Extreme Rainfall Amplified the Stimulatory Effects of Soil Carbon Availability on N2O Emissions

Ongoing climate change is predicted to increase the frequency and intensity of extreme rainfall, which will dramatically alter soil nitrous oxide (N₂O) emissions, especially changes in soil organic carbon (SOC) due to anthropogenic management. However, our ability to predict this effect is limited owing to a dearth of research. Therefore, we selected two croplands in Northeast China with the same quantity but contrasting availability of SOC to explore the in situ dynamics of N₂O fluxes and N-cycling microbes through 2-year field experiment and N₂O production pathways by laboratory ¹⁵N-tracing experiment. In a normal rainfall year, the croplands with high (HCA) and low (LCA) SOC availability emitted 0.66 and 0.25 kg N₂O-N ha^-1 without N-fertilization and 2.03 and 1.51 kg N₂O-N ha^-1 with N-fertilization, respectively. In a record-breaking wet year, multiple heavy rainfall events caused water supersaturation in the low-lying HCA cropland over 2 months. Consequently, the background N₂O emissions increased by 508% compared with the normal rainfall year, and the N-induced N₂O emission factor increased from 0.77% to 2.24%. Soil dissolved organic carbon (DOC) was identified as the primary driver of larger N₂O fluxes from HCA cropland which facilitated denitrification by fueling nirS- and nirK-denitrifiers metabolism. Furthermore, a greater N substrate supply via a faster mineralization-nitrification coupling process promoted the contribution of autotrophic nitrification to N₂O in HCA cropland. The N₂O pulses from HCA soils during the waterlogging period were derived from stimulated denitrification, which dominated N₂O production (> 90%). Simultaneously, C availability enhanced and nitrate was produced via archaeal nitrification, leading to an increased nirS/nosZII ratio that fostered N₂O production through incomplete denitrification. Overall, our findings highlight the importance of avoiding the amendment of exogenous organic materials with high C lability, particularly under climate extremes, to eliminate the potential positive feedback of SOC management on climate change by inducing N₂O emissions.