop and rye treatment had no or little N mineralized from residues during the season, the corn utilized N applied in fertilizer or mineralized from the soil resulting in a high apparent NUE (Figure 7) and lower residual soil inorganic N (Figure 3). In contrast, large amounts of N were mineralized from legume residues and that N was evidently taken up by corn (Figure 6) but resulted in a low NUE and higher residual soil inorganic N (Figure 3). Despite the equal amounts of fertilizer applied, treatments with high NUE (no cover, rye) had lower corn yields than those with low NUE (legumes). In this circumstance, a low NUE indicates a sufficient supply of N from the soil to meet crop demand which is desirable and allows fertilizer application to be reduced (Congreves et al., 2021). Thus, low NUE associated with legume treatments indicated that less or no fertilizer can be applied to corn following legume cover crops while maintaining yield as suggested by Rosecrance et al. (2000) and Teasdale et al. (2008). In succeeding years, the cover crop treatments will supply additional PAN from continued mineralization of persisting cover crop residues and soil organic matter enriched in N by prior N mineralization of plant residues (Chim et al., 2022).7FIGURENitrogen use efficiency (NUE) in corn following cover crops. Within a site‐year, bars sharing a common letter are not significantly different at the .10 probability level. No detectable amounts of aboveground biomass were found for sweet clover at Site Year 1CONCLUSIONSPredicting the supply of N by cover crops to a following corn crop is difficult because of annual variations in weather; however, we found that in‐situ measurements of N mineralization provided credible estimates for this value over two growing seasons with varied weather patterns. Legume cover crops generally produced higher seasonal amounts of mineralized N compared with rye or no cover crops and resulted in numerically higher corn yields. The cover crop treatments did not significantly decrease corn yields in either site‐year, despite N immobilization by a large rye cover crop in one of the years. Seasonal (100‐d) in‐situ N mineralization measurements better predicted yields across all treatments compared with other approaches that use cover crop biomass and N content. Regardless of treatment, the highest rates of N mineralization were observed during periods of high N demand by corn (V6–R3) and N mineralization during this period was a superior predictor of corn yield. The lower apparent NUE incorporating in‐season N mineralization indicated that less fertilizer N can be applied in the growing season following legume cover crops, while maintaining crop yield.ACKNOWLEDGMENTSThe authors thank Kurt Dagel and Max Pravecek for excellent technical assistance. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.AUTHOR CONTRIBUTIONSBee Khim Chim: Conceptualization; Formal analysis; Writing – original draft. Shannon L. Osborne: Conceptualization; Formal analysis; Methodology; Resources; Supervision; Writing – review & editing. R. Michael Lehman: Conceptualization; Formal analysis; Methodology; Writing – review & editing.CONFLICT OF INTERESTThe authors declare no conflict of interest.REFERENCESAbdalla, M., Hastings, A., Cheng, K., Yue, Q., Chadwick, D., Espenberg, M., Truu, J., Rees, R. M., & Smith, P. (2019). A critical review of the impacts of cover crops on nitrogen leaching, net greenhouse gas balance and crop productivity. Global Change Biology, 25, 2530–2543. https://doi.org/10.1111/gcb.14644Abendroth, L. J., Elmore, R. W., Boyer, M. J., & Marlay, S. K. (2011). Corn growth and development. PMR Ames.Aspiauzu, C., & Shaw, R. (1972). Comparisons of several methods of growing degree day unit calculation unit for corn. Iowa State College. Journal of Science, 46, 435–442.Aulakh, M. S., Walters, D. T., Doran, J. W., Francis, D. D., & Mosier, A. R. (1991). Crop residue type and placement effects on denitrification and mineralization. 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Journal

"Agrosystems, Geosciences & Environment" – Wiley

Published: Jan 1, 2022

References

  • A critical review of the impacts of cover crops on nitrogen leaching, net greenhouse gas balance and crop productivity
    Abdalla, M.; Hastings, A.; Cheng, K.; Yue, Q.; Chadwick, D.; Espenberg, M.; Truu, J.; Rees, R. M.; Smith, P.
  • Corn growth and development
    Abendroth, L. J.; Elmore, R. W.; Boyer, M. J.; Marlay, S. K.
  • Comparisons of several methods of growing degree day unit calculation unit for corn. Iowa State College
    Aspiauzu, C.; Shaw, R.
  • Crop residue type and placement effects on denitrification and mineralization
    Aulakh, M. S.; Walters, D. T.; Doran, J. W.; Francis, D. D.; Mosier, A. R.
  • Methods of soil analysis, Part 1
    Blake, G. R.; Hartge, K. H.
  • Managing cover crop profitably
    Bowman, G.; Shirley, C.; Cramer, C.
  • Cover Crop Effects on Cash Crops in Northern Great Plains No‐till Systems Are Annually Variable and Possibly Delayed
    Chim, B. K.; Osborne, S. L.; Lehman, R. M.; Schneider, S. K.
  • Nitrogen use efficiency definitions of today and tomorrow
    Congreves, K. A.; Otchere, O.; Ferland, D.; Farzadfar, S.; Williams, S.; Arcand, M. M.
  • Legume cover crop management on nitrogen dynamics and yield in grain corn systems
    Coombs, C.; Lauzon, J. D.; Deen, B.; Van Eerd, L. L.
  • Legume cover crop contributions to no‐tillage corn production
    Decker, A. M.; Clark, A. J.; Meisinger, J. J.; Mulford, F. R.; McIntosh, M. S.
  • A proposed use of ion exchange resins to measure nitrogen mineralization and nitrification in intact soil cores
    DiStefano, J. F.; Gholz, H. L.
  • Nonleguminus cover crops management for residual N recover and subsequent crop yields
    Ditsch, D. C.; Alley, M. M.
  • Nitrogen from legume cover crops for no‐tillage corn
    Ebelhar, S. A.; Frye, W. W.; Blevins, R. L.
  • Rye cover crops for no‐tillage corn and soybean production
    Eckert, D. J.
  • Estimation of Annual Soil Nitrogen Mineralization Rates using an Organic‐Nitrogen Budget Approach
    Geisseler, D.; Miller, K. S.; Aegerter, B. J.; Clark, N. E.; Miyao, E. M.
  • Determination of ammonia (salicylate) in 2 M KCl soil extracts Lachat QuikChem Method 12‐107‐06‐2‐A
    Hofer, S.
  • Release and recovery of nitrogen from winter annual cover crops in no‐till corn production
    Huntington, T. G.; Grove, J. H.; Frye, W. W.
  • Nitrogen dynamics in grain cropping systems integrating multiple ecologically based management strategies
    Isbell, S. A.; Bradley, B. A.; Morris, A. H.; Wallace, J. M.; Kaye, J. P.
  • C, N, and S mineralization of crop residues as influenced by crop species and nutrient regime
    Janzen, H. H.; Kucey, R. M. N.
  • Temperature dependence of organic matter decomposition: A critical review using literature data analyzed with different models
    Kätterer, T.; Reichstein, M.; Andrén, O.; Lomander, A.
  • Methods of soil analysis, Part 2: Chemical and Microbiological properties
    Keeney, D. R.
  • In situ core methods for estimating soil mineral‐N fluxes: Re‐evaluation based on 25 years of application and experience
    Khanna, P. K.; Raison, R. J.
  • Determination of nitrate in 2 M KCl soil extracts by flow injection analysis QuikChem Method 12‐107‐04‐1‐B
    Knepel, K.
  • Evaluation of an in‐situ net soil nitrogen mineralization method in dryland agroecosystems
    Kolberg, R. L.; Rouppet, B.; Westfall, D. G.; Peterson, G. A.
  • Effects of drying and rewetting on carbon and nitrogen mineralization in soils and incorporated residues
    Kruse, J. S.; Kissel, D. E.; Cabrera, M. L.
  • Nitrate leaching as influenced by cover cropping and nitrogen source
    McCracken, D. V.; Smith, M. S.; Grove, J. H.; Blevins, R. L.; MacKown, C. T.
  • Growing degree‐days: One equation, two interpretations
    McMaster, G. S.; Wilhelm, W.
  • Cover Crops for Clean Water
    Meisinger, J. J.; Hargrove, W. L.; Mikkelsen, R. L.; Williams, J. R.; Benson, V. W.
  • Winter‐annual cover crops for no‐tillage corn production
    Mitchell, W. H.; Tell, M. R.
  • Long‐term effects of harvesting maize stover and tillage on soil quality
    Moebius‐Clune, B. N.; Van Es, H. M.; Idowu, O. J.; Schindelbeck, R. R.; Moebius‐Clune, D. J.; Wolfe, D. W.; Abawi, G. S.; Thies, J. E.; Gugino, B. K.; Lucey, R.
  • Analysis and interpretation of factors which contribute to efficiency of nitrogen utilization 1
    Moll, R.; Kamprath, E.; Jackson, W.
  • Rye cover crop effects on soil quality in no‐till corn silage–soybean cropping systems
    Moore, E.; Wiedenhoeft, M.; Kaspar, T.; Cambardella, C.
  • Methods of Soil Analysis, Part 3 Chemical Methods
    Nelson, D. W.; Sommers, L. E.
  • The synchrony of cover crop decomposition, enzyme activity, and nitrogen availability in a corn agroecosystem in the Midwest United States
    Nevins, C. J.; Lacey, C.; Armstrong, S.
  • Climate normal in South Dakota
  • Methodology for studying fluxes of soil mineral‐N in situ
    Raison, R. J.; Connell, M. J.; Khanna, P. K.
  • Denitrification and N mineralization from hairy vetch (Vicia villosa Roth) and rye (Secale cereale L.) cover crop monocultures and bicultures
    Rosecrance, R.; McCarty, G.; Shelton, D.; Teasdale, J.
  • Base SAS 9.4 procedures guide: Statistical procedures
  • Field indicators of nitrogen mineralization
    Schepers, J.; Meisinger, J.
  • Conserving residual corn fertilizer nitrogen with winter cover crops
    Shipley, P. R.; Messinger, J. J.; Decker, A. M.
  • Estimating plant‐available nitrogen release from cover crops, A Pacific Northwest Extension Publication
    Sullivan, D. M.; Andrews, N.
  • Factors affecting nitrogen availability and variability in cornfields
    Tao, H.; Morris, T. F.; Kyveryga, P.; McGuire, J.
  • Sweet corn production and efficiency of nitrogen use in high cover crop residue
    Teasdale, J. R.; Abdul‐Baki, A. A.; Park, Y. B.
  • Catch crops and green manures as biological tools in nitrogen management in temperate zones
    Thorup‐Kristensen, K.; Magid, J.; Jensen, L. S.
  • Equations for estimating the amount of nitrogen mineralized from crop residues
    Vigil, M. F.; Kissel, D. E.
  • Cover crop management and nitrogen rate in relation to growth and yield of no‐till corn
    Wagger, M. G.
  • Modeling the contributions of nitrogen mineralization to yield of corn
    White, C. M.; Finney, D. M.; Kemanian, A. R.; Kaye, J. P.
  • Assessment of apparent N mineralization at field scale
    Zubillaga, M. M.; Redel, H. M.; Cabrera, M. L.