Abstract:

Objective: To quantitatively evaluate the variations in soil retention within the Chongqing section of the Three Gorges Reservoir area based on multi-source spatiotemporal data, this study analyzed the comprehensive driving mechanisms of various factors to reveal the impact mechanism of acid deposition on the numerical product of the cover-management factor (C) and the support practice factor (P) (hereafter referred to as the composite factor C×P, used to characterize the comprehensive surface anti-erosion capacity), aiming to provide a scientific basis for the prevention of soil erosion and the optimization of ecological barrier in the reservoir area. Method: Based on digital elevation model (DEM), land use, and acid deposition data from 2005 to 2020, the InVEST-SDR module was used to estimate soil retention. The geographic detector was applied to identify the dominant factors of soil retention and their interaction effects. Ordered Logistic regression and XGBoost-SHAP methods were used to quantitatively explain the linear and nonlinear impact mechanisms of acid deposition on the composite factor C×P. Result: 1) Total soil retention first increased and then decreased, rising from 6.74×10¹⁰ t in 2005 to a peak of 8.11×10¹⁰ t in 2015, and then falling to 7.87×10¹⁰ t in 2020. Spatially, it presented a pattern of “high in the east and low in the west, high in the northeast and low in the southwest”. High-value areas of soil retention per unit area (>3 000 t·hm^?2a^?1) were mostly located in steep slope zones with high canopy closure forests such as Wuxi and Yunyang, while low-value areas (<500 t·hm^?2a^?1) were located in densely constructed areas such as the main urban area of Chongqing and Fuling. 2) From 2005 to 2020, construction land expanded at an annual rate of 3.2% to 7.5%, cropland was mainly converted to forest land (decreasing by 3.9%), and open broadleaved forest land degraded significantly (decreasing by 90%). 3) Acid deposition showed significant spatiotemporal differentiation. The total sulfur deposition flux decreased from 41.41 kg·hm^?2a^?1 in 2005 to 30.23 kg·hm^?2a^?1 in 2020, while the total nitrogen deposition flux increased from 43.12 kg·hm^?2a^?1 in 2005 to 46.84 kg·hm^?2a^?1 in 2020. 4) The analysis results based on geodetectors indicated that land use was the dominant factor, and the interaction between acid deposition and land use had the strongest explanatory power (q=0.218 8). Logistic regression showed that the effect of nitrogen deposition on elevating the C×P level was approximately 8 times that of sulfur deposition. XGBoost-SHAP revealed that acid deposition exhibited nonlinear and spatially heterogeneous effects, with the main urban area and the southwest being hotspot areas. Conclusion: Acid deposition intensifies the spatial heterogeneity of soil retention in the reservoir area through the synergistic pathways of chemical erosion and biological inhibition with land use change. Ecological protection should prioritize reducing nitrogen deposition emissions, restoring sparse forest land, and improving forest land quality.

Key words: soil conservation, acid deposition, geodetector, Logistic regression, XGBoost-SHAP, Chongqing section of the Three Gorges Reservoir area