Abstract: To assess the potential of soil greenhouse gas emissions in citrus orchards of the Three Gorges Reservoir Region and to inform management optimization, we examined fertilized non-rhizosphere topsoil (0-10 cm) under four treatments: clean tillage (CK) and cover cropping with smooth vetch (Vicia villosa) established for 4, 7, and 11 years. Gas fluxes from soils were determined by the field in-situ measurement method, and soil physicochemical properties and microbial characteristics were analyzed to explore the driving mechanism of smooth vetch cover duration on nitrous oxide (N₂O) emissions. Smooth vetch cover significantly increased N₂O emission by 42.7%, and enhanced soil capillary porosity, β-glucosidase activity, and CO₂ emission. The N₂O emission presented a dynamic trend of rising first and then declining with increasing cover duration. Cover duration significantly affected soil pH and the mean weight diameter of water-stable aggregates (W_MWD). N₂O flux was negatively correlated with NH₃ and CH₄ fluxes but positively correlated with CO₂ flux (P<0.05). The primary drivers of N₂O flux were soil moisture, W_MWD, and N-acetyl-glucosaminidase activity. Partial least squares path modeling (PLS-PM) indicated that the duration of smooth vetch cover modulated N₂O emission by influencing soil moisture, enhancing aggregate stability, and optimizing the microbial threshold elemental carbon-to-phosphorus ratio (TER C∶P ratio). The synergistic effect of core driving factors explained 95.8% of the variation in N₂O emission. Overall, extending the duration of smooth vetch cover can effectively suppress N₂O emission by strengthening soil aggregate stability and optimizing TER C∶P ratio, providing a scientific basis for N₂O mitigation in citrus orchards of the Three Gorges Reservoir Region.

Key words: Three Gorges Reservoir Region, duration gradient, N₂O emission, smooth vetch cover, element threshold ratio