Elucidation of the dominant factors influencing N 2 O emission in water-level fluctuation zones in a karst canyon reservoir, southwest China

The water-level fluctuations zones (WLFZs) are crucial transitional interfaces within river-reservoir systems, serving as hotspots for N 2 O emission. However, the comprehension of response patterns and mechanisms governing N 2 O emission under hydrological fluctuation remains limited, especially in karstic canyon reservoirs, which introduces signiflcant uncertainty to N 2 O flux assessments. Soil samples were collected from the WLFZs of the Hongjiadu (HJD) Reservoir along the water flow direction from transition zone (T1 and T2) to lacustrine zone (T3, T4 and T5) at three elevations for each site. These soil columns were used to conduct simulation experiments under various water-fllled pore space gradients (WFPSs) to investigate the potential N 2 O flux pattern and elucidate the underlying mechanism. Our results showed that nutrient distribution and N 2 O flux pattern differed signiflcantly between two zones, with the highest N 2 O fluxes in the transition zone sites and lacustrine zone sites were found at 75 % and 95 % WFPS, respectively. Soil nutrient loss in lower elevation areas is influenced by prolonged impoundment durations. The higher N 2 O fluxes in the lacustrine zone can be attributed to increased nutrient levels resulting from anthropogenic activities. Furthermore, correlation analysis revealed that soil bulk density signiflcantly impacted N 2 O fluxes across all sites, while NO 3 and SOC facilitated N 2 O emissions in T1 -T2 and T4 -T5, respectively. It was evident that N 2 O production primarily contributed to nitriflcation in the transition zone and was constrained by the mineralization process, whereas denitriflcation dominated in the lacustrine zone. Notably, the annual N 2 O efflux from WLFZs accounted for 27 % of that from the water-air interface in HJD Reservoir, indicating a considerably lower contribution than anticipated. Nevertheless, this study highlights the significance of WLFZs as a vital potential source of N 2 O emission, particularly under the influence of anthropogenic activities and high WFPS gradient.