Nitrogen loss from agroecosystems is a leading to contributor to non-point source pollution in the northeastern United States and an inefficiency that increases agricultural production costs. Cover cropping is an effective and widely promoted strategy that increases N retention to counter these negative impacts of production. In recent years, a growing number of farmers have expressed interest in the use of cover crop “cocktails” (multi-species mixtures). It is recognized in both ecology and agriculture that plant diversity can positively impact productivity and ecosystem functions, making such mixtures a potential strategy to increase N retention by cover crops. In a two year field study of eighteen cover crop monocultures and mixtures, we found that the highest levels of N retention were provided by the winter hardy non-legumes, canola and cereal rye. In both years of the study, canola and rye monocultures and mixtures containing these species reduced nitrate leaching by 65-95% compared to the no cover crop control. From these data, we conclude that mixtures do not augment N retention compared to monocultures that already excel at providing this service. However, mixtures did benefit overall N management in that some were able to both retain N and provide N to a subsequent corn crop. Interestingly, the mixtures exhibiting this type of “multifunctionality” varied by year. In year one, the 4-species and 8-species mixes that combined red clover, hairy vetch, canola, and cereal rye led both to adequate N provision for the subsequent corn crop (yields of 9.2 and 9.9 Mg ha-1, respectively) and reduced nitrate leaching. Compared to the no cover crop treatment, red clover alone, and hairy vetch alone, which lost 35, 22, and 25 kg N ha-1, respectively, the 4-species mix lost 4 kg N ha-1 and the 8-species mix lost 12 kg N ha-1. In year two, the mixtures demonstrating the greatest degree of multifunctionality were a mix of two winter-killed legumes with winter-killed oats and forage radish (corn yield = 10.1 Mg ha-1, N loss reduced 55% compared to no cover crop) and a mix of red clover, hairy vetch, oats, and forage radish (corn yield = 10.1 Mg ha-1, N loss reduction of 80%). Our research provides evidence that cover crop mixtures are a viable strategy to improve N management by simultaneously enhancing both N retention and N provision.
The purpose of this project was to assess the potential of diverse cover crop mixtures to increase N retention in agroecosystems. Nitrate leaching is a significant contributor to non-point source pollution in the northeastern US. Loss of N from agroecosystems also represents the loss of an important nutrient required for crop production that must be replaced, often through fertilizer inputs that increase production costs. Developing management practices that optimize N retention in agroecosystems, therefore, will reduce the environmental risks involved in agriculture; promote improved water quality and natural resource protection; and support improved farm productivity, reduced costs, and increased farm income.
Cover cropping is an established strategy to reduce nitrate leaching and optimize N retention (1, 2). Our current understanding of the relationship between plant diversity and ecosystem processes indicates that increasing species richness leads to increases in functions such as biomass production and nutrient uptake efficiency (3). We hypothesized that cover crop mixtures of four or more species would exhibit higher biomass production and rates of inorganic N uptake, leading to greater assimilation of N in aboveground biomass in mixtures compared to monocultures (Objective 1).
Further, linkages between aboveground diversity and belowground processes indicate that some of the benefits offered by mixtures lie below the surface, making assessment of soil biological communities essential to cover crop mixture research (4). Though some aspects of cover crop effects on microbial communities, such as that of C:N ratio on decomposition, are well understood, research on linkages between plant diversity and microbial communities in agroecosystems is scant. Previous research has demonstrated that increases in aboveground biomass promote greater biomass and activity of microbial communities (5). We hypothesized that cover crop mixtures would increase N uptake by the soil microbial community by increasing microbial biomass and activity (Objective 2).
Together, increased N in aboveground biomass and increased N uptake by microbial communities will tighten N cycling and reduce the amount of N that is lost to leaching during the cover crop season (Objective 3). Further, the N retained in plant and soil microbial biomass is likely to increase the amount of N available to subsequent cash crops (Objective 4), making cover crop mixtures a potential strategy to not only retain N but also supply N to a subsequent cash crop.
A growing number of farmers are looking to put the power of diversity to work using cover crop “cocktails” to provide multiple agroecosystem functions. Farmers frequently list weed and erosion control, soil quality, and nutrient management among the main reasons for using cover crops, and many have expressed interest in using mixtures to boost these ecosystem services. Though a few scientific studies have examined cover crop mixtures (more than two species; (6–8)), there is a dearth of published research-based information to assist farmers seeking to adopt this practice. Given that cover crops require time and money, finding the “right mix” is critical. Research from ecology and agriculture suggests that the right mix is based on both on the number and identity of species in a mixture (9, 10). Therefore, explicitly testing the mechanisms by which cover crop diversity may impact N services will provide information essential to species selection (Objective 5).
Planting cover crop mixtures is an innovative strategy to provide multiple agroecosystem functions and enhance agricultural sustainability. This research will advance adoption of “cocktails” by contributing much-needed information on the benefits of and species selection for cover crop mixtures.
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The purpose of this project was to assess the potential of diverse cover crop mixtures to increase N retention in agroecosystems. To accomplish this, we established a two-year experiment at the Penn State agricultural research station with the following objectives:
Objective 1: To quantify the effect of cover crop mixtures on the quantity of N in aboveground cover crop biomass. Expected outcome 1: We hypothesize that increased diversity will lead to increased uptake of soil N by non-leguminous cover crop species and an overall increase in aboveground biomass N.
Objective 2: To assess the effects of cover crop mixtures on microbial N uptake. Expected outcome 2: We hypothesize that increased aboveground diversity will increased soil microbial biomass and activity and, therefore, microbial N assimilation during the cover crop growing season.
Objective 3: To measure the effects of cover crop mixtures on nitrate leaching during the cover crop season. Expected outcome 3: We hypothesize that increased aboveground diversity will reduce the amount of nitrate leached through the soil profile during the cover crop growing season.
Objective 4: To determine the effect of cover crop mixtures on nitrogen supply to a subsequent cash crop. Expected outcome 4: We hypothesize that increased aboveground diversity will lead to an increase in N available to a subsequent cash crop.
Objective 5: To increase understanding of the mechanisms through which diversity influences N retention and supply in order to assist farmers in choosing species for cover crop mixtures. Expected outcome 5a: We hypothesize that increasing the number of species present in a cover crop stand will positively impact cover crop N content, microbial uptake, nitrate leaching, and N supply, but that effect size will diminish as the number of species increases (richness mechanism). Expected outcome 5b: We hypothesize that mixing species that perform different N functions (N-fixing versus N-scavenging) will lead to higher quantities of N in aboveground cover crop biomass, greater N uptake by N-scavengers, and reduced nitrate leaching than mixtures with only one N functional group (N functional diversity mechanism). Expected outcome 5c: We hypothesize that the greatest effects of increased diversity on cover crop biomass N, microbial N assimilation, nitrate leaching, and N supply will be observed in cover crop mixtures that include species exhibiting temporal complementarity (winter kill versus winter hardy; temporal complementarity mechanism).
All of the above objectives were reached during the grant period, and we encountered no barriers to completion.
We established a randomized complete block experiment with four replications of seventeen (year 1) and eighteen (year 2) cover crop diversity treatments in September 2011 at the Penn State agricultural research station in central Pennsylvania. The two year experiment was carried out on adjacent fields managed in an oat-corn rotation. Each year, cover crops were planted in a field previously planted to spring oats. Oats were harvested in July 2011 (year 1) and July 2012 (year 2) and 12m x 6m (40ft x 20ft) cover crop treatment plots planted on August 30 (year 1) and August 24 (year 2). Cover crops were terminated in May prior to the planting of corn, and grain harvested the following November.
Cover crops included in this experiment were sunn hemp, soybean, red clover, hairy vetch, forage radish, oats, foxtail millet, sorghum sudangrass, cereal rye, canola, annual rye, and barley. To test our hypotheses regarding the mechanisms through which diversity influences N retention (Objective 5), cover crops are classified into niches according to two characteristics: nitrogen function (N-fixing versus N-scavenging) and temporal grouping (winter kill versus winter hardy; Table 1). These classifications (niches) are consistent with cover crop selection parameters used by farmers (11), and the selected cover crops are known to perform well in central PA and applicable to use in mixtures (12, 13). See Table 2 for additional treatment information.
Prior to cover crop planting, experimental plots were moldboard plowed and disked. Cover crop treatments were drilled using a Great Plains drill equipped with a cone distributor, and legume seed was mixed with dry inoculant. Seeding rates (Table 2) were based on those that have been successful in on-farm trials by Penn State extension and local farmers. For most species, the seeding rate in mixture was 50% of the monoculture rate. For species with established recommended seeding rates in mixture, the recommended rate was used in mixture (ie forage radish, hairy vetch, red clover, canola). Seeding rates and planting date represent potential limitations of this study, as best management practices for mixtures are not yet determined, and time and budgetary constraints do not allow multiple planting rates and dates. The results of this study will, however, contribute to more precise recommendations for mixture management.
One week before corn planting cover crops were mowed and incorporated by moldboard plowing followed by field cultivation. Grain corn was planted at 36,000 seeds/ha (32,000 seeds/A). Because the focus of this research is to understand N contributions from cover crop mixtures, no starter fertilizer was applied. At sidedress, half of each plot received 150 kg N/ha (135 lb N/ac) as urea ammonium nitrate (UAN), while the other half did not receive supplemental N in order to assess cover crop N contributions to corn.
Objective 1: Cover crop biomass and N content
Cover crop biomass and N content were quantified in October and May to coincide with peak cover crop biomass production. At each sampling time, we clipped aboveground biomass from three 0.5 m2 quadrats per plot, separated biomass by species, and dried and weighed biomass. A sub-sample of cover crop biomass was ground and analyzed for N concentration by combustion analysis.
Objective 2: Soil microbial community analysis
Soil for microbial analysis was collected in November and May. At each sampling event, we randomly collected and homogenized twelve 20cm soil cores from each plot for analysis. Samples were stored at 4oC until processing. Each sample was sieved to 2mm within 24 hours of collection. Samples were collected and processed used aseptic procedures.
We used chloroform fumigation extraction to determine soil microbial biomass N (SMB-N) (14). Following extraction with K2SO4, fumigated and non-fumigated samples were analyzed for total N following persulfate digestion (15). Differences in N between fumigated and non-fumigated samples represent the nitrogen content of the microbial biomass (SMB-N).
Objective 3: Nitrate leaching
We measured potential nitrate leaching using buried anion resin membranes and bags. Immediately following tillage and prior to cover crop planting in year 1, we buried three anion exchange resin membranes of a known area (2 cm x 4 cm) in each plot at a depth of 30 cm. Resin membranes are positively charged (and thus attract anions), and by using resins of a fixed area, we can calculate the flux of nitrate anions that passed by the resin membrane. The membranes were removed in November at the first killing frost, extracted with 2M KCl, and the quantity of nitrate accumulated on the membrane determined by colorimetric analysis (19). At the time the membranes were removed, three anion resin bags (5 cm x 5 cm) were buried in each plot at a depth of 30 cm. Resin bags function similarly to resin membranes by capturing nitrate anions that move through the soil profile with soil water. We decided to switch from membranes to bags due the difficulty in acquiring resin membranes and the greater capacity of resin bags. Bags were removed in May 2012 prior to cover crop termination, extracted with 3M KCl, and the quantity of nitrate accumulated in the bag determined by colorimetric analysis (16). In year two, four anion resin bags were buried in each plot immediately following cover crop planting. Two bags were removed in November 2012 and the remaining two removed in May 2013. Bags were extracted with 3M KCl and the quantity of nitrate accumulated determined by colorimetric analysis (16).
Objective 4: Corn yield
We harvested corn grain from the center two rows of each sub-plot (a total of 18m of row length) to determine yield. Yields were adjusted to 15.5% moisture.
Objective 5: Statistical analysis
Analysis of variance was performed on response variables measured for objectives 1-4 to test hypotheses under objective 5 regarding the overall impact of cover crop diversity and potential mechanisms for enhanced ecosystem function.
Additional funding was acquired to expand the scope of this project with further measurements relevant to N cycling and microbial communities. Namely, a 15N tracer was used in year two of the study to document the movement of N to cover crops, microbial communities, soil, and soil water. In addition, the impact of cover crop diversity on microbial community diversity is being investigated by phospholipid fatty acid analysis (years one and two) and pyrosequencing (year one) using soil collected from this study. Outcomes of these supplemental projects are not included in this report.
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- PSU Cover Crop Demonstration Network (2010) Available at: http://extension.psu.edu/cover-crops/trials/2010.
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Cover crop biomass and N content
Cover crop biomass production in year 1 (2011-12) ranged from 100 to 8000 kg ha-1 (figure 1A). We were not successful in establishing soybean and sunnhemp as winter killed legumes in monoculture or mixtures. As shown in figure 1, we successfully established 4 and 8 species mixtures in year one, though representation by winter killed legumes was nominal. Additionally, oats tended to dominate fall mixtures when present. In year two (2012-2013), we observed higher biomass production ranging from 500 to 10,000 kg/ha (figure 1B). We believe this was due in part to higher levels of fall production promoted by an earlier planting date and warmer fall in year two compared to year one. We observed lower production by the winter hardy legumes (red clover and hairy vetch) in both monoculture and mixture in year two compared to year one. We also saw a dominance of rye in mixtures in spring of year two. These differences in performance are likely due to environmental factors such as climate and soil that mediate not only individual species growth but also competition between species in mixtures. As shown in figure 2, the contributions of cover crop groups varied between years, particularly for those mixtures containing hairy vetch and red clover.
As we had predicted, aboveground biomass N in mixtures containing non-legumes and legumes, specifically those with winter hardy legumes (red clover and hairy vetch), was similar to biomass N in winter hardy legume monocultures and higher than in non-legume monocultures in year 1 (figure 3A; expected outcome 1). This trend was much less pronounced in year 2 due to the poor performance of hairy vetch and red clover (figure 3B). A comparison of the seeding rate ratio (seeding rate in mixture:seeding rate in monoculture) to the biomass N ratio (biomass N in mixture: biomass N in monoculture) for non-legume cover crops suggests that some species benefited from growing in a mixture with legumes (known as facilitation). A biomass N ratio that is higher than the seeding rate ratio would suggest a facilitation effect. For example, the seeding ratio for rye was 0.5. For mixtures in which rye was grown with a legume, the biomass N ratio was 0.5 or higher (Table 3). We see evidence of a similar facilitation effect for oats grown with soybean and sunnhemp in year 1 and with hairy vetch and red clover in year 2. Forage radish also benefitted from legume facilitation when grown with red clover and hairy vetch in both years and with sunn hemp and soybean in year 2.
Microbial N uptake
Measurements of N contained in the soil microbial biomass do not indicate any effects of cover crops or cover crop diversity on soil microbial N uptake and retention (figures 4 A&B). This in contrary to our expectation that diverse cover crop mixtures would promote greater microbial N uptake during the cover crop season (expected outcome 2) and previous studies that have shown increased microbial biomass (and, consequently, increased microbial N uptake) in the presence of cover crops (17). Our result may have been to do the short-term nature of our study in that cover crop effects on microbial communities may take more than one season to manifest. Additionally, our assessments of soil microbial biomass were performed on bulk soil. Changes in the microbial community, particularly during a single season may be detectable in the rhizosphere of cover crops (the soil within 1mm of cover crop roots), but not in bulk soil.
In year one, legume and oat monocultures experienced N leaching losses similar to the no cover crop control (figure 5A). This is consistent with previous findings that legume cover crops do not perform as well at retaining N as non-legumes (2). Nitrogen losses from oat were likely due to the fact that oats winter kill, make fields susceptible to N leaching in the spring. Monocultures of canola and rye and 4-species mixtures containing these species reduced N leaching by 85-97% compared to the control. The 8-species mix reduced leaching by 65%. The highest N losses were observed in the forage radish monoculture. We attribute this outcome to periods of above average spring temperatures that led to early mineralization of forage radish residues.
Soybean, sunnhemp, and red clover monocultures experienced N losses similar to those in the no cover crop control in year two (figure 5B). Though losses from hairy vetch were higher in year two than in year one, it reduced losses by approximately 50% compared to no cover crop in year two. Losses from forage radish were lower in year two than year one, supporting our spring mineralization hypothesis to explain high losses in year one. With the exception of one 4-species mix, all mixtures reduced nitrate leaching compared to no cover crop, with the greatest reductions (90-99%) by mixtures containing canola and cereal rye. These mixtures, however, did not lead to significantly greater reductions in N leaching compared to canola and cereal rye monocultures.
N provision to corn
As expected, the highest yields (and, therefore, the greatest N provision) were achieved in the red clover (9.7 Mg ha-1 and 10.2 Mg ha-1) and hairy vetch (9.8 Mg ha-1 and 10.8 Mg ha-1) monocultures in both years (figures 6A & 6B). In year one, three mixtures produced yields similar to the legume monocultures and higher than the no cover crop control (7.0 Mg ha-1): canola, cereal rye, hairy vetch, and red clover (9.2 Mg ha-1); forage radish, oat, red clover, and hairy vetch (10.2 Mg ha-1); and the 8-species mix (9.9 Mg ha-1). This outcome suggests that N provisioning by mixtures is largely dependent on the presence of winter hardy legumes. Data from year two further show that N provisioning is also influenced by the percent of total biomass contributed by winter hardy legumes. In year one, legumes contributed 40% of the cover crop biomass to the mix of canola, cereal rye, red clover, and hairy vetch; in year two legumes contributed only 15% of biomass and yields were 6.1 Mg ha-1, lower than the no cover crop control (8.7 Mg ha-1). Similarly, legumes contributed 50% of cover crop biomass to the 8-species mix in year 1, but less than 10% in year two. This mix yielded 5.9 Mg ha-1 in year two. In both of these mixes, we concluded that the high proportion of non-legume biomass, particularly cereal rye, led to N immobilization that reduced yields. The final high-yielding mixture in year one combined red clover and hairy vetch with forage radish and oats, and legumes contributed 70% of biomass to this mix. In year two, legumes contributed only 25% of biomass, but this mixture yielded 10.1 Mg ha-1, similar to the legume monocultures and higher than the no cover crop control. The fact that a mixture combining forage radish, oats, soybean, and sunnhemp produced an equivalent yield (10.2 Mg ha-1) suggests that N provisioning in both of these mixtures was not due primarily to legumes, but likely a combination of soil N reserves and N available from winter killed non-legumes. Strong N provisioning by the mixture of red clover, hairy vetch, forage radish, and oat in both years despite significant changes in biomass composition suggests that the combination of species in this mix provide an insurance effect for N provisioning.
N services provided by cover crop mixtures
It is clear from our findings that the N services of retention and provision are species and/or functional group dependent – canola and cereal rye (winter hardy non-legumes) lead to strong N retention in monoculture and when included in a mix; hairy vetch and red clover (winter hardy legumes) are strong N providers in monoculture and when they contribute at least 40% or more to mixture biomass; oats and forage radish (winter killed non-legumes) appear to contribute to both N retention and provision under certain conditions. Given that mixtures provide the opportunity to combine these species, it follows that mixtures will exhibit “multifunctionality”, the ability to provide not just one N service, but multiple N services. Using N leaching data as an indicator of N retention and corn yield as an indicator of N provision, our research shows trade-offs between N retention and N provision by monocultures (figures 7 A&B). Certain mixtures, however, were able to both retain and provide N. In year one, these mixtures were those that combined hairy vetch, red clover, canola, and cereal rye. In year two, the multifunctional mixtures were those that combined forage radish and oats with legumes (red clover and hairy vetch or sunn hemp and soybean).
2. Tonitto C, David MB, Drinkwater LE (2006) Replacing bare fallows with cover crops in fertilizer-intensive cropping systems: A meta-analysis of crop yield and N dynamics. Agriculture, Ecosystems & Environment 112:58–72.
17. Buyer JS, Teasdale JR, Roberts DP, Zasada IA, Maul JE (2010) Factors affecting soil microbial community structure in tomato cropping systems. Soil Biology and Biochemistry 42:831–841.
In light of the EPA goal to reduce nitrogen pollution in the Chesapeake Bay by 25% by 2025, reducing nitrate leaching from agricultural lands is a high priority for farmers and policymakers in the northeast. This project will provide critical information on the potential of cover crop mixtures to mitigate nitrate leaching and key aspects of mixture management that optimize this ecosystem service, including species selection criteria. For example, project results have been shared with the Chesapeake Bay Program’s Cover Crop Expert Review Panel as they update the nutrient and sediment reduction efficiencies that are assigned to cover crop practices in the Chesapeake Bay Model.
This project has also directly impacted efforts to promote cover cropping practices in Pennsylvania through a collaboration with the Northeast Sustainable Agriculture Research Education (NE-SARE) PA State Program. PA-SARE has identified cover crop mixtures as one of six cover crop innovations that will advance the adoption of cover cropping in PA. The results of my project have contributed to training programs for farmers, extension agents, and other agricultural professionals on the benefits and management of cover crop mixtures (see outreach/publications for further information).
In addition to these local impacts, this project will contribute to a growing body of biodiversity research exploring the links between diversity and ecosystem function. Our results demonstrated that individual ecosystem functions and services are species dependent and not necessarily optimized by increasing species richness. Diversity was necessary, however, to provide multiple ecosystem services. From our research it is clear that cover crop mixtures are promising strategy to improve N management in agricultural systems by simultaneously retaining and providing N.
Education & Outreach Activities and Participation Summary
This study has provided field demonstrations and results for several outreach events:
- Cover Crop Innovations Field Day supported by the Northeast SARE Pennsylvania State Program (October 2011). Field participants toured my project research site to observe cover crop mixtures and discuss the potential benefits of mixtures. More than fifty percent of participants (17 of 30) reported that their knowledge of cover crop mixtures had increased as a result of this field day. Seventeen of 30 participants also indicated that they were likely to adopt and/or recommend cover crop mixtures after the field day.
- Strategies for Soil Health and Nutrient Conservation Research Tour (June 2012). I co-led a hands-on workshop entitled “Cover Crop Mixtures for Corn Success” with PA-SARE coordinator Charlie White for this field day. Using data from two field experiments, including this NE-SARE project, we led participants through activities to assess N provision in corn and N retention under various cover crop regimes. Ninety-five percent of participants surveyed reported having some or a great deal of knowledge following the workshop, compared to 73% reporting to have this level of familiarity before attending.
- Penn State Extension Field Diagnostic Clinic (July 2013). Building on our 2012 workshop, Charlie White and I led more than 150 crop consultants, industry representatives, conservationists and educators through a series of hands-on activities to demonstrate N retention and N provision by cover crops and cover crop mixtures at this event. Our workshop was based at the field site for the second year of this experiment, which provided an outstanding visual of the effects of various cover crops on subsequent corn growth. All data presented at the workshop was derived from this NE-SARE funded project. Seventy-five percent of participants reported increased knowledge of the role of cover crops in supplying and retaining N, while 87% reported increased knowledge of the tools available to monitor nitrogen in agricultural fields. Fifty percent of participants reported that they were likely to use or to recommend the use of cover crops and cover crop mixtures to manage N after attending the workshop.
- Natural Resource Conservation Service Southwest Indiana Training (July 2013). I was invited to offer a presentation entitled “Cover Crops and Nitrogen Management” to 50 NRCS agents. The presentation included data derived from this research demonstrating the capacity of cover crop mixtures to balance N supply and retention.
Results of this research have also been distributed through a research brief (Appendix A) and an article in the Progressive Forage Grower magazine (Appenix B). Charlie White and I are preparing a manuscript for the Journal of Extension Education that outlines how we designed and implemented our workshop on nitrogen management with cover crops so that it can be replicated elsewhere. I am currently drafting two manuscripts for publication in peer-reviewed journals on this research project and presented the results from the first year of this study at the Annual Meeting of the Crop Science Society of America in November 2013. In January 2014 I received Penn State’s Intercollege Graduate Student Outreach Achievement Award for extension and outreach activities I carried out for this project.
Farmer adoption of cover crop mixtures was not explicitly tracked for this project. I have, however, had many opportunities to discuss this research with farmers through field days, site visits, and interactions with a farmer advisory panel for another cover crop mixture project at Penn State. Overall farmer response to this project has been positive, consistent with other surveys indicating that farmers are interested in cover crop mixtures, particularly for their perceived ability to provide multiple benefits to their cropping systems. Some farmers raised concerns about being able to replicate the system used in this experiment, as cover crops were tilled and planted earlier than many cropping systems allow. In this research, I worked to create conditions to maximize cover crop growth and performance, recognizing that this meant cover crops would follow a spring small grain, which is not typical of rotations in Pennsylvania, and would be tilled, which conventional no-till growers would not do. The feedback I received highlights the need to build on the “basic” research carried out in this project by applying the lessons learned in a variety of cropping contexts.
Areas needing additional study
The variability of mixture composition between years one and two observed in this study underscores the need for continued research on optimal planting rates for mixtures as well as adaptive practices for cover crop mixture management. This study was also limited in the number of species studied, and further research is needed to assess additional cover crops that may be suited to growth in mixture.