Episodic N2O emissions following tillage of a legume-grass cover crop mixture

Nitrogen (N) fertilizer inputs to agricultural soils are a leading cause of nitrous oxide (N2O) emissions. Legume cover crops are an alternative N source that can reduce agricultural N2O emissions compared to fertilizer N. However, our understanding of episodic N2O flux following cover crop incorporation by tillage is limited and has focused on single-species cover crops. Our study explores whether increasing cover crop functional diversity with a legume-grass mixture can reduce pulse emissions of N2O following tillage. In a field experiment, we planted crimson clover (Trifolium incarnatum L.), cereal rye (Secale cereal L.), a clover-rye mixture, and a no-cover control at two field sites with contrasting soil fertility properties in Michigan. We hypothesized that N2O flux following tillage of the cover crops would be lower in the mixture and rye compared to the clover treatment because rye litter can decrease N mineralization rates. We measured N2O for approximately 2 weeks following tillage to capture the first peak in N2O emissions in each site. Across cover crop treatments, the higher-fertility site, CF, had greater cover crop biomass, 2-foldhigher aboveground biomass N, and higher cumulative N2O emissions than the lower-fertility site, KBS (413.4 +/- 67.5 vs. 230.8 +/- 42.5 g N2O-N ha(-1); P = 0.004). There was a significant treatment effect on daily emissions at both sites. At CF, N2O fluxes were higher following clover than the control 6 d after tillage. At KBS, fluxes from the mixture were higher than rye 8 and 11 d after tillage. When controlling for soil fertility differences between sites, clover and mixture led to approximately 2-fold-higher N2O emissions compared to rye and fallow treatments. We found partial support for our hypothesis that N2O would be lower following incorporation of the mixture than clover. However, treatment patterns differed by site, suggesting that interactions between cover crop functional types and background soil fertility influence N2O emissions during cover crop decomposition.