Study of Soil Moisture Dynamics in Relation to Microtopography in the Loess Region of Northern Shaanxi, China

Soil moisture is the primary limiting factor of vegetation restoration and rehabilitation in the loess region of northern Shaanxi Province, China. A 5-year study was conducted in a microtopographically diverse landscape of China's loess region on the dynamics (monthly variations, vertical distribution, spatial variability) of soil moisture content in representative microtopographical units (including undisturbed slope as a control) as part of a long-term observational experiment at the Wuqi Ecological Station of Beijing Forestry University. One goal was to improve the efficiency of the use of soil moisture through a reasonable spatial distribution of planted vegetation. According to the 5-year average monthly variations in soil moisture content, we divided soil moisture conditions in the study area into four stages: slow moisture loss (March to May), moisture depletion (June to August), slow recovery (September to October), and stabilization (November to February). From January to December, soil moisture content varied with microtopography as follows: gullies > gently sloped terraces > collapsed soils > undisturbed slopes (control) > furrows > scarps. In terms of vertical distribution, soil moisture content varied obviously and was stratified; it also increased with increasing soil depth. Specifically, soil moisture content was generally low at 0-20 cm deep, varied considerably at 20-100 cm deep and the variation of soil moisture increased significantly at 100-160 cm deep. Spatial variability of soil moisture content in relation to microtopography can be categorized as follows: scarps > undisturbed slopes > furrows > collapsed areas > gentle-sloped terraces > gullies. These results provide reference information for implementing microtopographical and tree-oriented near-natural afforestation and optimizing vegetation selection for diverse microtopographical units of the loess region of northern Shaanxi.