Large-scale spatial variability of dried soil layers and related factors across the entire Loess Plateau of China

A dried soil layer (DSL) is generally formed in the soil profile at a particular depth owing to serious soil desiccation in water-limited ecosystems. The occurrence of DSLs potentially limits the development and sustainability of the ecological environment on the Loess Plateau of China. There is a need to address the spatial variations in DSL thickness (DSLT) and the DSL depth of formation (DSLFD) in this region. We pre-selected 382 sampling sites across the entire Loess Plateau region (620000km²) based on mapped information using an intensive sampling design. A total of 17906 disturbed soil samples from various soil depths were collected. Using classical statistics, principal component analysis, residual maximum likelihood (REML), and geostatistical methods, we investigated and characterized DSLs and their spatial distribution. There was strong spatial variation in DSLs, which had a mean thickness of 160cm occurring at a mean soil depth of 270cm. DSLT was highly correlated with a topographical factor (i.e., slope gradient) that dominated the first principal component (PC), which explained 39% of the variance. Geographical position (i.e., longitude) was important in the second PC. REML analysis demonstrated that land use, rainfall, soil type and slope gradient had a significant impact on DSLT, while only land use, rainfall, and soil type influenced DSLFD significantly.Semivariograms of DSLT indicated weak spatial dependence while DSLFD had moderate spatial dependence. The DSL was generally thicker (>170cm) in the western Loess Plateau region and in a central area (170 to 220. cm). Where irrigation was used along parts of the Yellow River and near rivers in the interior, DSLT was considerably thinner or non-existent due to the higher water inputs. Management practices utilizing knowledge of the spatial distribution of DSLs and the factors influencing them can improve land use management to improve the efficiency of vegetation restoration, water management, and to control or reclaim DSLs.