Balancing weeds, nitrogen, and soil health in cover crop mixtures

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Commodities

Practices

• Crop Production: cover crops, nutrient management
• Pest Management: weed ecology
• Soil Management: organic matter

Increasing functional diversity is essential to enhance ecosystem services and reduce agriculture’s environmental footprint while simultaneously meeting a growing demand for food. One way to achieve this is through the integration of cover crop mixtures into farming rotations. Cover crops provide a multitude of ecosystem services to cropping systems, including nitrogen retention and fixation, weed suppression, reduction in pest pressure, and increased profitability of cash crops. Here, we propose to evaluate how different cover crop treatments grown in mixtures and monocultures impact weed seed mortality, nitrogen fixation and uptake between species, and soil labile carbon for the following cash crop. These services are microbially mediated and likely enhanced by increasing species functional diversity. We will also investigate the relationship between species abundance (above and belowground) and ecosystem service functions to better inform grower mixture choice. Results from this study will be communicated to the grower community and agricultural professionals through an extension newsletter article, education modules, research field tours, and a Pennsylvania Sustainable Agriculture Conference presentation.

Objective 1: Evaluate how weed seed mortality differs among cover crop mixtures and monocultures.

Hypothesis 1: We expect that weed seed mortality will be greater in cover crop mixtures compared to monocultures. Presumably, the greater functional diversity and abundance of roots in mixtures will increase weed seed mortality by increasing microbial diversity and activity. Additionally, we expect that both legumes and brassicas will increase weed seed mortality relative to grasses; legumes increase N, and in turn microbial activity, and brassicas associate with saprophytic fungi which are primarily responsible for breaking down recalcitrate organic matter (such as weed seeds) in the soil.

Objective 2: Evaluate how biological nitrogen fixation (BNF) of legumes in mixtures compares to legume monocultures, and determine how much N from fixation is shared between species within mixtures.

Hypothesis 2: Relative to cover crop monocultures, legume species will have a greater percent of N derived from BNF when mixed with grass and/or brassica due to the drawdown in soil available N. Other species planted with legumes will also have higher N than those planted in monoculture or with other non-legumes due to acquisitioning BNF–derived N exuded from legume nodules.

Objective 3: Evaluate how labile carbon differs among cover crop mixtures and monocultures.

Hypothesis 3: Relative to cover crop monocultures, mixtures will promote higher labile carbon pools due to the greater diversity of cover crop residue inputs (varying in chemical composition) incorporated into the soil.

Objective 4: Evaluate whether cover crop mixtures can increase the diversity and breadth of the ecosystem services they provide (i.e. can mixtures increase multifunctionality).

Hypothesis 4: Relative to cover crop monocultures, cover crop mixtures will provide a greater suite of ecosystem services. We predict that the provisioning of any one service will be lower in mixtures compared to the best performing monoculture. However, when considering all of our measured services mixtures will exhibit greater performance.

Objective 5: Determine if there is a relationship between cover crop species abundance within mixtures and the provided ecosystem service(s) relative to monoculture treatments.

PI Rice has designed a molecular method to quantify relative root abundance of cover crop species in mixtures from field-collected soil cores. Prior to this method, it was impossible to determine the composition of cover crop species in the root biomass. This method is ready to be applied to the field to examine whether ecosystem services are linked to cover crop species composition and abundance.