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.