The Residual Effect of Temporal Heat Exposure Increases N 2 O Emissions by Accelerating Organic Matter Decomposition in Black Soil

ABSTRACT Black soil (Mollisol), characterized by high soil organic matter (SOM), is becoming more vulnerable to extreme heat events, yet the effects of heat on decomposition of SOM, thus N 2 O emission remain unclear. Here, we investigated the residual effect of temporal heat exposure on subsequent N losses and N 2 O production pathways in a cultivated black soil in Northeast China. Soil samples were exposed to 25°C, 35°C, 45°C, and 50°C for 12 h, followed by a 7‐day recovery period, after which N 2 O “hot moment” events (i.e., rewetting with N addition and aerobic‐to‐anaerobic transition) were set up using a robotized continuous‐flow incubation system. We then performed high‐frequency automated monitoring of CO 2 , N 2 O, and N 2 fluxes, and analyzed N 2 O isotopocules, bacterial community composition, and N‐cycling functional genes to elucidate the underlying mechanisms. Compared with the 25°C control, heat‐exposed soils exhibited 28%–94% higher N 2 O emissions during the hot moment events. The 15 N site preference values of N 2 O indicated that the short‐term heat exposure prior to rewetting enhanced the contribution of denitrification‐derived N 2 O. The N 2 O/(N 2 O + N 2 ) ratio increased in the extreme heat treatments, indicating a shift toward less complete denitrification. Heating caused modest shifts in bacterial community composition and selectively altered N‐cycling functional gene abundances, including increased AOB amoA and narG abundance in the 50°C treatment. The higher mineral N content and the reduced total N content (by 7%–13%) in heat‐exposed soils after incubation suggest that extreme heat accelerated the decomposition of soil organic N, thereby triggering greater N 2 O emissions during subsequent hot moment events. The findings have important implications for predicting greenhouse gas feedbacks in black soils under intensifying climate extremes, particularly the potential amplification of soil N 2 O emissions.