Nitrate Leaching Mitigation with Kura Clover and Rye Covers for Corn and Soybean in Irrigated Sands

Abstract: In addition to best nitrogen (N) management practices, integration of cover crops, such as winter rye (Secale cerale L.) and kura clover (Trifolium ambiguum), into annual row crops may be an effective mitigation strategy to reduce nitrate (NO₃^-- N) leaching from irrigated sandy soils. This study was conducted in the Central Sands region of Minnesota from 2016-2020. The objectives were to evaluate, at variable N rates, rye and kura’s ability to reduce NO₃^-- N leaching and determine the impacts of the covers on soil N availability, corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] N uptake, and grain yield in continuous corn (CC), corn-soybean (CSb) and soybean-corn (SbC) cropping systems. From 2017-2020, kura reduced NO₃^-- N leaching by 69% (42 kg NO₃^-- N ha^-1) compared to no cover crop but inter-crop competition resulted in reduction of 26% (2.3 Mg ha^-1) in corn and 21% (0.8 Mg ha^-1) in soybean grain yield. While inter-crop competition was successfully reduced with chemical suppression of kura, this also reduced the NO₃^-- N leaching benefit of kura. Inconsistent establishment and growth of rye resulted in variable results across years. Overall, however, rye had little effect on corn yield and reduced NO₃^-- N leaching compared to no cover by 11% (7 kg NO₃^-- N ha^-1) in CC and 26% (19 kg NO₃^-- N ha^-1) in CSb. In SbC, rye reduced yields 5% (0.2 Mg ha^-1) and increased leaching by 25% (15 kg NO₃^-- N ha^-1).  Regardless of cropping system or cover crop variables, applying N above optimum rates provides no agronomic benefit and increases risk of NO₃^-- N leaching. Restricting N applications below optimum rates provides little or no NO₃^-- N leaching  benefits and reduced grain yield. While best N management practices combined with cover crops can meet the need for grain production and minimize NO₃^-- N leaching in certain situations, minimizing inter-crop competition, and ensuring adequate annual rye establishment and N availability will prove essential to wide adoption of these alternative management systems.