Transient anoxic conditions boost N2O emissions by stimulating denitrification capacity and decreasing N2O reduction ratio in soils with different carbon substrates

Quantitative identification of the dominant N2O production and consumption processes following sudden shifts in soil aeration in well-aerated soils is limited, lagging the development of N2O mitigation strategies. Nitrous oxide emissions from agricultural soils, the largest source of anthropogenic N2O, are strongly influenced by lowmolecular-weight organic carbons from root exudates (RootEx). In this study, we conducted a laboratory incubation experiment to investigate the mechanisms of N2O emissions regulated by transient anoxic conditions under the effects of different carbon substrates. Three common components in RootEx, oxalic acid, serine, and glucose were individually added as carbon sources every 24 h to the sandy loam soil. The contribution of microbial processes to N2O production (fungal denitrification, nitrification, bacterial denitrification) and consumption (N2O reduction) was evaluated using a dual isotope mapping approach (SP/delta 18O MAP). The result indicated that transient anoxic conditions stimulated N2O production primarily through bacterial denitrification process facilitated by carbon substrates, and diminished the proportion of N2O reduction compared to oxic conditions. Among the three components of RootEx, serine and glucose induced higher N2O emissions and N2O reduction rates than oxalic acid during incubation. The differential effects of these three components on N2O emissions imply that the regulatory effect of carbon substrates on N2O reduction could not counterbalance the influence of transient anoxic conditions on N2O stimulation. Our findings provide a fundamental understanding of the effects of transient anoxic conditions and RootEx on N2O stimulation, facilitating the study of the spatial and temporal heterogeneity of N2O emissions in response to soil aeration changes in various upland soils.