Spatial and temporal evolution and factors influencing soil aggregate stability in the riparian zone during exposure: A case study of the water-level fluctuation zone of the Three Gorges Reservoir, China.

The water-level fluctuation zone (WLFZ) is a unique riparian region that forms in response to the operation of the reservoir. The periodic submersion-exposure conditions severely affect the properties of soil and vegetation, thus also change the spatial and temporal development of soil aggregate stability. Nonetheless, the changes in both space and time concerning soil aggregate stability and the factors influencing it during the exposure period in the WLFZ are still unclear. Consequently, the WLFZ within the Three Gorges Reservoir area (TGRA) was taken as the research object, soil and plant samples were obtained from various elevations along the slope at different times to capture the spatial and temporal changes in soil and plant characteristics throughout the exposure period. Moreover, the wet sieving method was utilized to measure the aggregate size distribution and stability of soil aggregates. To further analyze the effects of both individual and mutual variables on the spatial and temporal changes in aggregate stability, methods such as Pearson's correlation analysis, redundancy analysis (RDA), variation partitioning analysis (VPA), and partial least squares path modeling (PLS-PM) were employed. The findings revealed that aggregate stability increased with increasing elevation until peaking and then decreased spatially, whereas the passage of time contributed to enhanced aggregate stability. The spatial variation in aggregate stability was greater than temporal variation. RDA revealed that soil properties (bulk density, soil water content in mass, clay, silt, sand, pH and soil organic matter), environmental factors (elevation, water level, submersion duration and exposure duration) and plant characteristics (shoot and root biomass) contributed 53.6 %, 45.4 % and 0.9 %, respectively, to the spatiotemporal variability in aggregate stability. VPA and PLS-PM analysis further revealed that the main factors controlling the aggregate stability were soil chemical properties. These results provide a basis for further studies on the soil and water conservation in the WLFZ.