Effects of Biological Crust Development on Soil Saturated Hydraulic Conductivity in the Three Gorges Reservoir Area

[Objective] Soil saturated hydraulic conductivity (Ks) reflects soil infiltration capacity and erosion resistance potential, which significantly affects hydrology and erosion processes. Economic fruit forest is an important land use type in the Three Gorges Reservoir area, biological crust wide-developed under the forest, which will inevitably drive the change in soil physical and chemical properties, thereby affecting Ks, but the effect of biological crust development on soil saturated hydraulic conductivity and its driving mechanism in reservoir area is still unclear. [Methods] Therefore, in order to investigate the effect of biological crust development on Ks in the Three Gorges Reservoir area. One site without biological crust (as control) and five moss crust sites with different coverage (0～20%, 20%～40%, 40%～60%, 60%～80%, 80%～100%) were selected under the economic fruit forest. The soil saturated hydraulic conductivity and basic soil physical and chemical properties of biological crust with different coverage gradients were determined by field sampling and laboratory analysis, combined with nonlinear regression, partial least squares regression and structural equation model methods. The changes in Ks with biological crust coverage and main factors attributed to these changes were clarified. [Results] (1) The development of biological crust significantly increased Ks, and compared to control, Ks of economic fruit forest covered by 0～20%, 20%～40%, 40%～60%, 60%～80%, 80%～100% biological crust increased by 36.98%, 338.09%, 407.17%, 900.66%, 713.11%, respectively. (2) Ks increased in an approximate logarithmic trend with the increase of biological crust coverage. When biological crust coverage was greater than 60%, Ks tended to be stable. (3) Ks was significantly correlated with the biological crust characteristics and the differences in soil physical and chemical properties driven by biological crust. The thickness of biological crust, soil bulk density, total porosity, saturated water content and water-stable aggregate content were the main influencing factors leading to the differences in Ks with different biological crust coverages. (4) The result of structural equation model showed that the development of biological crust was mainly through its indirect effect in improving soil structure (indirect path coefficient 1.655 bigger than direct path coefficient 0.887) to increase Ks. [Conclusion] These results provide a theoretical basis for understanding the hydrologic and erosion processes of biological crust development in the Three Gorges Reservoir area, and also provide practical guidance for the prevention and controlling of soil and water loss in the economic fruit forest land.