In the U.S. Midwest, excess applications of nitrogen (N) fertilizers have resulted in decades of nitrate (NO3-) leaching and nitrous oxide (N2O) emissions from grain farms. Agroecological nutrient management practices, such as planting cover crops during the winter season, can reduce N losses, while maintaining other important ecosystem functions like yield. We designed a field experiment to ask: How does cover crop litter chemistry (e.g. C:N) affect soil N mineralization rates following tillage, N2O flux, and subsequent corn N assimilation? We used a randomized, complete block design with four replicates to compare a legume monoculture (crimson clover), a grass monoculture (cereal rye), and a grass-legume cover crop mixture, to a weedy fallow control. Grain corn was planted following the cover crop treatments. We measured aboveground biomass C and N prior to cover crop termination to calculate litter C:N. Following cover crop termination, we used the static chamber method to measure net soil N2O flux six times over three weeks, and we incubated soil cores in the field in polyethylene bags every 28 days for three months to measure N mineralization rates. Prior to harvest, we measured N in corn biomass to calculate corn N assimilation.
Results/Conclusions
Aboveground biomass C:N was significantly lower in the rye-clover mixture (21.7 ± 1.8) than the rye monoculture (34.7 ± 1.6; p < 0.001). Cumulative N2O losses were lower in the mixture (105.8 ± 12.2 g N2O-N/ha) compared to clover alone (182.7 ± 52.8 g N2O-N/ha; p = 0.0553). After incorporating the cover crops into soil, N2O peaked sooner and higher after clover than after the mixture or rye. Timing of N mineralization also varied by treatment. Three weeks after tillage, N mineralization rates were highest after clover (1.17 ± 0.15 mg N/g soil/day), followed by the mixture (0.92 ± 0.16 mg N/g soil/day). N mineralization rates were lowest after rye (0.83 ± 0.08 mg N/g soil/day), peaking seven weeks after incorporation. Resulting corn N assimilation was significantly greater following the mixture (120.5 ± 38.3 kg N/ha) and clover (116.0 ± 22.8 kg N/ha) treatments, compared to the rye monoculture (43.6 ± 10.1 kg N/ha; p = 0.016). The grass-legume cover crop mixture reduced N losses compared to clover, while increasing corn N assimilation compared to rye, providing multiple ecosystem functions with fewer tradeoffs. Mixtures may reduce fertilizer inputs by providing a N source that better synchronizes N supply with demand.