Contribution of reforestation to soil aggregate stability and shear strength in hilly red soil region of southern China

In response to the effectiveness of reforestation in controlling soil erosion, there has been a dramatic increase in forest coverage in the hilly red soil region of southern China. Aggregate stability and soil shear strength are indicators that reflect soil resistance to erosion and its ability to prevent shallow landslides, respectively. However, limited research has focused on the response of soil aggregate stability and shear strength to reforestation. We selected three types of reforestations (Phyllostachys edulis forest, Cunninghamia lanceolata (Lamb.) Hook. forest, Citrus sinensis (L.) Osbeck. orchard), a natural forest (mixed coniferous and broadleaf forests), and a fallow land as study plots, and measured root traits, and soil physicochemical traits, i.e., pH, soil organic matter (SOC), Soil water content (SWC), soil bulk density (BD), soil cohesion (c), soil internal friction angle (φ) and analyzed their multiple interactions. The soil aggregate stability traits, refer to the mean weight diameter (MWD) and geometric mean diameter (GMD), exhibited a significant increase in reforested plots, approximately 200% compared to fallow land and 50% compared to natural forests. For soil shear strength the values were approximately 20% higher than in fallow land and approximately 10% lower than in natural forests. Soil aggregate stability and soil shear strength did not exhibit a significant positive correlation across all plots, and the underlying drivers of these traits were variable. For instance, in natural forest and timber stands, soil aggregate stability was mainly influenced by soil organic carbon, while soil shear strength was primarily affected by root length density. In economic forest, aggregate stability and shear strength are mainly affected by organic carbon. Overall, we found that vegetation restoration enhances soil erosion resistance, however, the primary drivers for the improvement of aggregate stability (soil organic carbon) and shear strength (root length density) are different. Therefore, in future benefit assessments of vegetation restoration projects aimed at soil erosion control, different indicators should be considered based on specific conditions.