Winter N2O accumulation and emission in sub-boreal grassland soil depend on clover proportion and soil pH

Abstract

Inclusion of legume species into grass leys reduces nitrogen (N) fertilizer need but increases the risk of freeze-thaw induced N2O emissions. We investigated how liming and presence of clover affect N2O accumulation under snowpack and its emission during freeze-thaw cycles in autumn and spring under sub-boreal conditions. A field experiment was performed in southern Norway in limed and control plots containing grasses only (fertilized with 270 kg N ha−1 yr−1), a grass-red clover mixture (fertilized with 140 kg N ha−1 yr−1) and unfertilized pure red clover. Soil air samples were collected at 8, 24, and 40 cm depths and analyzed for gas concentrations including N2O, and N2O fluxes measured by a fast-chamber robot. Red clover produced more N2O than the grass-only plots during freeze-thaw cycles in autumn and spring and accumulated more N2O under snow cover (emissions were not measured during this period). Contrary to expectations, limed red clover plots emitted more N2O than control plots during freeze-thaw cycles. Liming reduced subnivean N2O accumulation in grass-only but not in grass-clover or pure clover plots. After spring fertilization, grass-only plots had larger N2O emissions than red clover plots. Our data suggest that winter-sensitive, N-rich clover biomass fuels decomposition and nitrification, thereby increasing NO3 - and depleting O2, resulting in increased N2O emissions from denitrification. Although liming of pure clover leys exacerbated the risk of high N2O emissions during freeze-thaw, this effect was not observed in grass-clover mixtures. Interestingly, grass-clover mixtures also emitted less N2O than expected from their proportions and the emissions recorded in pure grass and clover stands. This warrants further studies into off-season functional diversity effects on N cycling and N2O loss in temperate and boreal forage production.