Aggregate stability thresholds and sediment sorting governed by soil net force under different woodland restoration types

Different types of vegetation restoration, particularly forests, are well known to affect soil erosion resistance differently. However, the underlying mechanisms for these differences remain unclear. Most existing studies focus on macro-scale soil properties, overlooking the fundamental interparticle forces that govern soil structure. We propose that the net force between soil particles is this key mechanistic link. To test this, we compared soil properties and erosion processes across four woodland types (one broadleaf, three coniferous) after 23 years of restoration. Our results show that tree species alter soil organic matter (SOM) content and functional group composition (e.g., aromatic-C, polysaccharide-C). This enhancement, in turn, improves soil surface charge properties. Crucially, this process preferentially strengthens van der Waals attraction, which leads to a significant reduction in the net repulsive force. The reduction in net force showed a strong nonlinear correlation with improved aggregate stability. We identified a critical threshold of approximately 12 atm, which governs the transition of aggregates from gradual release to explosive breakdown. Consequently, soils with a lower net force (e.g., under Quercus wutaishanica forest) had significantly lower sediment concentration and a distinct sediment sorting pattern. This pattern showed reduced enrichment of fine particles and a shift from silt-dominated to a more balanced particle loss. This mechanistic understanding reveals how soil net force mediates the differential erosion outcomes, providing a scientific basis for tree species selection in restoration projects.