The influence of tree species composition on soil microstructure stability and its relation with erosion resistance

Vegetation restoration can improve soil structural stability and erosion resistance, However, the effects of forest composition on the rheology-based stability of soil microstructure and its relationship with erosion are still unclear. Therefore, this study investigated typical plantation soils on the Loess Plateau to explore differences in soil rheological properties and their stratification under different afforestation models. Our results show that: (1) Afforestation significantly altered soil physicochemical properties. Mixed forests of Robinia pseudoacacia (RP) and Platycladus orientalis (PO) (1:1) notably enhanced the accumulation of organic carbon in the surface layer (0–20 cm). (2) PO plantations promoted the retention of water-stable macro-aggregates (> 0.25 mm). Despite mechanical and water-induced disruption, these soils maintained a higher proportion of macro-aggregates, with significantly greater mean weight diameter and lower aggregate disruption rates (p < 0.05). (3) With increasing strain, soil structure progressively approached its shear strength limit until failure occurred. Mixed forest soils exhibited both a wider linear viscoelastic region (γ_LVR) and higher integral Z (Iz), suggesting an elastic and tough structure. In contrast, PO soils showed the highest γ_LVR but the lowest Iz, indicating structural rigidity with weak internal cohesion. (4) Modeled soil erodibility (K) was lowest in mixed forests and highest in PO soils. In fine-textured, low-organic-matter soils, high mean weight diameter may accompany high rigidity and brittleness, resulting in poor erosion resistance. Overall, high aggregate stability does not invariably indicate strong erosion resistance. Through rheological analysis can identify healthy soil structures that combine strength and resilience. This study elucidates the intrinsic relationships among soil rheological properties, aggregate stability, and soil erodibility, providing new insights into the soil conservation functions of forests from a mechanical perspective.