Managing Soil Nitrogen Surplus: The Role of Winter Cover Crops in N₂O Emissions and Carbon Sequestration

Abstract. Cover crops are acclaimed for enhancing the environmental sustainability of agricultural practices by aiding in carbon (C) sequestration and reducing losses of soil mineral nitrogen (SMN) after harvest. Yet, their influence on nitrous oxide (N₂O) emissions—a potent greenhouse gas—presents a complex challenge, with findings varying across different studies. This research aimed to elucidate the effects of various winter cover crops—winter rye (frost-tolerant), saia oat (frost-sensitive grass), and spring vetch (frost-sensitive legume)—against a control of bare fallow on SMN dynamics, N₂O emissions and C sequestration.

While cover crops efficiently lowered SMN levels during their growth, they also increased N₂O emissions in comparison to bare fallow conditions. Notably, winter frost events triggered significant emissions from the frost-sensitive varieties. Moreover, the practices of residue incorporation and soil cultivation were associated with increased N₂O emissions across all cover crop treatments. Winter rye, distinguished by its high biomass production and nitrogen (N) uptake, was linked to the highest cumulative N₂O emissions, highlighting the impact of biomass management and cultivation techniques on N cycling and N₂O emissions.

Cover crop treatment lead to a slight increase in direct N₂O emissions (4.5±3.0, 2.7±1.4, and 3.1±3.8 kg N₂O-N ha^-1 for rye, oat, and vetch, respectively) compared to the fallow (2.6±1.7 kg N₂O-N ha^-1) over the entire trial period (16 months). However, the potential of non-legume cover crops to reduce indirect N₂O emissions compared to fallow (0.3±0.4 and 0.2±0.1 kg N₂O-N ha^-1 a^-1 for rye and oat respectively) and their contribution to carbon sequestration (120–150 kg C ha^-1 a^-1 over a period of 50 years when growing cover crops every fourth year) might partially counterbalance these emissions. Thus, while cover crops offer environmental benefits, their net impact on N₂O emissions necessitates further exploration into optimized cover crop selection and management strategies tailored to specific site conditions to fully leverage their ecological advantages.