Fall cover crop nitrogen uptake drives reductions in winter-spring leaching.

Cover crops can reduce nitrate leaching after cash crop harvest. Despite widespread cover crop implementation there has been a limited impact on water quality in the Chesapeake Bay watershed. We hypothesize that typical timing for Maryland cover crop planting after cash crop harvest is too late to allow roots to take up substantial nitrate from the soil profile before it is leached by winter drainage water. Across four site-years (including sandy and silty soils) we compared various planting dates for a radish (Raphanus sativus, L)-crimson clover (Trifolium incarnatum, L.)-triticale (Triticosecale) cover crop mixture. Also, across two site-years we compared early-planted pure rye, radish, and a 3-species mixture to no cover. We measured cover crop biomass and N content and used tension lysimeters to measure deep soil porewater nitrate concentrations. Cumulative nitrate leaching was estimated from these concentrations and weather-based drainage estimates. Cover crops were planted on four dates over a six-week period. Overall, cover crops planted first, second, third, fourth, and no cover crop (just weeds) resulted in 3340, 3160, 1600, 303, and 164 kg ha-1 of biomass, biomass N accumulation of 65.5, 68.6, 44.0, 9.88, and 4.79 kg N ha-1 , and mean porewater concentrations of 2.71, 2.57, 4.72, 10.0, 17.1 mg L-1 of nitrate-N, respectively. Over two site-years, the three-species mix performed as well or better than pure rye or radish. Early planting altered cover crop species proportions, increased cover crop productivity and reduced nitrate leaching from agricultural fields. This article is protected by copyright. All rights reserved.