The 2025 goal of the Chesapeake Bay Program is to reduce agriculture’s nitrogen (N) loading by 20% from 2014 values. Cover cropping is an important best management practice for recycling and conserving N in cropping systems that might otherwise be lost to groundwater via leaching and runoff. The reduction of N leaching by winter cover crops depends largely on precipitation, timing of planting, and the selection of the appropriate crop species. We compared nitrate-nitrogen (NO₃^-–N) leaching losses among forage radish (Raphanus sativus L.), cereal rye (Secale cereal L.), a forage radish+cereal rye mixture, and no-cover control. Replicated field trials were conducted at the University of Maryland Central Maryland Research and Education Center over 2016–2018. We collected porewater from 60 cm below the ground surface using porous cup lysimeters following rainfall events and used NO₃^-–N concentrations paired with the HYDRUS 1-D soil moisture model to compare N leaching losses (in kg N ha⁻¹) among cover crop treatments. We show that mean soil porewater NO₃^-–N concentrations were higher (by 5x) in the no-cover control compared to rye and radish+rye treatments (P < 0.0001). Overall, N leaching losses (kg N ha⁻¹) were highest in the no-cover control plots (P < 0.0001), but the majority of N leaching losses in rye plots occurred during the fall while the majority of radish N leaching losses occurred during the winter and spring (after they winter-killed). The rye and radish+rye mixtures reduced N leaching losses by 80% in both years. Understanding how different cover crop species affect N leaching losses, can help us design cropping systems to minimize N losses to the Chesapeake Bay especially as climate change alters rainfall patterns across the region.

切萨皮克湾计划的 2025 年目标是将农业的氮 (N) 负荷比 2014 年的值减少 20%。覆盖种植是一种重要的最佳管理实践，可在种植系统中回收和保存氮，否则这些氮可能会通过浸出和径流流失到地下水中。冬季覆盖作物对 N 浸出的减少在很大程度上取决于降水、种植时间和适当作物种类的选择。我们比较了硝酸盐氮（NO ₃ ^- -N）浸出草料萝卜（间损失萝卜L.），黑麦谷物（黑麦谷物L.)，一种草料萝卜+谷物黑麦混合物，以及无覆盖控制。2016-2018 年在马里兰大学中央马里兰研究和教育中心进行了重复的田间试验。我们在降雨事件后使用多孔杯式蒸渗仪从地表以下 60 cm 处收集孔隙水，并使用 NO ₃ ^- –N 浓度与 HYDRUS 1-D 土壤水分模型配对，以比较覆盖层之间的N 浸出损失（以 kg N ha ^{-1 为单位}）作物处理。我们表明，与黑麦和萝卜 + 黑麦处理相比，无覆盖控制的平均土壤孔隙水 NO ₃ ^- -N 浓度更高（5 倍）（P < 0.0001）。总体而言，N 浸出损失 (kg N ha ^-1) 在无覆盖对照小区中最高 (P < 0.0001)，但黑麦小区的大部分 N 浸出损失发生在秋季，而大部分萝卜 N 浸出损失发生在冬季和春季（在它们越冬后）。在这两年中，黑麦和萝卜+黑麦混合物使氮浸出损失减少了 80%。了解不同的覆盖作物物种如何影响氮浸出损失，可以帮助我们设计种植系统，以最大限度地减少切萨皮克湾的氮损失，尤其是在气候变化改变整个地区的降雨模式时。