Episodic N 2 O emissions following tillage of a legume–grass cover crop mixture

Nitrogen (N) fertilizer inputs to agricultural soils are a leading cause of nitrous oxide (N 2 O) emissions. Legume cover crops are an alternative N source that can reduce agricultural N 2 O emissions compared to fertilizer N. However, our understanding of episodic N 2 O 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 N 2 O 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 N 2 O 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 N 2 O for approximately 2 weeks following tillage to capture the first peak in N 2 O emissions in each site. Across cover crop treatments, the higher-fertility site, CF, had greater cover crop biomass, 2-fold-higher aboveground biomass N, and higher cumulative N 2 O emissions than the lower-fertility site, KBS (413.4±67.5 vs. 230.8±42.5 g N 2 O-N ha −1 ; P =0.004). There was a significant treatment effect on daily emissions at both sites. At CF, N 2 O 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 N 2 O emissions compared to rye and fallow treatments. We found partial support for our hypothesis that N 2 O 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 N 2 O emissions during cover crop decomposition.