Unraveling runoff‐sediment and soil physicochemical responses via indoor rainfall simulation: Insights from water‐eroded Northeast China for cropland conversion forest

Rainfall‐induced soil erosion drives land degradation and nutrient loss in Northeast China. The Grain for Green Program, a large‐scale afforestation initiative in China, provides an important context for understanding erosion risks under land‐use conversion. In this study, controlled rainfall simulations were conducted on disturbed and repacked cropland and afforested soils in laboratory flumes at 0, 50, and 100 mm h − 1 to quantify runoff generation, sediment yield, and associated soil physicochemical changes. The results showed that under 100 mm h − 1 rainfall, sediment yield from repacked afforested soil was significantly greater than that from cropland soil under the same conditions, indicating differences in structural response. Sediments derived from cropland soil exhibited greater enrichment of fine particles and higher contents of carbon and calcium carbonate compared with forest soil. Increasing rainfall intensity markedly enhanced soil compaction and shear strength in both soil types, with a stronger response observed in forest soil. In cropland soil, rainfall intensification was associated with pronounced declines in clay content and soil nutrients, whereas forest soil showed comparatively limited changes. Exploratory multivariate analyses indicated that sediment yield in forest soil was primarily associated with rainfall intensity and soil structural properties, whereas sediment yield in cropland soil was more strongly linked to fine particle redistribution and soil compaction. Rainfall intensity was identified as the dominant factor controlling runoff generation in both soil types. These findings provide mechanistic insights into rainfall–soil interactions in disturbed cropland and afforested soils under controlled conditions and contribute soil‐process‐level understanding of land‐use transitions.