Scavenging and recycling deep soil nitrogen using cover crops on mid-Atlantic, USA farms

Abstract

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.