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.