Nitrate losses and nitrous oxide emissions under contrasting tillage and cover crop management

Authors: O'Brien, Peter; Emmett, Bryan; Malone, Robert - Rob; NUNES, MARCIO - Orise Fellow; Kovar, John; KASPAR, THOMAS - Retired ARS Employee; MOORMAN, THOMAS - Retired ARS Employee; JAYNES, DANIEL - Retired ARS Employee; PARKIN, TIMOTHY - Retired ARS Employee

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/7/2022
Publication Date: 4/20/2022
Citation: O'Brien, P.L., Emmett, B.D., Malone, R.W., Nunes, M.R., Kovar, J.L., Kaspar, T.C., Moorman, T.B., Jaynes, D.B., Parkin, T.B. 2022. Nitrate losses and nitrous oxide emissions under contrasting tillage and cover crop management. Journal of Environmental Quality. 51:683-695. https://doi.org/10.1002/jeq2.20361.
DOI: https://doi.org/10.1002/jeq2.20361

Interpretive Summary: Soils in the central United States are highly productive, but if only corn and soybeans are grown there is a risk to environmental quality. Two tools that farmers can use to lower that risk are cover cropping and no-till systems. Cover cropping is growing plants when corn and soybean are not growing, and no-till is growing crops without any tillage. In this study, researchers in Iowa measured water quality, greenhouse gas emissions, and crop yields when growing corn and soybean with different cover crops and tillage systems. Growing a rye cover crop was determined to be the best way to maintain good water quality because it reduced unwanted nutrients in drainage water. However, neither cover crops nor tillage affected greenhouse gas emissions. No-till lowered corn yields in some years but not soybean yields, which may limit the number of farmers that adopt a no-till system. Overall, these results show that no single tool can reduce the environmental impacts of corn and soybean croplands, but rather they highlight the need to find the right combination of tools to maintain environmental quality and high crop yields. This information is important for scientists, crop advisors, conservation professionals, policy makers, and producers that are trying to find ways to have high crop production with low environmental impacts.

Technical Abstract: Agroecosystems in the upper Mississippi River Basin (UMRB) are highly productive but often contribute to deterioration of water and air quality. Cover cropping and no-till are conservation strategies implemented to reduce environmental impact of these agroecosystems. However, using multiple strategies can lead to systemwide interactions that are not fully understood. These interactions can affect not only environmental quality metrics, such as subsurface drainage nitrate losses or nitrous oxide (N2O) emissions, but also may be influence crop production potential. A field trial was initiated comparing nitrate losses, N2O emissions, and crop production under systems with fall tillage, fall tillage with an oat (Avena sativa L.) cover crop (CP-oat), no-till (NT), no-till with a rye (Secale cereale L.) cover crop (NT-rye) and no-till with zero N fertilizer (ZN). Pathways for nitrate losses and N2O emissions did not appear linked and were not tied to cover crop or tillage practices. Nitrate losses were linked with drainage volumes, and cover crops and tillage had limited effect on cumulative drainage volumes. Notably, NT-rye altered the relationship between drainage volume and nitrate losses by reducing nitrate concentrations, lowering nitrate losses by 59% (±9%) compared to CP-oat and 67% (±9%) compared to NT. Neither cover crop nor tillage consistently impacted N2O emissions or crop yield. Rather, N2O emissions were closely tied with fertilizer N application and seasonal weather patterns. These findings indicate that nitrate leaching and N2O emissions are regulated by separate mechanisms, so conservation management may require stacking multiple practices to be effective.