Interseeded cover crop impacts on ecosystem carbon and water fluxes in no-till, rainfed corn in the Northern Plains

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Nicanor Z. Saliendra, Mark A. Liebig and David W. Archer, USDA ARS Northern Great Plains Research Laboratory, Mandan, ND

Background/Question/Methods
Cover crops can minimize erosion, boost soil health, increase biodiversity, and aid in controlling weeds, pests and diseases, but their effects on net carbon and water balance in semiarid regions are largely unknown. As participants in the Long-Term Agroecosystem Research (LTAR) – Croplands Common Experiment, we compared business-as-usual (BAU, corn, Zea mays L.) and aspirational (ASP, corn with cover crop) practices in two fields (>20 ha) near Mandan, North Dakota, USA. In 2020, we quantified carbon and water fluxes in both fields by using eddy covariance methods in conjunction with remote sensing through satellite (Landsat 8) and drone (multi-spectral camera). On four dates during the growing season when Landsat 8 images were available, we calculated the simple ratio, Blue:Near Infra-Red (B:NIR), of surface reflectance to quantify the relative greenness and temperature of the vegetation. We also measured a suite of agronomic and environmental attributes during the growing season and calendar year.
Results/Conclusions
For several weeks after the peak growth of corn (Julian day 211−246), ecosystem carbon uptake (a negative value of NEE, net ecosystem exchange for CO2), gross ecosystem production (GEP), and evapotranspiration (ET) were higher in the ASP than the BAU field. During this similar period, weekly ecosystem respiration (ER) was similar between fields. Annual sum of NEE (g C m-2 yr-1 ± standard error) indicated a CO2-sink for ASP (−71 ± 42) whereas a CO2-source for BAU (23 ± 38). Additionally, annual sum of ET (mm yr-1 ± standard error) was greater in ASP (522 ± 36) than BAU (416 ± 31). In a period near peak growth of corn (Julian day 209−241), variations in NEE and ET were strongly associated to changes in B:NIR ratio with R2 values of 0.92 and 0.94 for ASP, and 0.89 and 0.99 for BAU, respectively. However, above-ground biomass (AGB, 1115 ± 23 vs. 963 ± 37 g m-2) and grain yield of corn (524 ± 13 vs. 359 ± 17 g m-2) were higher in BAU than ASP, respectively. The AGB of cover crop was not sufficient to balance the difference in AGB between ASP and BAU practices, indicating that greater C-uptake in ASP than BAU could be attributed to higher below-ground C storage due to the presence of a cover crop in the ASP field.