Abstract Diverse crop rotations can enhance productivity and yield stability, perhaps by promoting soil microbial communities or increasing carbon sequestration and soil water availability. We used a decade of maize ( Zea mays L.) yield data from the Minnesota Long‐Term Agricultural Research Network to investigate legacy effects of crop‐rotation diversity on total and yearly maize grain productivity and stability. Cropping systems evaluated included a maize–soybean [ Glycine max (L.) Merr.] rotation (MS), a maize–soybean–wheat ( Triticum aestivum L.) rotation (MSW), and a maize–soybean rotation plus a fall‐planted rye ( Secale cereale L.) cover crop (MS + CC). Average maize yield over a 10‐year period was greatest in the MSW, followed by MS and then MS + CC rotation at the Waseca and Lamberton sites, while there was no difference in yield among systems at the Grand Rapids site. During drought years, however, maize yield was significantly lower in the MSW rotation at the Waseca and Lamberton sites. Maize yield stability was lower in the MSW compared to the MS with or without a cover crop at the Waseca and Lamberton sites. Only some of the results from this research support the general hypothesis that crop productivity and stability benefit from systems with greater crop rotational diversity. Our findings show the importance of long‐term, multi‐location research for understanding how cropping systems respond to increasingly variable weather and how strategies for diversification need to be site‐specific.

IntroductionCover crop adoption in U.S. crop rotations is steadily increasing. In the upper Midwest, where the conventional maize (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation is mostly rainfed, there is legitimate concern that cover crops may affect available soil water and the establishment of the subsequent main crop.MethodsA study was conducted to evaluate 1) the effect of interseeded cover crops on soil moisture at seeding and termination, and subsequent maize and soybean yields, and 2) seasonal evapotranspiration (ET) or water use of the main crops and cover crops. Field trials were conducted from 2016 to 2019 at three locations in the upper Midwest using four treatments: monoculture cereal rye (Secale cereale L.), two-species rye + crimson clover (Trifolium incarnatum L.), three-species rye + clover + forage radish (Raphanus sativus L.), and a fallow (no-cover planted) as the control.ResultsThe ET of cover crops varied between 52 and 110 mm, 70% of which was attributed to its evaporation component. Meanwhile the ET for maize and soybean ranged from 364–516 mm and 378–503 mm, respectively, 20% of which was attributed to evaporation. Regardless of the interseeding strategy, the biomass of cover crops was low in two out of the three experimental years due to weather conditions, resulting in little to no effect on soil water content or crop yield.DiscussionOur findings suggest that late interseeded cover crops for conditions in the northern U.S. may have limited impact on soil available water or the productivity of the subsequent crop when cover crop growth is low.

Soil health management systems use agricultural practices incorporating living roots, persistent surface cover, diverse crop rotations, and minimal soil disturbance such as tillage. These systems are widely thought to improve soil hydraulic functions. However, intense rainfall can cause physical slaking of aggregates, loss of surface pores, and reduced hydraulic functions. Soil health management systems correlate with stable aggregates and large soil pores, but it is not clear how these properties change with rainfall in fine-textured soil profiles. Therefore, quantifying hydraulic function in soil health systems is important as climate change intensifies growing season rainfall. We investigated the effects of soil health systems on volumetric soil water content (VWC), soil aggregates, soil pore size distributions, and a suite of soil health indicators in response to rainfall. During 2021 and 2022, we collected data from five tillage and cover crop treatments in replicated plots at the Southern Research and Outreach Center (SROC) in Waseca, MN (tillage treatments included rip/chisel plow, strip till, no till, and cover crop treatments included no cover crops and cereal rye), and three paired systems (conventional and soil health management, which differ in tillage and cover crop use) at long-term (>= 5 years), on-farm sites with finetextured soils. We monitored volumetric soil water content and soil aggregates within 24 h before and after select rainfall events. Across all locations, few differences in water capture, evidenced by an increase in VWC after rain, were evident. Aggregate responses to rainfall were observed between the paired on-farm treatments. Generally, conventional sites had 5-20 % more < 0.053 mm and 0.053-0.25 mm aggregates following rainfall than soil health sites, but this effect was inconsistent across locations. Soil health systems on-farm generally retained 10-30 % more > 2 mm water-stable aggregates than conventional systems in response to rainfall. Based on soil water retention curves, on-farm soil health sites had 2.5-12.5 % more macroporosity (pore diameter > 75 mu m) than conventional systems, despite having similar water capture. At the on-farm sites, greater microporosity and pore connectivity are attributed to an observed 0.25-2 cm/hr greater unsaturated hydraulic conductivity relative to soil health sites, validating the greater macroporosity observed in the soil health sites. Despite long-term treatment history at SROC plots, there were no differences in unsaturated hydraulic conductivity. At one onfarm site, the soil health system had higher soil health indicators than the conventional system, where the soil health system included 30 years of no-till and 12 years of cover crops compared to moldboard plowing in the conventional system. This research indicates the importance of holistically incorporating soil health practices into field systems for achieving enhanced soil functions.

Corn ( Zea mays L.) and soybean [ Glycine max (L.) Merr.] account for much of the arable land in the Upper US Midwest during the summer. Land is left fallow in late autumn after harvest through early spring leaving valuable growing degree days unused. Temporal intensification is a concept that considers planting crops such as winter camelina ( Camelina sativa L.) during these fallow periods. Winter camelina is a freeze‐hardy winter annual oilseed that can provide an economic benefit to farmers the following spring. However, there are significant agronomic and economic trade‐offs associated with integrating camelina into the corn–soybean rotation. The objectives were to assess the yield potential and seed quality of a corn–camelina–soybean rotation using (1) a range of corn hybrid maturities, (2) corn stover presence or absence, and (3) calculate the economic trade‐offs compared with a typical corn–soybean rotation. This study was conducted over the 2019 and 2020 growing seasons at two locations in Minnesota. Corn and soybean seed yield was maximized in treatments where camelina performed poorly and vice versa. Late corn harvest and stover presence had a negative effect on camelina establishment and yield but were favorable to soybean production. Based on both the agronomic and economic analyses for the aggregated cropping system, treatments that began with 90‐ and 95‐day relatively mature corn hybrids performed equally well, regardless of stover presence. This indicates there are multiple options to move forward with a corn–camelina–soybean cropping rotation.

Due to the environmental consequences of annual-dominated cropping systems, there is an increasing need to identify agronomic strategies that incorporate perennial crops. One strategy for increasing perennial cover is through the targeted use of annually harvested perennial food and bioproduct crops in buffer strips, which has the potential to create new revenue streams for farmers and substantially mitigate agricultural nutrient pollution from conventional cropping systems. As buffers are typically installed on marginal land, it is critical to understand how landscape position influences the success of perennial crops. The objectives of this study were to determine the relatively early influence of landscape position on the productivity of a variety of perennial crops and their subsequent soil nutrients and soil water storing capabilities. In this experiment, nine perennial (alfalfa, alsike clover, indiangrass, switchgrass, big bluestem, prairie cordgrass, intermediate wheatgrass, high-diversity polyculture, low-diversity polyculture) and two annual (corn, soybean) crops were planted across two landscape positions (hillslope and deposition). Plant biomass, plant tissue nitrogen, soil moisture, and soil NO3-N and NH4-N were measured and compared at two different locations in Minnesota. Overall, the polyculture mixes, and to some extent intermediate wheatgrass, performed the best with respect to biomass production while also providing ecosystem services across most soil by landscape position combinations tested in this study. However, there were some important findings specific to each soil and landscape position combination, mainly oriented toward biomass production. We also observed temporal patterns in soil moisture and depth-related patterns in soil N reductions. This study presents an opportunity to optimize the use of perennial crops on marginal agricultural lands for improved environmental and economic benefit.

Summer annual species are the most widely grown crops in the Upper Midwest, but reliance on a summer annual system has led to excessive soil and nutrient loss when crops are not present. Integrating winter rye (Secale cereale L.), known for its hardiness and environmental benefits, into the rotation can address these issues. However, there's limited understanding of the phenotypic diversity in winter rye during key growth stages that overlap with the typical planting dates for corn (Zea mays L.), soybean [Glycine max (L.) Merr], and late-planted crops like dry bean (Phaseolus vulgaris L.). Thus, the objectives of this study were to evaluate changes in biomass accumulation and quality of winter rye and triticale (x Triticosecale Wittmack) cultivars at three growth stages. Twenty cultivars were assessed over the 2014-2015 and 2015-2016 growing seasons across three environments in Minnesota. Winter rye and triticale were harvested for biomass at the tillering, booting, and soft dough growth stages and rye biomass yield and quality [i.e., crude protein, neutral detergent fiber (NDF), and 48-hour digestibility (NDFD)] were determined following harvest. Overall, there was little variation in each parameter among the cultivars within a location and growth stage. NDFD had some variation at the soft dough stage where digestibility ranged from 273 to 324 g kg-1 at the Crookston site and 332 to 406 g kg-1 at the St. Paul site. The lack of difference between cultivars indicates a wide range of choice when selecting a winter rye cultivar.

Growing multiple crops in rotation can increase the sustainability of agricultural systems and reduce risks from increasingly adverse weather. However, widespread adoption of diverse rotations is limited by economic uncertainty, lack of incentives, and limited information about long-term outcomes. Here, we combined 36,000 yield observations from 20 North American long-term cropping experiments (434 site-years) to assess how greater crop diversity impacts productivity of complete rotations and their component crops under varying growing conditions. Maize and soybean output increased as the number of species and rotation length increased, while results for complete rotations varied by site depending on which crops were present. Diverse rotations reduced rotation-level output at eight sites due to the addition of lower-output crops such as small grains, illustrating trade-offs. Diverse rotations positively impacted rotation-level output under poor growing conditions, which illustrates how diverse cropping systems can reduce the risk of crop loss in a changing climate.

In the Upper Midwest, corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] are the most prominent agricultural crops, occupying land for only half the year. Their fallow periods are associated with nutrient leaching and soil erosion, but they also represent an opportunity to establish winter-hardy crops, like pennycress (Thlaspi arvense L.), that can reduce negative environmental impacts while contributing to farm profitability. A lack of agronomic best management practices remains a barrier to pennycress adoption. The objectives of this study were to evaluate the impact of stover presence on pennycress stand establishment and seed yield after seeding pennycress into R4 corn. This study was conducted over the 2014-2015 and 2015-2016 growing seasons with MN106 pennycress at three locations in Minnesota. At maturity, corn plants were removed from plots in 25% increments between 0% and 100% to simulate stover removal scenarios. Pennycress seedlings were unable to compete with corn for available light resources and did not have enough time following corn harvest to establish a dense stand in the autumn with only 5% and 10% green cover in 2014 and 2015, respectively. Despite poor early season emergence, the average seed yield, 1223 kg ha-1, was high relative to other studies using MN106, indicating that challenges of growing pennycress under adverse conditions may not be a barrier to competitive seed yields. Experimentation on the corn-pennycress system should continue in the future to further explore and establish best management practices for this double crop. Rate of stover removal minimally impacted pennycress germination. Pennycress seedlings provided little autumn soil coverage. Stover removal rate did not affect pennycress seed yield.

Reliance on summer annual crops in the Upper Midwest results in fallow land from late fall through early spring, providing opportunities to integrate winter crops, such as pennycress (Thlapsi arvense L.), onto the landscape. Pennycress agronomics have primarily been studied using unimproved wild-type lines prone to seed shatter, resulting in significant yield loss if not harvested early. However, high plant and seed moisture complicates harvest and seed storage. A new breeding line with a reduced-shatter mutation made it possible to use harvest aids to reduce plant moisture without the risk of seed loss. The objectives of this study were to quantify the reduction in pennycress seed and biomass moisture after applying a harvest aid and to assess the seed yield, oil content, and crude protein of the reduced-shatter line. This study was conducted over the 2018-2019 and 2019-2020 growing seasons with '"IO217" pennycress in Rosemount, MN. Seed moisture decreased to a similar level by harvest maturity regardless of treatment while swathing was the most effective method of reducing biomass moisture. Natural senescence decreased pennycress moisture content to a harvestable level at the same rate as treated plants, indicating that a harvest aid is not required at this time. Seed yield was two to six times higher than in studies using unimproved pennycress lines. Challenges associated with wild-type pennycress lines, such as uneven germination and late maturation, were prevalent in this study and further genetic improvement will be necessary to ensure successful pennycress production in the Upper Midwest.

Intercropping forages with corn can improve cropping system productivity relative to single crop systems. However, limited light resources in 76 cm corn rows may impede successful forage establishment. This study assessed whether the combination of intercropped high value forage cover crops and wider corn rows could result in economically viable crop production systems in the Upper Midwest. A high value forage mixture was interseeded into standing corn at three working farms in the Rice and Goodhue Counties, MN, USA. Treatments were comprised of four row widths: 76 cm with no forage cover crop (best management practices, BMP), 76 cm with a forage cover crop (BMP + CC), 76 cm + CC, and two skip rows every fourth row (Balanced), and 152 cm + CC (WIDE). The WIDE, Balanced, and BMP + CC corn treatment reduced corn yields relative to the 76-cm treatments. However, the forage cover crop yields for all treatments optimized for light resources (Balanced and WIDE) ranged from 945 to 1865 kg ha−1 a forage quality (CP and RFV) equivalent to alfalfa. Our economic analysis revealed that high yielding, quality forage crops can offset up to 12.6% of economic losses caused by grain reductions. Wide-row intercropped systems may be economically viable for producers looking for opportunities to reintegrate their crop and livestock production systems, but further work is needed to refine this system for farm use.

Ecosystem benefits of winter-killed annual cover crops are less studied than winter hardy annuals. The objectives of this study were to determine (1) the effects of tillage practices and cover crop mixtures on biomass and soil cover of cover crops seeded into nearly mature maize (Zea mays L.) and soybean [Glycine max (L.) Merr.] and (2) if cover crop biomass can be estimated from soil surface coverage measurements. The study was conducted within two long-term tillage trials from 2016 to 2018. Tillage included conventional-(CT), strip-(ST), and no-till-(NT). Cover crops included annual ryegrass (AR; Lolium multiflorum L.); AR and crimson clover (CC; Trifolium incarnatum L.) mixture (ARCC); AR, CC, and forage radish (FR; Raphanus sativus L.) mixture (ARCCFR); and no-cover (NC). Seeding rates for AR, ARCC, and ARCCFR were 28, 36, and 40.5 kg ha(-1), respectively. The ARCCFR mix produced the most biomass (256 kg ha(-1)), followed by AR (174 kg ha(-1)) and ARCC (165 kg ha(-1)). Cover crop biomass was 260 and 136 kg ha(-1) in maize and soybean; and 218, 201, and 177 kg ha(-1) in CT, ST, and NT, respectively. Cover crop soil coverage between rows was 25% in maize and 14% in soybean. Cover crop biomass was associated with soil surface coverage (R-2 = 0.841). Given that cover crop biomass drives ecosystem services associated with cover crops, limited biomass and ground coverage of winter killed cover crops seeded into nearly mature maize and soybean may not add ecosystem services in the upper Midwest.

Soil organic carbon (SOC) is closely tied to soil health. However, additional biological indicators may also provide insight about C dynamics and microbial activity. We used SOC and the other C indicators (potential C mineralization, permanganate oxidizable C, water extractable organic C, and beta-glucosidase enzyme activity) from the North American Project to Evaluate Soil Health Measurements to examine the continental-scale drivers of these indicators, the relationships among indicators, and the effects of soil health practices on indicator values. All indicators had greater values at cooler temperatures, and most were greater with increased precipitation and clay content. The indicators were strongly correlated with each other at the site-level, with the strongest relationship between SOC and permanganate oxidizable C. The indicator values responded positively to decreased tillage, inclusion of cover crops, application of organic nutrients, and retention of crop residue, but not the number of harvested crops in a rotation. The effect of decreased tillage on the C indicators was generally greater at sites with higher precipitation. The magnitude and direction of the response to soil health practices was consistent across indicators within a site but measuring at least two indicators would provide additional confi-dence of the effects of management, especially for tillage. All C indicators responded to management, an essential criterion for evaluating soil health. Balancing the cost, sensitivity, interpretability, and availability at commercial labs, a 24-hr potential C mineralization assay could deliver the most benefit to measure in conjunction with SOC.

Cover crops are widely advocated for increasing soil organic carbon (SOC) levels, thereby benefiting soil health improvement and climate change mitigation. Few regional-scale studies have robustly explored SOC stocks under cover cropping, due to limited long-term experiments. We used the unique experimental data from the North American Project to Evaluate Soil Health Measurements conducted in 2019 to address this issue. This study included 19 agricultural research sites with 36 pairs of cover cropping established between 1896 and 2014. Explanatory variables related to site-specific environmental conditions and management practices were collected to identify and prioritize contributing factors that affect SOC stocks with cover crops, by coupling the Boruta algorithm and structural equation modeling. Overall, cover crops significantly (P < 0.05) improved several indicators of soil health, including greater SOC (concentration: +8%; stock: +7%), total nitrogen (+8%), waterstable aggregates (+15%), and potential carbon mineralization (+34%), on average, compared to no cover crop control. Likewise, on average, cover crops sequestered SOC 3.55 Mg C ha-1 (0-15 cm depth), with a sequestration rate of 0.24 Mg C ha-1 yr-1. In addition, we found climate (Hargreaves climatic moisture deficit) was important in explaining the variation of SOC stocks with cover crops, followed by soil properties (e.g., soil clay content). In terms of management practices, cover crop type had a significant positive (0.36) effect on SOC stocks, with non-legumes showing a greater impact, compared to legumes and mixtures. Crop rotational diversity also had a positive (0.28) effect on SOC accumulation. Our findings suggested that integrating non-legume cover crops into diverse crop rotation is likely to be a promising strategy to maximize SOC stocks with cover crops across North America.

Abstract The use of residual herbicides as a part of preemergence (PRE) and in‐season layered treatments has proven to be an effective strategy for controlling late‐emerging broadleaf weed species in annual crops. However, the use of residual PRE herbicides has the potential to negatively affect other crops within an annual two‐crop rotation, such as fall‐planted cover crops. The intent of this study was to determine if PRE herbicides commonly used in the upper Midwest United States would affect stand density, height, and biomass production of fall‐planted cover crops. Field studies were conducted at three locations that differed in soil type and climate. Four PRE herbicides were applied in silage corn (Zea mays L.) in the spring: dimethenamid‐P (single application), dimethenamid‐P + saflufenacil, acetochlor + clopyralid + mesotrione, and a layered treatment of dimethenamid‐P at planting and 30 days after first application. Three cover crops were planted in the fall following silage corn harvest with a no‐till drill: winter cereal rye (Secale cereale L.), winter camelina (Camelina sativa L.), and red clover (Trifolium pratense L.). Although some of the PRE HBs tested affected cover crop spring plant density and height in sandy soils, there was no difference in cover crop biomass production at termination between herbicide treatments, regardless of soil type. These results indicate that the application of these residual PRE herbicides for control of late‐emerging weed species did not interfere with cover crop biomass production as long as soil moisture was not limiting.

In the Upper Midwest, corn (Zea mays L.) and soybean (Glycine max [L.] Merr.) dominate the landscape, but only for six to seven months of the year. Thus, opportunities exist to establish crops that can utilize the remainder of the growing season and contribute to overall farm profitability. One species of interest is pennycress (Thlaspi arvense L.), but a lack of established agronomic best management practices is a barrier to successful crop production. The objectives of this study were to identify a range of cumulative growing degree days (CGDD) corresponding to pennycress physiological maturity, determine the optimal harvest window that maximizes pennycress seed yield and oil content, and characterize changes in pennycress seed attributes over seed maturation. This study was conducted over the 2016-2017 and 2017-2018 growing seasons with 'MN106' pennycress at two locations in Minnesota, USA. Seed dry weight stabilized within the window of maximum seed yield, but oil content did not maximize until after this period. However, there was minimal loss of oil content when pennycress was harvested within the seed yield maximization window. Based on these parameters, as well as seed moisture, it was estimated that pennycress reached physiological maturity between 2230 and 2250 degrees C d CGDD, or about a week prior to harvest maturity in terms of crop phenology. Delaying harvest to harvest maturity resulted in a 26% loss in harvestable seed due to seed shatter compared with the average maximum seed yield of 928 kg ha-1. Ensuring maximum pennycress seed yield and oil content at harvest is imperative to successful production and contribution to farm economic viability.

Farmers, scientists, and other soil health stakeholders require interpretable indicators of soil hydraulic function. Determining which indicators to use has been difficult because of measurement disconformity, spatial and temporal variability, recently established treatments, and the effect of site characteristics on management practice differences. The North American Project to Evaluate Soil Health Measurements includes 124 sites uniformly sampled across a range of soil health management practices in North America in 2019. We compare and recommend indicators of hydraulic function that best characterize soil health. We assessed the relationship of each indicator to a suite of soil inherent properties and climate variables, the response of each indicator to soil health management practices, the effect that soil inherent properties (clay content, sand content, and pH) and climatic variables (10-yr mean annual precipitation and temperature) had on response to management practices, and the relationship among the responses of the indicators to soil health management practices. Field capacity measured on intact cores (theta(FC_INTACT)) was the best measure of soil hydraulic function, because it responded to management, represents a direct measure of soil hydraulic function, is proximal to stakeholder values, and its response to management was not significantly influenced by inherent and climatic variables. Other suitable indicators are bulk density, soil organic carbon (SOC), and aggregate stability, which are not direct measures of soil hydraulic function but do respond to management and may be practical in situations in which measuring theta(FC_INTACT) is not. This study informs selection of soil health indicators to measure soil hydraulic function.

Currently accepted pedotransfer functions show negligible effect of management-induced changes to soil organic carbon (SOC) on plant available water holding capacity (theta(AWHC)), while some studies show the ability to substantially increase theta(AWHC) through management. The Soil Health Institute's North America Project to Evaluate Soil Health Measurements measured water content at field capacity using intact soil cores across 124 long-term research sites that contained increases in SOC as a result of management treatments such as reduced tillage and cover cropping. Pedotransfer functions were created for volumetric water content at field capacity (theta(FC)) and permanent wilting point (theta(PWP)). New pedotransfer functions had predictions of theta(AWHC) that were similarly accurate compared with Saxton and Rawls when tested on samples from the National Soil Characterization database. Further, the new pedotransfer functions showed substantial effects of soil calcareousness and SOC on theta(AWHC). For an increase in SOC of 10 g kg(-1) (1%) in noncalcareous soils, an average increase in theta(AWHC) of 3.0 mm 100 mm(-1) soil (0.03 m(3) m(-3)) on average across all soil texture classes was found. This SOC related increase in theta(AWHC) is about double previous estimates. Calcareous soils had an increase in theta(AWHC) of 1.2 mm 100 mm(-1) soil associated with a 10 g kg(-1) increase in SOC, across all soil texture classes. New equations can aid in quantifying benefits of soil management practices that increase SOC and can be used to model the effect of changes in management on drought resilience.

Potential carbon mineralization (Cmin) is a commonly used indicator of soil health, with greater Cmin values interpreted as healthier soil. While Cmin values are typically greater in agricultural soils managed with minimal physical disturbance, the mechanisms driving the increases remain poorly understood. This study assessed bacterial and archaeal community structure and potential microbial drivers of Cmin in soils maintained under various degrees of physical disturbance. Potential carbon mineralization, 16S rRNA sequences, and soil characterization data were collected as part of the North American Project to Evaluate Soil Health Measurements (NAPESHM). Results showed that type of cropping system, intensity of physical disturbance, and soil pH influenced microbial sensitivity to physical disturbance. Furthermore, 28% of amplicon sequence variants (ASVs), which were important in modeling Cmin, were enriched under soils managed with minimal physical disturbance. Sequences identified as enriched under minimal disturbance and important for modeling Cmin, were linked to organisms which could produce extracellular polymeric substances and contained metabolic strategies suited for tolerating environmental stressors. Understanding how physical disturbance shapes microbial communities across climates and inherent soil properties and drives changes in Cmin provides the context necessary to evaluate management impacts on standardized measures of soil microbial activity.

Aggregate stability is a commonly used indicator of soil health because improvements in aggregate stability are related to reduced erodibility and improved soil-water dynamics. During the past 80 to 90 years, numerous methods have been developed to assess aggregate stability. Limited comparisons among the methods have resulted in varied magnitudes of response to soil health management practices and varied influences of inherent soil properties and climate. It is not clear whether selection of a specific method creates any advantage to the investigator. This study assessed four commonly used methods of measuring aggregate stability using data collected as part of the North American Project to Evaluate Soil Health Measurements. The methods included water stable aggregates using the Cornell Rainfall Simulator (WSACASH), wet sieved water stable aggregates (WSAARS), slaking captured and adapted from SLAKES smart-phone image recognition software (STAB10), and the mean weight diameter of water stable aggregates (MWD). Influence of climate and inherent soil prop-erties at the continental scale were analyzed in addition to method responses to rotation diversity, cash crop count, residue management, organic nutrient amendments, cover crops, and tillage. The four methods were moderately correlated with each other. All methods were sensitive to differences in climate and inherent soil properties between sites, although to different degrees. None measured significant effects from rotation diversity or crop count, but all methods detected significant increases in aggregate stability resulting from reduced tillage. Significant increases or positive trends were observed for all methods in relation to cover cropping, increased residue retention, and organic amendments, except for STAB10, which expressed a slightly negative response to organic amendments. Considering these results, no single method was clearly superior and all four are viable options for measuring aggregate stability. Therefore, secondary considerations (e.g., cost, method availability, increased sensitivity to a specific management practice, or minimal within-treatment variability) driven by the needs of the investigator, should determine the most suitable method.

This study quantified root biomass distribution; root accrual of C, N, P, and K; and changes in soil organic carbon (SOC) associated with alley-cropped switchgrass (Panicum virgatum L.), prairie cordgrass (Spartina pectinata Bosc ex Link), intermediate wheatgrass [Thinopyrum intermedium (Host) Barkworth and Dewey 'Rush'], and a native polyculture planted between rows of 'NM6' poplar (Populus maximowiczii x P. nigra) and 'Fish Creek' willow (Salix purpurea) at two Minnesota sites (Empire and Granada). After 4 yr since establishment, SOC declined at each site but was not influenced by species selection. NM6 poplar-prairie cordgrass systems had among the highest root biomass, C, and nutrient accrual, with up to 16.3 Mg root biomass, 7.0 Mg C, 175 kg N, 31 kg P, and 97 kg K ha(-1). Fine roots were the largest fraction of belowground biomass, although course roots were also a large fraction for poplar and prairie cordgrass. Tree roots extended to 6 m into the crop alley, although 85-89% were within 1 m of tree rows, depending on tree species. Crop fine root biomass was reduced up to 67% at 1 m from tree rows and 20% at 3.5 m and was up to 142% greater in willow than in poplar alleys. Total root C was predominated by poplars at Empire regardless of herbaceous crop type, whereas the proportions of tree and crop root C varied by crop at Granada. These results suggest that prairie cordgrass is well suited to alley cropping and that, due to the competitive ability of poplars, productivity, C sequestration, and nutrient accrual may be greater in willow systems in the long term.