Vertical variations of soil water and its controlling factors based on the structural equation model in a semi-arid grassland.

Soil water content (SWC) of a vertical profile plays an important role in the soil-plant-atmosphere continuum system through eco-hydrological process, which was controlled by multiple factors. Previous studies ignored soil water from a systematic perspective because of the lack of suitable methods to deal with interrelated factors. We developed a meta-model based on structural equation model (SEM) to identify the factors contributing to soil water, and the interactions among these factors, in a semi-arid grassland system. The model was based on the hypothesis that soil water is affected by hydrological variables (precipitation: P, evapotranspiration: E and underground water: GW), vegetation (vegetation coverage: VC and above ground biomass: AGB), and soil parameters (soil organic matter: SOM and bulk density: BD). E and AGB decrease soil water content, while VC and SOM help to retain soil water content. The proportion of explained variation in soil water increased with depth due to increasing stability. The most important contributors were AGB (r∂ = −0.15) and VC (r∂ = 0.39), and their contributions were opposite because their mechanisms differed. The accumulation of AGB in the growth season consumed soil water through root uptake. The contribution of AGB increased with depth, inferring that grassland species are xerophytes with deep roots to access soil water during drought. Coverage positively contributed to soil water, but its influence decreased with depth because its main effects (intercepting rainfall and providing shade) were at the surface. This systematic perspective of how hydrological, vegetation, and soil properties affect soil water will be useful to guide the management of semi-arid grasslands.