Nitrogen provisioned and recycled by cover crops in monoculture and mixture across two organic farms

Cover crops can supply and recycle soil nitrogen (N) in agroecosystems. With increasing interest in cover crop mixtures, farmers have questions about how N cycling is affected by individual species when planted alone or in mixtures. Field trials were conducted on two organic vegetable farms during the spring and summer of 2014–2016 to quantify biomass tissue N provisioned, soil nitrate (NO3-N) scavenged, and soil NO3-N mineralized from 12 species of cover crops (six spring and six summer species) planted in monoculture or five-way mixtures. All cover crops were compared to weedy and weeded (no cover crops or weeds) controls. Cover crops provisioned 28–33% more biomass tissue N than fallow weeds. Among spring cover crops, field pea produced the most biomass tissue N (131 ± 13 kg N ha−1; mean ± 1 SE) but that benefit disappeared when pea abundance was reduced in mixtures. Soil NO3-N to a depth of 0.2 m was reduced by 50–67% during cover crop and weed growth, regardless of treatment. Soil NO3-N availability approximately 1 month after incorporation of cover crop and weedy residues varied by treatment. Post-incorporation soil NO3-N was 17–19% greater in pea and soybean monocultures compared to mixtures; conversely, soil NO3-N was 17–19% lower following incorporation of buckwheat and sudangrass monocultures compared to mixtures. Results of this study demonstrate that cover crop N recycling services in near-surface soil are largely independent of species or mixture, but N provisioning services require careful management and species selection to ensure maximum biomass tissue N and timely N mineralization.