Long-term conservation practices reduce nitrate leaching while maintaining yields in tile-drained Midwestern soils

Authors: Rogovska, Natalia; O'Brien, Peter; Malone, Robert - Rob; Emmett, Bryan; Kovar, John; JAYNES, DAN - Retired ARS Employee; KASPAR, THOMAS - Retired ARS Employee; MOORMAN, THOMAS - Retired ARS Employee; KYVERYGA, PETER - Iowa State University

Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/14/2023
Publication Date: 8/15/2023
Citation: Rogovska, N.P., O'Brien, P.L., Malone, R.W., Emmett, B.D., Kovar, J.L., Jaynes, D., Kaspar, T., Moorman, T., Kyveryga, P. 2023. Long-term conservation practices reduce nitrate leaching while maintaining yields in tile-drained Midwestern soils. Agricultural Water Management. 288. Article e108481. https://doi.org/10.1016/j.agwat.2023.108481.
DOI: https://doi.org/10.1016/j.agwat.2023.108481

Interpretive Summary: Excessive nitrate in surface waters is a key factor causing degradation of water quality locally and a major cause of the hypoxic zone in the northern Gulf of Mexico. A study was initiated to quantify the long-term effectiveness of conservation practices on nitrate-N removal rates from agricultural tile drain. Corn and soybeans were grown with three different treatments: 1) Control: no-till crop production, 2) no-till with rye cover crop (RC), and 3) no-till with in-situ woodchip denitrification wall/ bioreactor (DW) where trenches were excavated parallel to the tile and filled with woodchips to serve as additional carbon sources to increase denitrification. Over a period of 19 years (2002-2020), the rye cover crop and in-situ bioreactor were effective in reducing N leaching by 59% and 58% compared to the Control, respectively. The effectiveness of both conservation practices remained constant over duration of the study and was affected by annual rainfall; effectiveness of rye cover increased in dry years, while effectiveness of bioreactor increased in wet years. Although both treatments were effective in reducing nitrates in drainage, N loss was also affected by rainfall. The unpredictability of rainfall may make it difficult to reduce nitrate concentrations at a consistent rate in surface water, but it does not diminish the effectiveness of both practices. These long term observations also suggest minimal or no yield penalty for adoption of cereal rye cover crop and/or woodchip bioreactor conservation practices, which is important for wider acceptance of conservation practices by the production agriculture community.

Technical Abstract: Nitrate losses from artificially drained agricultural fields lead to acceleration of eutrophication and hypoxia in aquatic ecosystems. Adoption of conservation practices, such as cover crops and woodchip bioreactors has been shown to significantly reduce nitrate loss and improve water quality. However, the long-term performance of these conservation practices and their effect on water quality has not been sufficiently quantified. A replicated plot experiment was initiated to quantify the long-term effectiveness of such conservation practices on nitrate-N removal rates from subsurface tile drain. Corn and soybeans were grown with three different treatments: 1) Control: no-till crop production, 2) no-till with rye cover crop (RC), and 3) no-till with in-situ woodchip denitrification wall (DW) where trenches were excavated parallel to the tile on both sides and filled with woodchips to serve as additional carbon sources to increase denitrification. Over a period of 19 years (2002-2020), all three treatments received the same total annual N fertilization in corn years with rates ranging from 168 to 247 kg N/ha, depending on the production year and management. Averaged across the 19 years, the RC and DW treatments were effective in reducing N leaching by 59 and 58% compared to the Control, respectively. Both conservation practices were effective over the duration of the study and both were affected by annual rainfall. Effectiveness of RC increased in dry years, while effectiveness of DW increased in wet years. Overall, treatment and annual precipitation had greatest effects on annual N loss in drainage. This suggests that the unpredictability of precipitation may make it difficult to reduce nitrate concentrations at a consistent rate in surface water, but it does not diminish the effectiveness of both practices. Minimal or no yield penalty was observed following adoption of rye cover crop and woodchip bioreactor conservation practices, which is important for wider acceptance of conservation practices by the production agriculture community.