In the mid-Atlantic USA region, nitrogen uptake by crops ceases about four weeks prior to harvest maturity, leaving substantial mineral N in the soil profile, which is prone to leach during the winter. Deep-rooted cover crops planted by early-September can potentially take up residual N and recycle some of it for following cash crops. We performed experiments on 19 minimum-tillage, grain farms investigating four unfertilized cover crop systems (forage radish (Raphanus sativus L.), winter cereal or grass, forage radish + winter cereal + crimson clover (Trifolium incarnatum L.), and a no cover crop control). We measured cover crop biomass, N uptake, and inorganic N distribution within the upper 210 cm of soil in late-fall and early-spring, and the following corn (Zea mays L.) crop’s growth and yield. In late-fall, radish reduced soil NO₃ in the upper 90 cm by 66 %, while winter cereal or mix cover crops reduced NO₃ in the upper 60 cm by 67 % and 56 %, respectively, compared to a no cover crop control. In the spring, the radish and mix cover crops resulted in comparable nitrate levels to the no cover crop control in the topsoil layer (> 30 kg ha⁻¹) and less nitrate than the no cover crop control in subsoil layers. The winter cereal cover crop had low nitrate levels in the topsoil (∼20 kg ha⁻¹) layer and subsoil layers. The biomass and N content of corn seedling (5 leaf) were influenced by the previous cover crop treatment in the order radish (4.0 g biomass plant⁻¹) > mix (3.0 g biomass plant⁻¹) = control (3.5 g biomass plant⁻¹) > winter cereal (2.4 g biomass plant⁻¹). At the farmers’ standard N fertilizer application rate, corn yield following radish was higher than following the winter cereal or mixed species cover crop but corn yield following radish was not different than following no cover crop. Corn yield following the winter cereal cover crop was lower than following no cover crop. Cover crops can be fit within the framework of existing cropping systems to scavenge residual N, therein reducing subsoil inorganic N. Radish and mixed species cover crops can be used prior to corn without reducing the overall short-term N use efficiency of the cropping system.

在美国中部大西洋地区，农作物在收获成熟前约四周就停止吸收氮，从而在土壤剖面中留下大量的矿质氮，该氮素在冬季易于浸出。到9月初种植的根深蒂固的遮盖作物可能会吸收残留的氮，并将其中的一些氮再循环用于随后的经济作物。我们在19个最小耕作的谷物农场进行了试验，调查了四种未施肥的覆盖作物系统（萝卜萝卜（Raphanus sativus L.），冬季谷物或草，萝卜萝卜+冬季谷物+深红色三叶草（Trifolium incarnatum L.））和没有覆盖作物控制）。我们测量了晚秋和早春以及随后的玉米（玉米）的上部210 cm土壤中的覆盖作物生物量，氮吸收量和无机氮分布L.）作物的生长和单产。在后期下落，萝卜还原土壤NO ₃ 66％在上90厘米，而冬季谷物或混合覆盖作物NO减少₃在上部60厘米由分别为67％和56％，与无覆盖作物控制。在春季，萝卜和混合覆盖作物的硝酸盐水平与表土层（> 30 kg ha ^-1）的无覆盖作物控制水平相当，而硝酸盐水平低于地下土壤层的无覆盖作物控制水平。冬季谷物覆盖作物的表土层（〜20 kg ha ^-1）和下层土壤的硝酸盐含量较低。玉米幼苗（5片叶子）的生物量和氮含量受到萝卜覆盖（4.0 g生物量植物^-1）的覆盖的影响。）>混合（3.0 g生物质工厂^-1）=对照（3.5 g生物质工厂^-1）>冬季谷物（2.4 g生物质工厂^-1）。以农民标准的氮肥施用量计算，萝卜后的玉米单产高于冬季谷物或混合种覆盖作物，但萝卜后的玉米单产与不覆盖作物后的玉米无差异。冬季谷物覆盖作物之后的玉米单产低于无谷物作物之后的玉米单产。覆盖作物可以适合现有种植系统的框架，以清除残留的氮，从而减少土壤中的无机氮。萝卜和混合物种覆盖作物可以在玉米种植之前使用，而不会降低整个种植系统的短期氮素利用效率。