Soil responses to inclusion of corn, soybean, and cover crops under rainfed conditions in the northern Great Plains

Abstract Crop rotations in the northern Great Plains of North America increasingly include corn (Zea mays L.) and soybean (Glycine max (L.) Merr.). Use of cover crops, while less extensive, is also increasing given their purported agronomic and environmental benefits. To date, soil responses to the inclusion of corn, soybean, and cover crops in rainfed cropping systems have not been well documented in the region. Therefore, soil properties were evaluated 6 years after establishment of three crop rotations (spring wheat (Triticum aestivum L.)–soybean (SW–S), spring wheat–corn–soybean (SW–C–S), and spring wheat–corn–cover crop (SW–C–cc)) each split by no and minimum tillage on a Dark Brown Chernozem near Mandan, ND, USA. Soil responses to treatments were subtle and exclusive to the 0–7.6 cm depth. Soil pH was lower in SW–S than SW–C–cc (5.28 vs. 5.48; P = 0.05), SO4-S was greater under SW–C–cc than SW–C–S (13.4 vs. 11.6 g S kg−1; P = 0.03), exchangeable K was greater under SW–C–S and SW–C–cc than SW–S (0.83 cmol kg−1 vs. 0.52 cmol kg−1; P = 0.05), and water-stable aggregates were greater in SW–S than SW–C–S (26% vs. 19%; P = 0.08). Soil organic carbon (SOC) and total N did not differ among crop rotations or between tillage treatments, while particulate organic matter N was greater under no tillage compared to minimum tillage (P = 0.08). Between 2012 and 2018, soil pH decreased and SOC increased under SW–C–S. Frequent monitoring of near-surface soil conditions in rotations with soybean every other year is recommended. Furthermore, innovative management practices are needed to enhance soil C and N fractions in rotations with full-season cover crops.


Introduction
Over the past three decades, cropping systems in the northern Great Plains (NGP) have transitioned away from small grain-based rotations to those increasingly dominated by corn (Zea mays L.) and soybean (Glycine max (L.) Merr.) (Aguilar et al. 2015).In North Dakota alone, harvested area dedicated to corn and soybean grain production increased by 6-and 14fold, respectively, between 19876-and 14fold, respectively, between and 20176-and 14fold, respectively, between (NASS 2023)).This change in cropland use occurred in response to more favorable weather conditions for warm-season crops coupled with scientific advances and policy incentives that have facilitated their increased adoption (Turner et al. 2013;O'Brien et al. 2020).Projections over the next three decades suggest warmseason crops (e.g., corn, soybean, sunflower (Helianthus annuus L.), sorghum (Sorghum bicolor L.), millet (Setaria italica (L.) P. Beauv.)) will become even more prevalent in the region in response to warmer and generally wetter--but more variable-growing conditions (Conant et al. 2018).
Increased prevalence of corn and soybean has impacted the delivery of ecosystem services from agricultural lands in the NGP (O'Brien et al. 2020).Such service-related impacts are often manifested through alterations in soil condition (Robinson et al. 2013).Corn, with its high nutrient demand, frequently requires supplemental synthetic N to maximize grain production (Franzen 2022), increasing N losses by leaching and denitrification and subsequent acidification in nearsurface soil (Tarkalson et al. 2006).Soybean, with its labile residue (Vachon and Oelbermann 2011;Stewart et al. 2015), has been found to accelerate decomposition of soil organic carbon (SOC) (Huggins et al. 2007;Hall et al. 2019), increase erosion susceptibility (McCracken et al. 1986;Gantzer et al. 1987), and decrease indicators of soil health (Agomoh et al. 2020).
Including annual forages in NGP cropping systems may offer producers a form of production with multiple uses that concurrently improves soil condition.As reviewed by Carr et al. (2021), annual forages in the north and central Great Plains were found to improve near-surface soil physical conditions, increase SOC and mineralizable N, and enhance labile pools of soil C and N when included in wheat-based cropping systems.However, improvements to soil from annual forages took a long time to be detected and were easily lost follow-ing changes in management and occurrence of drought (Carr et al. 2021).How annual forages affect soil condition in cornand soybean-based cropping systems has not been previously documented in the NGP.
In 1993, a long-term cropping system experiment was initiated near Mandan, ND, USA to investigate effects of six spring wheat (Triticum aestivum L.)-based crop sequences under minimum and no tillage on grain and biomass yield, precipitation use, and soil properties (Halvorson et al. 2016(Halvorson et al. , 2019)).In 2012, three crop sequences were modified to include corn, soybean, and annual forages (i.e., full-season cover crop mixture).Here, we report soil property responses to the updated treatments following 6 years of deployment.Hypotheses tested in the study were as follows: (i) inclusion of a full-season cover crop mixture in a crop sequence would increase particulate organic matter (POM) and aggregate stability compared to crop sequences without cover crops, and (ii) inclusion of corn in a crop sequence would increase near-surface SOC between 2012 and 2018.

Site and management description
The research site was located approximately 6 km southwest of Mandan, ND, USA (46.77,; 591 m elevation) on the Area IV Soil Conservation Districts Cooperative Research Farm.The site has nearly level topography (0%-1% slope) and is composed of Temvik-Wilton silt loam (finesilty, mixed, superactive, frigid Typic, and Pachic Haplustolls) (USDA 2023), which is similar to a Dark Brown Chernozem (Soil Classification Working Group 1998).Climate is semiarid to sub-humid Continental, with cold and dry winters, warm to hot summers, and erratic precipitation (Bailey 1995).Longterm (1989Longterm ( -2018) ) mean annual precipitation (MAP) and mean annual temperature (MAT) was 454 mm and 5.9 • C, respectively (NOAA 2023).
Minimum tillage involved one pass with a John Deere 550 Mulch Master tillage implement in the spring prior to planting (Deere and Company, Moline, IL, USA).Maximum depth of soil disturbance by minimum tillage was approximately 0.08 m.No tillage plots were not disturbed except at planting.Plot size was 9.1 by 30.1 m.
Planting of spring wheat typically occurred in early May using a John Deere 750 no-till drill at 3.2 million seeds ha −1 in 0.19 m rows.Corn and soybean were planted in midto late-May with a John Deere 1750 planter at 60 515 and 444 600 seeds ha −1 in 0.76 m rows.The cover crop mixture was planted mid-June in 0.19 m rows at 2.5 million seeds ha −1 for spring triticale, 1.6 million seeds ha −1 for millet, 1.5 million seeds ha −1 for canola, 59 000 seeds ha −1 for sunflower, 740 000 seeds ha −1 for pea, 321 000 seeds ha −1 for soybean, and 1.3 million seeds ha −1 for turnip.Grain harvest for spring wheat, soybean, and corn typically occurred in August, September, and October, respectively, using a John Deere 4420 combine.Harvested residue was spread uniformly over the soil surface using a chaff spreader.The cover crop mixture was cut and swathed near peak biomass (mid-August), baled, and removed from the plots for use as livestock feed.
Weed and fertilizer management followed practices used by area producers.All plots were sprayed with glyphosate prior to planting and again with appropriate post-emergent herbicides.Fertilizer (N, P) was applied during planting.Spring wheat received 67 kg N ha −1 as urea.Corn received 101 kg N ha −1 , while the cover crop mixture received 67 kg N ha −1 .Soybean received 25 kg P ha −1 as monoammonium phosphate, while spring wheat, corn, and the cover crop mixture received 11 kg P ha −1 .
Precipitation and air temperature were recorded at a North Dakota Agricultural Weather Network station approximately 2 km east of the study location (NDAWN 2023).Precipitation received as snowfall was recorded from a National Oceanic Atmospheric Administration weather station 5 km north of the study location (NOAA 2023).

Sample collection and processing
Soil sampling occurred in April 2018 prior to field operations.Within each plot, samples were collected at three locations (east, middle, and west) along a northwest-southeast transect to a 152.4 cm depth in increments of 0-7.6, 7.6-15.2, 15.2-30.5, 30.5-61.0, 61.0-91.4, 91.4-121.9, and 121.9-152.4cm using a 3.3-cm (i.d.) Giddings hydraulic probe (Giddings Machine Co., Windsor, CO, USA).At each location, two soil cores were collected at 0-7.6, 7.6-15.2,and 15.2-30.5 cm, and one soil core was collected from the lower depths.Cores across a transect were composited by depth in each plot.Separate samples for aggregate stability analysis were collected with a trowel from the 0-7.6 cm depth.Samples were saved in double-lined plastic bags and placed in cold storage at 5 • C until processing.
Soil samples for aggregate stability analyses were carefully broken along natural fracture lines and dried at ambient temperature for 7 days.Soil core samples were first evaluated for gravimetric soil water content using a 15-20 g subsample by measuring the difference in mass before and after drying at 105 • C for 24 h (Gardner 1986).Bulk samples were dried at 35 • C for 3-4 days and then ground to pass a 2.0 mm sieve.Samples from the 0-7.6 cm depth were ground by hand with a rolling pin, whereas remaining depths were mechanically ground with a BICO grinder (Bico Inc., Burbank, CA, USA).Identifiable plant material (>2.0 mm diameter, >10 mm length) was removed prior to grinding.

Soil analyses
Electrical conductivity and soil pH were measured using a 1:1 soil-water mixture (Watson and Brown 1998;Whitney 1998).Exchangeable cations (Ca 2+ , Mg 2+ , K + , and Na + ) were estimated by atomic absorption spectrometry (Sumner and Miller 1996).Soil NO 3 -N was estimated from 1:10 soil-KCl (2 M) extracts using cadmium reduction followed by a modified Griess-Ilosvay method (Mulvaney 1996).Plant-available soil P was estimated by bicarbonate extraction (Kuo 1996).Copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn) were estimated via DTPA extract method, boron (B) by the hot water extract method, and sulfate-sulfur (SO 4 -S) by the Mehlich 3 ICAP method as outlined by Nathan and Gelderman (2012).Total soil C and N were determined by dry combustion (Nelson and Sommers 1996).For soils with pH ≥ 7.2, soil inorganic C was estimated by difference following HCl fumigation (Harris et al. 2001).POM was estimated using the method of Gregorich and Ellert (1993), where material retained on a 0.053 mm sieve from a 30 g air-dried subsample was collected and analyzed for C and N content by dry combustion.Analyses for POM were conducted for the 0-7.6 cm depth only.Data for soil chemical properties were expressed on an ovendry basis.
Wet aggregate stability of the 1-2 mm fraction was determined on duplicate 4 g air-dry samples following the method of Kemper and Rosenau (1986).Soil bulk density of core samples was determined using the oven-dry mass and known volume of the composited samples (Blake and Hartge 1986).Soil C and N were expressed gravimetrically and on an equivalent soil mass basis following the method of Ellert and Bettany (1995).For purposes of evaluating treatment effects on soil C and N stocks in surface depths, soil profile masses of 1120, 2320, and 4559 Mg ha −1 were approximately aligned with the 0-7.6, 0-15.2, and 0-30.5 cm depths, respectively.

Soil cover and aboveground biomass
The percent of soil covered by crop residue was measured annually in each plot immediately prior to spring field operations.Presence of crop residue on the soil surface was counted along two 25 point transects equally spaced on a 7.6 m cable (Laflen et al. 1981;Larney et al. 2017).Transects were placed in the east and west sections of the plot following a northeast-southwest orientation.Crop residue intersecting with a point on the cable was counted as a contact, and the total number of residue contacts within a plot was recorded.Residue contact data were converted to percent soil coverage by residue prior to statistical analyses.Due to labor constraints, soil coverage by residue was not measured in 2014.
Total aboveground biomass in spring wheat, corn, and soybean crop phases was measured immediately prior to combine harvest using two 0.33 m 2 frames placed in east and west sections of each plot.Aboveground biomass of cover crops was measured similarly at peak growth (typically early August).Cover crop regrowth following haying was negligi-ble and not measured.All biomass samples were collected by hand-clipping standing plant material at the soil surface within each frame.Samples were dried at 65 • C for 72 h in a forced air oven then weighed.Wheat, corn, and soybean grain were threshed and weighed.Mass of non-grain aboveground biomass was calculated by subtracting grain from total aboveground biomass.

Statistical analyses
Fixed effects of crop rotation and tillage on soil properties were evaluated by depth with a mixed model using PROC GLIMMIX (SAS Institute 2019).A similar model was used to analyze soil cover and non-grain aboveground biomass within years.Repeated measures were used to evaluate change in soil C and N between 2012 (as reported in Halvorson et al. 2016) and 2018.Multiple pairwise comparisons were conducted using least squares means with differences deemed significant at P ≤ 0.05, or where noted, P ≤ 0.1.Shared values represent arithmetic means ± the standard error of the mean (Steel and Torrie 1980).

Weather conditions
Weather conditions between 2012 and 2018 were typical of a cold, semiarid Continental climate.MAP over the 6-year period was 437 mm, or about 30 mm less than the long-term mean (Table S1).Four of 6 years had lower MAP than the longterm mean (2012, 2014, 2015, and 2017).Greatest monthly precipitation fell between May and August while the nongrowing season was generally dry, except in 2016 when substantial snowfall was received in November and December.
MAT over the study period was 6.4 • C, or approximately 0.6 • C greater than the long-term mean (Table S1).Four of 6 years had greater MAT than the long-term mean, with 3 of 4 warmer years aligning with those that were drier than normal (2012, 2015, and 2017).Mean monthly air temperature during the study period peaked in July-August (19.7-23.5 • C) and was lowest in December-February (−1.3 to −14.0 • C).

Soil responses to crop rotation and tillage in 2018
Soil responses to the updated crop sequences were limited to the 0-7.6 cm depth, where crop rotation affected soil pH, SO 4 -S, exchangeable K, and water-stable aggregates (Table 1).Tillage effects on soil properties were limited to POM-N (Table 1), where it was greater under no tillage compared to minimum tillage (Fig. 1; P = 0.08).Stocks of SOC and total N were not affected by crop rotation or tillage at soil masses of 1120, 2320, and 4559 Mg ha −1 (Table 2).P values for tillage and rotation effects on soil properties below 7.6 cm are presented in Table S2.

Soil cover and aboveground biomass
Crop rotation and tillage affected soil coverage by residue in 2012 and 2013, but not thereafter (Table 5).In 2012, soil coverage by residue was greater in SW-S (72%) compared to SW-C-cc and SW-C-S (58% and 49%, respectively), and greater under no tillage than minimum tillage (68% vs. 48%).Treatment responses were similar in 2013, though percent cover was lower overall (57%, 41%, and 37% for SW-S, SW-Ccc, and SW-C-S, respectively, and 51% and 39% for no-and minimum-tillage).A significant crop rotation by tillage interaction in 2012 (P < 0.01) was attributed to dissimilar decreases in soil cover among rotations between tillage treatments, with coverage decreasing between no-and minimumtillage by 14% for SW-C-cc, 16% for SW-C-S, and 42% for SW-S.Mean values for soil coverage by residue in 2015, 2016, and 2017 were 90%, 67%, and 90%, respectively.
Non-grain aboveground biomass was generally unaffected by applied treatments, ranging from 1076 to 4573 kg ha −1 for spring wheat, 4284 to 7433 kg ha −1 for corn, and 1330 to 3045 kg ha −1 for soybean over the 6-year period (data not shown).Only twice were significant crop rotation effects on non-grain aboveground biomass observed for individual crops (Table S4).Spring wheat biomass was greater in SW-C-S and SW-C-cc (3716 and 3801 kg ha −1 , respectively) than SW-S (3130 kg ha −1 ) in 2012 (P = 0.01), while in 2014 soybean biomass in SW-S (2439 kg ha −1 ) was greater than SW-C-S (1877 kg ha −1 ) (P = 0.03).Tillage did not affect non-grain aboveground biomass in any year.Across the 6 year study period, annual aggregate non-grain aboveground biomass for spring wheat, corn, and soybean differed among crop rotations, with SW-C-S greatest (3794 kg ha −1 ), SW-S least (2673 kg ha −1 ), and SW-C-cc intermediate (3237 kg ha −1 ) (P < 0.01) (Table 6).Aboveground biomass of cover crops in the SW-C-cc rotation ranged from 2194 to 5650 kg ha −1 , and generally increased over the 6-year period (Fig. 3).Aboveground biomass of cover crops did not differ between tillage treatments in any year.
The percentage of water-stable aggregates was significantly associated with soil coverage by crop residue (P = 0.05; r = 0.75).Moreover, non-grain aboveground biomass was positively associated with gravimetric expression of total N (P = 0.05; r = 0.29) and SOC (P = 0.02; r = 0.34) but was negatively associated with soil pH (P = 0.01; r = −0.36)at the 0-7.6 cm depth (data not shown).

Discussion
Findings from this study suggest inclusion of corn, soybean, and cover crops to previous spring wheat cropping systems will have subtle impacts on soil properties, soil cover, and non-grain aboveground biomass under semiarid, rainfed conditions in the short-term.Factors that limit immediate detection of strong treatment effects--such as an inherently fertile soil coupled with limited treatment replication (n = 3) -could have played a role in the soil and crop responses to rotation and tillage.Moreover, soil responses to treatments in this study were exclusive to the surface depth (0-7.6 cm), where effects of crop management are often concentrated (Franzluebbers 2002).However, detection of any treatment responses over the 6-year period is noteworthy given the typical slow rate of change in soil properties under rainfed cropping systems in the region (Mikha et al. 2006).
Among contemporary categories of critical soil functions (Bünemann et al. 2018), findings from this study provided insight into rotation and tillage effects on fertility (i.e., pH, macronutrients, exchangeable cations, trace elements), soil structure maintenance (i.e., soil bulk density, water-stable aggregates), and organic matter cycling (i.e., soil C and N pools and stocks).Increased acidification of near-surface soil is common under rainfed cropping throughout the NGP (Jones et al. 2019), underscoring challenges faced by producers to mitigate its effects on crop production.Findings in this study suggest a buffering effect from cover crop use, as the SW-C-cc rotation had greater soil pH than SW-S after 6 years despite receiving twice the amount of N fertilizer over that time.Nutrient conservation benefits from cover crops are well documented (Blanco-Canqui et al. 2015;Nouri et al. 2022), as their growth outside the traditional growing season can recover NO 3 -N throughout the soil profile, thereby slowing "permanent" acidification associated with N loss (Bolan et al. 1991).While cover crops have been purported to increase basic cations in surface soil under certain edaphic and climatic conditions (Koudahe et al. 2022), no such effect was observed in this study.
Increased soil acidification over the 6-year period in SW-C-S aligned with previous findings evaluating soil pH over time under continuous crop, no-tillage management (Reeves and Liebig 2016).Surface application of ammoniacal N fertilizer, coupled with limited physical disturbance by tillage, can ex-acerbate soil acidification in surface soil depths (Tarkalson et al. 2006).Deployment of management interventions to mitigate soil acidification will eventually be needed to avoid negative impacts on crop growth through reduced nutrient cycling efficiency, herbicide efficacy, and increased metal toxicity (Enesi et al. 2023).
Though soil potassium levels were relatively high across study treatments (Range = 146-486 mg K kg −1 ; Mean = 294 mg K kg −1 ; 0-7.6 cm), increased frequency of soybean in rotation likely contributed to lower exchangeable K in SW-S compared to SW-C-S and SW-C-cc.Enhanced uptake of K by soybean in vegetative growth phases is well documented (Pettigrew 2008), frequently resulting in elevated grain K levels at harvest (Salvagiotti et al. 2021).While K loss from soybean residue has been documented in the U.S. Cornbelt (Oltmans and Mallarino 2015), limited rainfall coupled with negligible erosion potential at the study site make this possibility unlikely.
Elevated SO 4 -S in surface soil of SW-C-cc may have been influenced by pasja turnip in the cover crop mix.Roots of brassica plant species contain high concentrations of indole glucosinolates (Krumbein et al. 2005) that if not harvested can decompose in place, concentrating S-containing organic compounds in surface soil depths.Under on-farm conditions in the southcentral US, Tubaña et al. (2020) found 2%-20% greater S in near-surface soil where brassica and legume cover crops were present compared to farms where these cover crops were not present.As pasja turnip bulbs were below the cut height of the mower in this study, soil SO 4 -S may be expected to increase under the SW-C-cc treatment over time.While soil sulfur supply is known to affect wheat grain quality (Moss et al. 1983), it is unclear whether such an effect could be resolved given the relatively narrow range of soil SO 4 -S levels measured in this study.
A lack of soil disturbance coupled with surface accumulation of crop residue can improve soil structural attributes in near-surface depths of rainfed cropping systems (Arshad et al. 1999).However, detection of crop rotation effects on structural attributes generally, and water-stable aggregation specifically, can be difficult under wheat-based cropping systems (Klopp et al. 2023).Moreover, spring wheat and soybean in rotation have been found to have opposite effects on waterstable aggregation, with it increasing under spring wheat and decreasing under soybean (Zuber et al. 2015).In this study, SW-S was found to increase water-stable aggregation compared to SW-C-S, despite overall lower non-grain aboveground biomass in the former.The addition of pulse crops in rotation have been found to increase water-stable aggregation through the enhancement of decomposition-induced binding agents (Yan et al. 2022). In Ontario, Canada, van Eerd et al. (2014) found aggregate stability in a winter wheat (WW)-S rotation not different from S-WW-C, but greater than continuous C, continuous S, and C-S after 14 years of cropping.Retention of spring wheat residue on the soil surface is not uncommon given its high C:N ratio (Kamkar et al. 2014) coupled with limited soil-residue contact in low disturbance systems, resulting in slowed decomposition (Douglas et al. 1980).We postulate that retained spring wheat residue over time in SW-S served to initially increase and then maintain high soil coverage, thereby mitigating degradation of near-surface physical properties.Increased cover and minimal disturbance also likely contributed to increased soil porosity in SW-S, as reflected by the decrease in soil bulk density at 0-7.6 cm between 2012 and 2018.These results align with a recent meta-analysis, where the addition of grain legumes coupled with reduced soil disturbance was found to improve aggregate stability and soil porosity (Iheshiulo et al. 2023).
Soil C and N pools have been found to increase when cover crops are included in rainfed cropping systems (Hao et al. 2023), but responses can be variable depending on weather conditions (Blanco-Canqui et al. 2013), cover crop composition (Dube et al. 2012), duration of stand length (Alonso-Ayuso et al. 2014), and termination method (Wayman et al. 2015).Results from this study did not detect a positive effect from cover crops on SOC, total N, and POM-C or N, thereby rejecting the first null hypothesis.As aboveground biomass in the cover crop phase of the SW-C-cc rotation was removed as hay, C and N inputs to soil were largely restricted to root biomass.Resolving net effects between C and N inputs and decomposition under biomass-restricted conditions will likely take more time under semiarid conditions (Blanco-Canqui et al. 2015).Even so, findings from the first 18 years of this study found crop rotation and tillage to have no effect on SOC and total N in the surface 30.5 cm of soil (Halvorson et al. 2016).Accordingly, investigations into labile fractions of soil C and N would seem more fruitful in discerning treatment effects.Greater POM-N under no than minimum tillage at 0-7.6 cm aligns with previous research under semiarid conditions (Álvaro-Fuentes et al. 2008), reflecting greater root biomass inputs in near-surface depths under no tillage (Cambardella and Elliott 1992).
The decrease in volumetric expression of total N under SW-S appeared largely driven by the reduction in soil bulk density over time, whereas the increase in gravimetric expression of SOC under SW-C-S aligned with greater biomass inputs among the three rotations.Corn in rotation with spring wheat has been found to increase SOC over 16-22 years in temperate zones (Zhang et al. 2010), with SOC stocks positively associated with biomass C inputs (Wang et al. 2015).
The absence of a SOC increase in SW-C-cc in this study reflected the complexity of the second null hypothesis, as outcomes were rotation specific.Given the amount of biomass removed in the cover crop phase of the SW-C-cc rotation, the absence of a SOC increase in that rotation appears to be a significant tradeoff.Investigations into management strategies that provide abundant, high-quality forage through fullseason cover crops while concurrently increasing SOC are warranted.

Conclusion
Soil change under rainfed cropping systems in the NGP will be important to document as cropping patterns evolve to more frequent use of warm season crops.Quantifying changes following cover crop use will also be important to document given their multifunctional roles in addressing resource concerns.Collectively, how changes in properties and processes affect soil functions over time will provide critical insights into the sustainability of deployed practices.
Soil responses to corn, soybean, and full-season cover crops over 6 years were subtle and concentrated at the soil surface.Detected changes in this study suggest frequent inclusion of spring wheat and soybean may create a tradeoff between enhanced soil cover and near-surface structural stability with increased soil acidification and depletion of exchangeable K. Frequent monitoring of near-surface soil condition in rotations with soybean every other year appears justified to guide potential adjustments in crop choice and/or rotation length.The absence of a positive response from cover crops on soil C and N pools highlights an opportunity for future agronomic research.Efforts focused on developing full-season cover crop practices that provide adequate soil cover, ample forage, and suitable regrowth in the fall seem worthwhile.
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Fig. 1 .
Fig. 1.Particulate organic matter nitrogen at 0-7.6 cm under no and minimum tillage.Error bars reflect standard error of the mean.

Fig. 3 .
Fig. 3. Cover crop aboveground biomass under no and minimum tillage treatments, 2012-2017.Error bars reflect standard error of the mean.

Table 1 .
P values for crop rotation and tillage effects on soil properties at 0-7.6 cm.P values given for gravimetric expression of soil property (not applicable for soil bulk density, electrical conductivity, and soil pH).POM, particulate organic matter. *

Table 2 .
P values for crop rotation and tillage effects on soil nitrogen and carbon stocks adjusted to equivalent soil mass.Equivalent soil masses of 1120, 2320, and 4559 Mg ha −1 align with depths of 0-7.6, 0-15.2, and 0-30.5 cm, respectively. *

Table 3 .
Crop rotation effects on soil properties at 0-7.6 cm.

Table 4 .
Change in soil properties between 2012 and 2018 for two crop rotations at 0-7.6 cm.Values between years differ at P ≤ 0.1.Total nitrogen stock using an equivalent soil mass of 1120 Mg ha −1 .
*Values in parentheses are mean standard error.†

Table 5 .
P values for crop rotation and tillage on soil coverage by residue.
* Soil coverage by residue not collected in 2014 due to labor constraints.