Spatial distribution characteristics and influencing factors of shallow and deep soil moisture under ecological restoration in the loess plateau, China

Deep soil moisture is one of the primary factors restricting vegetation growth on the Loess Plateau (LP) of China. Investigating the spatial variations and influencing factors of shallow and deep soil moisture is of great significance for ecological restoration on the LP. In this study, we sampled disturbed soil up to a depth of 10 m with a soil drill. A 200 × 200 m grid was superimposed on a digital topographic map, and 78 sampling sites were selected across the Zhifanggou (ZFG) watershed. According to the results, the vertical profile of soil could be subdivided into four layers: the shallow active layer (SAL; 0–0.5 m), the shallow rainfall recharge layer (SRL; 0.5–2.2 m), the deep excessive layer (DEL; 2.2–4 m), and the deep stable layer (DSL; 4–10 m). The horizontal distribution characteristics of soil moisture in various soil layers were similar, with a generally high concentration in the south and a low concentration in the north. In addition, a relatively low soil moisture content (SMC) occurred in the center of the SRL. Soil moisture was negatively correlated with slope and slope aspect between 0 and 2.2 m ( p < 0.05), whereas slope and slope aspect had no significant effects on deep soil moisture (>2.2 m). The SMCs of the four soil layers, from lowest to highest, were as follows: forestland, shrubland, and grassland. The SMC of forestland was negatively correlated with restoration age, whereas the SMC of grassland was positively correlated with restoration age. This study deepens our understanding of shallow and deep soil moisture distribution characteristics and the corresponding ecological restoration on the LP and provides a reference for the rational use of deep soil moisture.