Evaluating ecosystem services of summer cover crop mixtures for organic cabbage production

Abstract Cover crops are an important management tool for organic vegetable farmers, but selection of individual cover crop species can lead to a tradeoff between agroecosystem services provisioned. In contrast, cover crop mixtures may provide multiple ecosystem services simultaneously, known as multifunctionality. This study evaluated the performance of browntop millet ( Urochloa ramosa ; BTM), buckwheat ( Fagopyrum esculentum ; BW), cowpea ( Vigna unguiculata ; CP), and sunnhemp ( Crotalaria juncea ; SH) in monocultures, two three-way mixtures (3-CP = browntop millet, buckwheat, and cowpea; 3-SH = browntop millet, buckwheat, and sunnhemp), and a four-way mixture containing all evaluated cover crop species (4-W). An autumn cabbage vegetable crop ( Brassica oleracea var. Caraflex) was grown following cover crop termination. To evaluate the cover crop treatments and explore the applicability of some tenets of biodiversity theory to cover crop mixtures, we monitored the ecosystem services of weed suppression, inorganic nitrogen provisioning, vegetable yield, and habitat for microorganisms. Overall, the cover crop mixtures evaluated were able to combine the benefits of the individual species in the mixture and provision the sought ecosystem services, although they did not exceed the performance of the best monocultures. Weed suppression was similar between mixtures and the top performing monoculture, BTM in 2022 and BW in 2023. The high productivity of browntop millet in mixtures, accounting for on average 74% of 3-CP biomass and 56% of 4-W biomass, when seeded at 20% and 25% its full rate, respectively, likely drove weed suppression in mixtures. In 2022, cabbage yield following 3-SH and 4-W was similar to the legume monocultures. Due to the suppression of cowpea in 3-CP from interspecific competition, 3-CP plots had a lower cabbage yield than the legume monocultures and were similar to BTM and Control. Soil microbial biomass, used to measure habitat for microorganisms, was 18% higher following mixtures compared to monocultures in the first year, although no differences were found in the second year. The seeding rate proportions used in the three-way mixtures, 60% of the legume full rate and 20% of the full rate of both browntop millet and buckwheat, achieved the target of a C:N ratio ≤30:1 and can be recommended when based on appropriate seeding rates for a given area. Tailoring future cover crop mixture research to questions of seeding rate thresholds and interspecific competition will improve complementarity and the provisioning of multiple ecosystem services in mixtures, offering valuable, practical guidance to growers.