Investigation of Factors Controlling the Regional-Scale Distribution of Dried Soil Layers Under Forestland on the Loess Plateau, China

The effects of drought on plants have been extensively documented in water-limited systems. However, its effects on soil are seldom considered because of the lack of comparative data on profile soil water content (SWC). A dried soil layer (DSL) within the soil profile is a typical indication of soil drought caused by climate change and/or ill-advised human practices. The regional spatial variability, dominant factors, and predictive models of DSL under forestland were explored in the present study. SWC at 0-600 cm of 125 pre-selected sites across the entire Loess Plateau was measured, and then two evaluation indices of DSL (the thickness of DSL, DSLT; SWC within the DSL, DSL-SWC) were calculated. The corresponding soil, topography, plant, and meteorology factors (a total of 28 variables) for each site were also measured. Most of the forestlands across the Plateau had DSL formation within the soil profile (102 of 125 study sites). The DSL levels were considered to be serious, with DSLT generally exceeding 300 cm with a mean DSL-SWC of only 7.9% (field capacity (FC) = 18.1%). DSLT and DSL-SWC indicated a moderate and strong spatial dependence with ranges of 69 and 513 km, respectively. Thicker DSLs were mainly distributed in the center of the Plateau, whereas thinner DSLs were observed in the southern and southeastern parts. In contrast, DSL-SWC distributions demonstrated an obvious decreasing trend from the southeast to the northwest. Dominant factors affecting DSLT under forestlands were FC, bulk density, slope gradient, slope aspect, and capillary water content; while dominant factors for DSL-SWC were FC, aridity, sand content, altitude, vegetation coverage, and evaporation. Moreover, predictive models developed by multiple regressions were relatively accurate when predicting DSLs, especially DSL-SWC. Understanding these associations with DSLs formation in forestland is helpful for efficient water resource management, silviculture, and eco-environment restoration on the Loess Plateau and in other water-limited regions around the world.