Does the Capacity for Nitrogen Fixation in Cowpea Increase When Planted in Diverse Cover Crop Mixtures?.

Legume cover crops are often used to build soil N in agroecosystems and there is increasing interest in cover crop mixtures to maximize ecosystem services. The objective of this study was to determine the effect of cover crop community diversity and soil fertility on the potential for N₂-fixation in cowpea (Vigna unguiculata L. Walp.) Two consecutive greenhouse trials were conducted in 2013 and 2014. Treatment levels included cover crop community (cowpea monoculture and two possible two- and four-species mixtures) and soil fertility level (high vs. low). Cover crops were grown for 42 to 53 days, aboveground biomass was harvested, and nitrogenase activity was estimated with the acetylene reduction assay. Roots were then excavated to determine nodule and root biomass. Nitrogenase activity and nodule biomass were greatest in the cowpea monoculture and reduced by 71-98% in the four-species mixtures. Reductions in N₂-fixation were driven in part by lower cowpea biomass in the mixtures. The ratio of root nodule:shoot biomass increased by 81-297% in low- relative to high-fertility soils, which contributed to increased nitrogenase activity. Despite the potential benefits of cover crop mixtures, results suggest that a cowpea monoculture planted in low fertility soils has the greatest potential for N₂-fixation. Field studies were initiated in 2014 to determine the capacity for N₂-fixation in cowpea and five other legume cover crop species planted alone and in diverse mixture combinations.