Soil surface electrochemical properties and aggregate stability under wind-water compound erosion

Soil aggregate is one of the most important clusters of colloidal particles after land erosion. Its critical physicochemical properties are defined by the specific surface area (S), surface charge density (σ₀), surface electric field intensity E₀), and surface potential (Φ). The physical, chemical, and biochemical processes can be regulated on the formation and stabilization of the soil aggregate in soil systems. This study aims to investigate the impact of wind-water compound erosion on the stability of the soil aggregates. The 2-5 mm aggregates of the loessial soil were collected from the Yunwushan Nature Reserve in Guyuan City, Ningxia Hui Autonomous Region, China. A series of simulations were conducted on the treatments of wind erosion (W), water erosion (R), and sequential compound erosion (wind followed by water: WR; water followed by wind: RW). A systematic measurement was also made on the surface electrochemical properties (S, σ₀, E₀, Φ) and stability indices of the soil aggregates under different erosion forces after erosion. Surface physicochemical characterization and wet-sieving method were finally integrated to clarify the properties of the soil particles and aggregate stability after the wind-water compound erosion. The results demonstrated that: 1) There were more pronounced variations in the surface electrochemical properties of the aggregates under the wind-water compound erosion, compared with the single-erosion treatments. Specifically, the compound erosion reduced the surface charge density (σ₀) and surface electric field intensity (E₀) by 6.25%-32.43% and 6.38%–31.65%, respectively, while the specific surface area (S) and surface potential (Φ) increased by 1.10%-16.36% and 0.63%–5.60%, respectively. The WR sequence (wind followed by water) exerted stronger effects on the electrochemical properties than the RW sequence (water followed by wind). Among them, the σ0 and E0 under WR were 16.67% and 14.70% lower than those under RW, respectively. 2) Wind-water compound erosion shared the greater destabilizing effect on the aggregates, compared with the single-erosion treatments. Particularly, the macroaggregate content, mean weight diameter (MWD), and geometric mean diameter (GMD) significantly decreased by 5.26%–10.89%, 3.76%-18.10%, and 6.25%-17.50%, respectively, under compound erosion. Furthermore, the MWD and GMD declined by 18.10% and 17.50%, respectively, in the WR sequence. Whereas the reductions were only 3.76% and 6.25%, respectively, in the RW sequence. As such, the aggregates under RW exhibited higher water stability than those under WR. 3) There was a similar relationship between electrochemical properties and aggregate stability under different erosion forces. But the magnitude of impacts after wind-water compound erosion exceeded those of the single-erosion forces. The destabilization was dominated by the direct effects of the compound erosion on the aggregate stability (-0.922 for E₀; -0.448 for S), compared with its indirect effects (0.1 and -0.429). There were much stronger mediating effects of the surface electric field intensity (E₀) under compound erosion (-0.464 to -0.468), compared with the single erosion (-0.050 to -0.423). The total effects of the specific surface area (S) on the aggregate stability (0.929 and 0.759) were greater under compound erosion than those under single erosion (0.129 and 0.769). Therefore, E₀ served as the key indicator to explain the impact of the wind-water compound erosion on aggregate stability, which also acted as the dominant indirect factor. While S was the primary direct factor in the stability under compound erosion. The E₀ attenuation and S expansion were synergistically combined to exacerbate the aggregate destabilization under wind-water compound erosion. The WR sequence induced more severe disruptions in both electrochemical properties and stability. The soil electrochemical parameters were regulated to enhance the erosion resistance. The content of the soil organic carbon increased after organic amendments in practice. Straw incorporation elevated the specific surface area and aggregation capacity of the soil particles, in order to stabilize their electrochemical characteristics. Such measures reduced the risk of aggregate fragmentation under compound erosion. A framework of “Electrochemical Modulation-Structural Reinforcement” can be offered to prevent wind-water alternating erosion in the Loess Plateau.