Response of soil functions to erosion/deposition in sloping farmland within the Three Gorges Reservoir Area: A study using magnetic susceptibility tracing across the whole hillslope.

Evaluating soil function is crucial for mitigating soil degradation in sloping farmland, improving soil environment, and promoting sustainable agricultural development. However, a systematic understanding of the spatial differentiation mechanisms and main controlling factors of soil function in sloping farmland, particularly under whole hillslope erosion and deposition environments, remains insufficient. This restricts the scientific formulation of soil function improvement and sustainable utilization strategies for regional sloping farmland. Hence, we took the purple soil sloping farmland in the Three Gorges Reservoir Area as the research object, and used field in-situ experiments, magnetic susceptibility tracing, and modified tillage homogenization model methods to determine the distribution of erosion-deposition across the whole hillslope. Meanwhile, we elucidated the differential characteristics and influencing mechanisms of soil functions (primary productivity, PP; water purification and regulation, WPR; climate regulation and carbon sequestration, CRCS; nutrient cycling and provision, NCP; provision of habitat for biodiversity, PHB) on sloping farmland. The results showed that the boundary between the erosion and deposition zones of the sloping farmland was located at the mid-slope position of the whole hillslope (30-50m away from the uphill boundary). The area above the boundary was dominated by erosion, whereas the area below the boundary was dominated by deposition. The soil functions of PP, WPR, CRCS, NCP, and PHB in sloping farmland exhibited an overall increasing trend from the upper to lower slope positions, peaking in the mid-lower or lower slope positions, and were significantly higher than those in the upper and mid-upper slope positions (P < 0.05). Soil erosion was the dominant process that significantly decreased soil function (P < 0.05), mainly through the cascading effects of damaging soil structure, reducing organic matter content, decreasing nutrient availability, and inhibiting enzyme activity. The five functions in the erosion zone decreased by 43.38%, 42.77%, 50.94%, 39.63%, and 53.90%, respectively, compared with those in the deposition zone. Ten indicators, including cation exchange capacity, total nitrogen, soil organic matter, and microbial biomass carbon, were the dominant factors influencing changes in soil functions of sloping farmland, accounting for a cumulative contribution rate of 68.81%. These findings offer a theoretical basis for synergizing crop production with soil erosion control on sloping farmland, which is conducive to promoting the sustainable use of sloping farmland resources and improving the ecological environment.