Extending the Cover Crop Growing Season to Reduce Nitrogen Pollution

Abstract

Maryland currently has the highest rate of cover crop use in the United States. The Cover Crop Program, started as an initiative to clean nutrients from the Chesapeake Bay, has made it a common practice to plant a cereal cover crop after cash crop harvest in fall, and kill it several weeks before cash crop planting in spring. In Maryland, this practice does not allow enough growing time with warm conditions for optimal cover crop growth. Planting earlier in fall and killing a cover crop later in spring could improve soil N cycling. We hypothesized that interseeding into a cash crop in early fall, and delaying spring cover crop termination could increase cover crop biomass, carbon accumulation, and nitrogen uptake and decrease nitrate leached. We tested these hypotheses over four years with five field experiments, consistently using a brassica-legume-cereal cover crop mix. We evaluated the relationships between cover crop planting date and fall cover crop N uptake and reduction in nitrate leaching. In spring, we tested termination timing effects on cover biomass C and N, soil mineral N concentration, soil moisture, and corn yield. We tested multiple dates for broadcast interseeding cover crops into standing soybean cash crops. We partnered with farmers on Maryland’s Eastern Shore to test if our methods are feasible at a realistic scale. We measured nitrous oxide emissions to test if our recommended cover crop practice has the negative drawback of increasing emissions of nitrous oxide, a powerful greenhouse gas. The nitrate leached under late drilled and early interseeded methods were comparable under conditions which favored late drilling, but interseeding outperformed drilling when there was adequate rainfall for seed germination. The result was lower nitrate porewater concentrations under early planted cover crops. Nitrous oxide emissions increased slightly with cover crops relative to no cover crop, but the increase was negligible when compared to the nitrous oxide produced from applying N fertilizer. Our research showed that extending the cover crop growing season of a brassica-legume-cereal mix has multiple environmental benefits and few drawbacks.