In situ N-15-N2O site preference and O-2 concentration dynamics disclose the complexity of N2O production processes in agricultural soil

Arable soil continues to be the dominant anthropogenic source of nitrous oxide (N2O) emissions owing to application of nitrogen (N) fertilizers and manures across the world. Using laboratory and in situ studies to elucidate the key factors controlling soil N2O emissions remains challenging due to the potential importance of multiple complex processes. We examined soil surface N2O fluxes in an arable soil, combined with in situ high-frequency measurements of soil matrix oxygen (O-2) and N2O concentrations, in situ N-15 labeling, and N2O N-15 site preference (SP). The in situ O-2 concentration and further microcosm visualized spatiotemporal distribution of O-2 both suggested that O-2 dynamics were the proximal determining factor to matrix N2O concentration and fluxes due to quick O-2 depletion after N fertilization. Further SP analysis and in situ N-15 labeling experiment revealed that the main source for N2O emissions was bacterial denitrification during the hot-wet summer with lower soil O-2 concentration, while nitrification or fungal denitrification contributed about 50.0% to total emissions during the cold-dry winter with higher soil O-2 concentration. The robust positive correlation between O-2 concentration and SP values underpinned that the O-2 dynamics were the key factor to differentiate the composite processes of N2O production in in situ structured soil. Our findings deciphered the complexity of N2O production processes in real field conditions, and suggest that O-2 dynamics rather than stimulation of functional gene abundances play a key role in controlling soil N2O production processes in undisturbed structure soils. Our results help to develop targeted N2O mitigation measures and to improve process models for constraining global N2O budget.