Enhancing Cover Crop Nitrogen Fixation, Weed Suppression, and Soil Health through Fall Amendments in Organic Vegetable Systems

Commodities

Practices

Background soil fertility is a major determinant of cereal-legume cover crop outcomes and services provided across farms. Combining cover crops with fall amendments can help create growing conditions that promote the desired benefits of cover crops and reduce nitrogen losses, but more research is needed to fully understand the tradeoffs of these interactions. "Enhancing Cover Crop Nitrogen Fixation, Weed Suppression, and Soil Health through Fall Amendments in Organic Vegetable Systems," will explore cereal, legume, and mixture responses to fall amendments that directly alter nitrogen availability and how these practices can then better supply nutrients to a cash crop. Manure, for example, can improve biomass on non-legume cover crops, but may also allow them to dominate mixtures when grown with legumes. Similarly, reduced nitrogen can improve legume nitrogen fixation at the expense of cereal and weed growth. This experiment will increase our understanding of cover crop competitive dynamics while allowing us to explore a novel strategy in cover crops: "reverse fertilization" through high organic matter, low nitrogen amendments. Using amendments to temporarily immobilize soil nitrogen within the biomass of the microbial community has been applied for altering plant community composition in restoration ecology and to manage weed growth during cash crop production. While it has not been investigated in cover crops, this strategy may pose fewer risks to cash crop yields and has the potential to improve legume performance through increased nitrogen fixation rates and weed suppression.

At the MSU Horticulture Teaching and Research Center in Holt, MI, vetch, rye, and vetch/rye mixtures will be planted in the fall into soils amended with: manure (supplying 60 lbs/acre of Plant Available Nitrogen), wheat straw, manure + wheat straw at a 20:1 C:N ratio, or bare soil. The resulting gradient of nitrogen availability/immobilization will allow us to evaluate changes in:

1. cereal-legume cover crop performance via biomass production, nutrient content, mixture proportions, and land equivalency ratios
2. legume biological nitrogen fixation via N15 natural abundance and root nodulation, and
3. nitrogen supply to a cash crop via inorganic N release patterns, potentially mineralizable N, and sweet corn yield and quality.

Additionally, rate response treatments using a high-nitrogen organic fertilizer will be included to evaluate the fertilizer nitrogen equivalence rate of cover crop + amendment treatments and to compare the profitability of various fertility management strategies. This work will be shared through Extension publications, class tours, field days, presentations at grower conferences, on-farm demonstrations, and academic conferences.

This systems-level experiment will increase our understanding of how fall amendments impact nutrient cycling, biological nitrogen fixation, weed suppression, and nitrogen crediting to a high-demand cash crop. This will help growers optimize benefits from both practices.

Learning Outcomes

1. We will better understand the impacts of directly manipulating soil nitrogen levels on cover crop monoculture performance and competition in mixtures.
2. Our understanding of how stacking organic amendments and cover crops can improve soil health and supply nitrogen to a subsequent cash crop will increase.
3. Carryover effects from fall weed suppression and reduced nutrient availability on spring legume nitrogen fixation will be better understood.
4. Growers and researchers will better understand how novel soil health and remote-sensing tools can help both manage and predict nitrogen availability in-season.

Action Outcomes

1. More growers will adopt cover crops as part of their nutrient management plan and be able to maximize their benefits through strategic use of fall-amendments.
2. High organic matter amendments can be an additional non-herbicide management tool in growers' toolkits.

Ecological/ Production Outcomes

1. Large amounts of nitrogen can be supplied while reducing erosion and phosphorus overapplication, which supports both yield and waterways.
2. Synchronizing nitrogen release with crop uptake promotes greater profitability by reducing yield and nutrient losses.
3. Increased cover crop biomass helps build soil organic matter.