Mainstream Planting Systems Influence Spatiotemporal Variations in the Soil Quality of Watershed Sloping Farmland

Mainstream planting systems of watershed sloping farmland span the globe and are closely related to the variations in the soil quality of watershed sloping farmland. However, little information is available about how mainstream planting systems influence spatiotemporal variations in the soil quality of watershed sloping farmland. The soil of 0–20 cm was collected at fixed points in three mainstream planting systems (a low-altitude citrus orchard system, a mid-altitude double-cropping system, and a high-altitude single-cropping system) at a fixed time each year for 15 years in a typical agricultural watershed of the Three Gorges Reservoir area of China. Fourteen physicochemical properties of the sampled soil were measured. We found that (1) the soil quality indexes of the citrus orchard system, double-cropping system, and single-cropping system decreased from 0.75, 0.71, and 0.67 in 2004 to 0.68, 0.57, and 0.55 in 2019, respectively; (2) the order of the six master control factors influencing soil quality was sand content > bulk density > total nitrogen > clay content > pH > total phosphorus in the citrus orchard system, sand content > bulk density > clay content > pH > total phosphorus > total nitrogen in the double-cropping system, and sand content > clay content > total phosphorus > pH > bulk density > total nitrogen in the single-cropping system; and (3) the total effects of soil erosion and fertilization on soil quality were −0.496 and −0.308 in the citrus orchard system, −1.254 and 0.371 in the double-cropping system, and −0.844 and 0.013 in the single-cropping system, respectively. We suggest that the three mainstream planting systems influence soil quality through variations in their master control factors caused primarily by soil erosion and secondarily by fertilization. These findings are important for controlling soil degradation through controlling soil erosion and rational fertilization in watersheds.