Soil oxygen depletion and corresponding nitrous oxide production at hot moments in an agricultural soil

Hot moments of nitrous oxide (N2O) emissions induced by interactions between weather and management make a major contribution to annual N2O budgets in agricultural soils. The causes of N2O production during hot moments are not well understood under field conditions, but emerging evidence suggests that short-term fluctuations in soil oxygen (O2) concentration can be critically important. We conducted high time-resolution field observations of O2 and N2O concentrations during hot moments in an upland agricultural soil in Northern China. Three typical management and weather events, including irrigation (Irr.), fertilization coupled with irrigation (Fer.+Irr.) or with extremely high precipitation (Fer.+Pre.), were observed. Soil O2 and N2O concentrations were measured hourly for 24 h immediately following events and measured daily for at least one week before and after the events. Soil moisture, temperature, and mineral N were simultaneously measured. Soil O2 concentrations decreased rapidly within 4 h following irrigation in both the Irr. and Fer.+Irr. events. In the Fer.+Pre. event, soil O2 depletion did not occur immediately following fertilization but began following subsequent continuous rainfall. Peak soil N2O concentrations occurred when soil O2 concentrations declined to their lowest point in the 24 h period following each event. The soil O2 concentration dropped to as low as 0.2% (with the highest soil N2O concentration of up to 180 ppmv) following the Fer.+Pre. event, but only fell to 11.7% and 13.6% after the Fer.+Irr. and Irr. events, which were associated with soil N2O concentrations of 27 ppmv and 3 ppmv, respectively. During the hot moments of all three events, the soil N2O concentration was negatively correlated with soil O2 concentration (r = −0.5, P < 0.01), showing a quadratic increase as the soil O2 concentrations declined. Our results provide new understanding of the rapid short response of N2O production to O2 dynamics driven by changes in soil environmental factors during hot moments. This mechanistic understanding of the underlying processes could help improve soil management in order to avoid transitory O2 depletion and reduce the risk of N2O production.