Nitrogen rate, cover crops, and living mulches: Their impact on irrigated corn and soybean production and the environment

Minnesota has over 600,000 acres of irrigated sands that are predominantly under corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] cultivation. While highly productive and important to Minnesota’s economy, these soils are also vulnerable to nitrate-N leaching and groundwater contamination. As indicated by the recent groundwater protection rule, groundwater contamination is an important issue in Minnesota as many cities and households depend on this resource for their drinking water supply. This proposal will expand on the work that the PI in partnership with The Minnesota Department of Agriculture (MDA) and the Pope County Soil Water and Conservation District (SWCD) have been conducting since 2017 with Minnesota Clean Water, Land and Legacy Amendment funds. 

The study site is in an irrigated coarse-textured soil that typifies over 200,000 acres of similar soils within an adjacent six-county region as well as 400,000 acres with similar challenges due to the need of supplemental irrigation, high nitrate-N leaching potential, and groundwater contamination that are common to outwash sands of west-central and central Minnesota. During an earlier phase of this project, we established a winter rye (Secale cereal L.) cover crop, kura clover (Trifolium ambiguum M. Bieb.) perennial living mulch (a cover that grows between the crop rows), and no cover crop arrangement within three long-term (since 2011) continuous corn (CC), corn-soybean (CSb), and soybean-corn (SbC) cropping systems.

The objectives of the original study were to evaluate under these cropping systems and cover crop arrangements the effect of nitrogen (N) rate on grain yield and nitrate-N leaching concentration and load. We observed that while cover crops (especially kura clover) can reduce nitrate-N leaching, corn required additional N to optimize grain yield and soybean yield was reduced when grown with kura clover. Although we found meaningfully reduction of nitrate-N loss via leaching, which directly impacts groundwater contamination, the economic forces that drive adoption of cover crops can be a large obstacle if their use results in additional inputs (N and water) and reduced yields. For these reasons, using the current established cropping system with rye cover crop, kura clover, and no cover crop, we propose three main goals.

  • First, develop innovative strategies in nutrient and crop management to maintain—and hopefully increase— the gains in nitrate-N reduction we have obtained and improve grain yields and N use efficiency.
  • Second, quantify key N cycle processes (gaseous N losses, immobilization, and mineralization) that directly affect nitrate-N leaching and N bioavailability and crop grain yield.
  • Third, compute water depletion and needs for supplemental irrigation to improve water resource use and minimize nitrate-N loss via leaching.