White Clover does not Increase Soil N2O Emissions Compared to Ryegrass in Non-Frozen Winter, but Increases CH4 Uptake

As one of the most important forage species in Europe, white clover ( Trifolium repens ) is a legume that is well recognized for its potential to increase productivity especially under reduced N input. It is hypothesized that legumes have the potential to decrease overwinter soil greenhouse gas (GHG) emissions due to more efficient N recycling as compared to non-legume forbs. We conducted a field experiment recording high-resolution soil nitrous oxide (N 2 O) and methane (CH 4 ) fluxes during the winter months (December 2019 to March 2020) on a five-year-old grassland in central Germany with white clover, fertilized and unfertilized perennial ryegrass ( Lolium perenne ), and bare soil. White clover and fertilized ryegrass stimulated soil N 2 O emissions by 174% and 212% as compared to bare soil, and by 36% and 56% as compared to unfertilized ryegrass, respectively, due to their greater N availability and higher water-filled pore space (WFPS). The estimated cumulative CH 4 fluxes under white clover were a net CH 4 sink, whereas ryegrass and bare soil were net CH 4 sources. Soil N 2 O fluxes were predominantly regulated by both mineral N and WFPS, while CH 4 fluxes were mainly explained by WFPS. N-fertilization during the growing season did not affect off-season N 2 O and CH 4 fluxes in perennial ryegrass plots. The combined non-CO 2 global warming potential highlighted the possible mitigation effect of white clover on overwinter GHG emissions. Our findings suggest that GHG emissions from legumes are not offsetting their productive benefits during the non-frozen winter seasons.