Three Decades of Tillage Driven Topsoil Displacement and Soil Erosion Attenuation on Loess Plateau Slope Farmlands

The slope lands of the Loess Plateau represent a critical region impacted by soil erosion, which directly contributes to the globally recognized high sediment concentration in the Yellow River. However, the extent to which sloped farmland contributes to soil loss remains scientifically contentious. In this study, farmland with an initial slope gradient of 20° was selected for the experiment, and three decades of field monitoring data (1990s–2020s) and the Universal Soil Loss Equation (USLE) model were used for comparative calculation. The data indicated that the model-predicted soil loss rate in sloped farmland from the 1990s to the 2020s was calculated to be 62.48 t·ha−1·yr−1. Field-measured values averaged 45.67 t·ha−1·yr−1, whereas the current value is approximately 15.00 t·ha−1·yr−1. Anthropogenic disturbances, including tillage, manual weeding, and ovine grazing, mean that the topsoil of slope farmland has undergone cumulative displacement of 450~870 cm in 30 years, which is resulting in progressive slope gradient reduction from 20° to 5°. The soil erosion rates exhibited exponential decay characteristics, and finally gradually reached the level of flat farmland. When using the USLE model, the evolving slope gradient must be incorporated, rather than the slope angle extracted by DEM. Therefore, the key finding of this study is that the primary sources of soil loss in the Loess Plateau are non-agricultural slopes and gullies. Conversely, soil erosion on slope farmlands does not constitute a critical problem requiring urgent intervention. This finding should attract the attention of the local agricultural sector.