Impact of Cover Crop Species Diversity on Soil Nutrient Availability and Crop Productivity

Cover crops provide multiple environmental benefits that improve both soil and water quality; however, farmers only utilize them on approximately 5% of harvested U.S. cropland. Low adoption rates are attributed to yield impact concerns, seed and planting costs, and lack of advocacy. This study, which began in October 2019, assessed the effects of nitrogen rate and cover crop diversity on weed biomass, soil coverage, in-situ residue decomposition, soil nitrogen and phosphorus availability, and cash crop yield to better understand the costs and benefits of cover crop adoption at two locations in Tennessee (Milan and Spring Hill). Treatments were replicated 4 times in a split-plot design and consisted of four nitrogen rates (0, 112, 164, 224 kg N ha^-1) and five cover crop treatments (fallow, winter wheat, crimson clover, wheat-clover bi-culture, and a five-species “soil health mix” that consisted of wheat, oats, clover, hairy vetch, daikon radish). Results indicated that increased cover crop diversity provided better weed control and soil coverage as well as providing more consistent biomass with less annual variability. Diverse mixtures improved residue decomposition, which controlled overall nutrient release, and provided a source of supplemental nitrogen comparable to the clover monoculture. Of all cover strategies, none provided enough nitrogen or phosphorus to justify reducing fertilizer application, and there were significant, negative impacts on yield under nitrogen-limited conditions to impact profitability. Cover cropping alone did not increase profitability alone; however, participation in cost-share programs and additional revenue such as forage sales have the potential to improve financial sustainability. Regardless of cover crop selected, sufficient biomass production was the most influential factor in cover crop success and was most impacted by planting and termination date; however, increased diversity did somewhat alleviate these constraints.