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 (620 000 km2) based on mapped information using an intensive sampling design. A total of 17 906 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 160 cm occurring at a mean soil depth of 270 cm. 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 (> 170 cm) 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.

Research Highlights
► Soil-water scarcity results from the excessive depletion of deep soil water by artificial/natural vegetation, strong evapotranspiration, and long-term insufficient rainwater supply, may lead to soil desiccation, and serious soil desiccation will gradually lead to the formation of a dried soil layer (DSL). DSL has great negative effect on ecological and hydrological process, e.g., impacts the water cycle in soil–plant–atmosphere systems by cutting off water interchange between upper soil and groundwater, leads to soil degradation, regional microclimate environment aridity, failure to afforestation and reduction of vegetation, and poor renewal by natural germination, and so on.
► Recently, many literatures have reported that DSL distributed widely in the Loess Plateau of China. However, very little work has been done on the spatial structures and patterns of DSL and its related factors on the entire Loess Plateau.
► Therefore, we pre-selected 382 sampling sites across the entire Loess Plateau region (620 000 km2) based on mapped information using an intensive sampling design. A total of 17 906 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.
► A comprehensive knowledge of DSL variability can help scientists and policy makers take effective measures to improve the efficiency of vegetation restoration, water management, and DSL control/restoration.