In the U.S. Midwest, excess applications of nitrogen (N) fertilizers to grain fields have resulted in decades of nitrate (NO3-) leaching from farms, producing toxic algal blooms. Excess reactive N also leads to increased emissions of nitrous oxide (N2O), a potent greenhouse gas. Ecological nutrient management practices, such as planting legume cover crops during the winter season, can reduce synthetic fertilizer inputs by adding biologically fixed N to the system, improving nitrogen cycling efficiency and reducing losses to surrounding ecosystems. In this study, we asked, do cover crop mixtures of legumes and grasses alter litter chemistry (e.g. C:N) to affect hot moments of N2O flux following soil disturbance through tillage? Using a randomized, complete block design with four replicates and four treatments, we compared a legume monoculture (cowpea), grass monoculture (sorghum-sudangrass), and weedy fallow to a legume-grass mixture. We measured C and N in above- and below-ground biomass prior to cover crop termination to calculate litter C:N. Using the static chamber method, we measured net soil N2O flux five times over a week, once before and four times after tilling cover crops into the soil. At each sampling point, soil cores were taken to measure soil NO3- and ammonium concentrations.
Results/Conclusions
Mixing cowpea with sorghum-sudangrass significantly increased the C:N ratio of the above- and below-ground cover crop litter from 16.8 ± 1.1 to 27.9 ± 2.2 (p < 0.001). The C:N ratio for sorghum-sudangrass (26.7 ± 1.4) and the mixture were similar. The litter chemistry of the mixture was likely driven by the sorghum-sudangrass biomass, which was a strong competitor under drought conditions, contributing more biomass to the mixture than cowpea. Contrary to our hypothesis, C:N was not a significant predictor of net soil N2O flux, although higher C:N corresponded with lower N2O flux across sites. Soil NO3- concentrations were a significant predictor of N2O flux (p < 0.05). Following tillage, N2O flux peaked sooner and higher in cowpea than in sorghum-sudangrass or the mixture, indicating higher rates of denitrification and faster rates of N mineralization with a higher litter N concentration. Results of this study suggest that adjusting the quality of cover crop litter inputs with legume-grass mixtures may slow litter decomposition and N mineralization rates following soil disturbance through tillage. Ecological nutrient management using cover crop mixtures that combine distinct plant functional groups can be used to manage N cycling and potentially increase N use efficiency in agroecosystems.