Legume cover crop impacts on soil nitrogen cycling and soil fertility on organic vegetable farms

Collaborators: Daniel Brainard, Michigan State University; Sieglinde Snapp, Michigan State University

Funding: The Ceres Trust Organic Research Initiative

In this project, we investigated how cropping system diversification through legume cover crops affects nitrogen (N) fixation, soil organic matter (SOM) pools, and the sustainability of organic vegetable farms. We partnered with 10 small-scale organic vegetable farmers to conduct on-farm research testing different legume and grass species grown in monocultures and mixtures for biomass production, N inputs from legume N fixation using stable isotope methods, and impacts on soil nutrient cycling and SOM pools. This research addressed key knowledge gaps in ecological nutrient management on organic farms.

We found that legume N sources increased soil health, especially by increasing N storage in labile SOM fractions, such as particulate organic matter. We also showed that cover crop mixtures with greater functional diversity can increase multifunctionality, or the provisioning of multiple ecosystem services at once. Mixtures with legumes maintained levels of nutrient retention and weed suppression comparable to the most commonly planted cover crop in the Midwest (cereal rye), while also providing N through biological N fixation. This work has also shown how cover crop root functional traits vary with intercropping, and with soil properties, across a soil organic matter gradient. Our results identified niche partitioning between cereal rye and hairy vetch, demonstrating that acquisitive root traits (e.g., root length) increased for hairy vetch grown in mixture, while root diameter, a more conservative trait, increased in monoculture. Finally, through this project, we tested how plant functional diversity affects decomposition dynamics with a year-long soil incubation experiment on two soils with contrasting management legacies.