Cover Crops to Enhance Nitrogen Management in Organic Cropping Systems.

Cereal-legume cover crop blends have the potential to supply all or a significant portion of N to crops grown in rotation. Despite their benefits, cover crops often require adjustments to the timing of field operations. The rate of release of N from cover crops is also uncertain due to variations in climate, residue quality and nutrient content. We conducted field trials from 2005 to 2010 to evaluate the effect of rye-hairy vetch blends and planting and harvest date on cover crop establishment, biomass yield and N accumulation. An additional 160 d laboratory incubation was completed to assess N release patterns from cover crop residues. Five rye-hairy vetch blends were seeded on optimal and late planting dates in the fall and harvested for biomass yield on early and late planting dates in the spring and analyzed for C and N concentration by dry combustion. Cover crop residue and soil was collected from the field for the incubation. Inorganic N (NO₃^- + NH₄⁺)-N was extracted with KCl during the incubation and analyzed on an autoanalyzer. We found that planting early and harvesting late yielded 2 to 4 times more biomass than late planting and early harvest. Plant N was about 10 g kg^-1 greater at early harvest and 7 g kg^-1 greater for rye-hairy vetch blends than pure rye. Nitrogen release patterns were affected by the proportion of rye and were well correlated with cover crop N concentration and C:N ratio. At day 70, pure rye, rye-hairy vetch blends, and pure hairy vetch had released 36, 49-56, and 62% of total N that was applied to incubation vessels, respectively. Our results indicate that rye-hairy vetch blends planted by early October will provide significant biomass and N benefits to farmers.