Effect of man-made forest on soil moisture of different slopes in upper and lower reaches of small watershed of Loess Plateau

Abstract: Soil moisture is a key limiting factor of agricultural production and ecological restoration on the Loess Plateau. The comparative study on the soil moisture under the influence of different ecological managements at a small watershed scale as the basic unit of ecological restoration is of great significance to evaluate ecological effects and predict soil water accurately. Most importantly, it is essential for ecological planning and layout. In order to compare the soil water content (SWC) in the artificial forest watershed and natural restoration watershed and determine the relative deficit situation of the former, soil samples within the 120 cm profile in the 2 basins named Yangjiagou (Y) and Dongzhuanggou (D) were simultaneously taken to measure SWC of 9 layers every 4 days in August 2016. Both Y and D were inside the Nanxiaohegou watershed located in the Loess Gully Area of eastern Gansu Province and the former is of Robinia pseudopodia plantation, whereas the latter was of natural restoration. Of the 2 basins, the basic conditions such as zone, slope aspect and slope position of sampling slopes and sampling points were well contrasted. The SWC of different zones, slope aspects, slope positions and soil layers in each basin was analyzed by classical statistical methods. And a new indicator, difference ratio of soil moisture (DRSM) between the 2 different basins was used to quantitatively compare the SWC in Y and D. The results showed that: 1) In D, the SWC in the upper reaches was higher than that in the lower reaches, the SWC in the eastern slope was larger than that in the western slope, its ranking in the different slope positions from highest to lowest was toe-slope (16.96%) > waist-slope (14.34%) > shoulder-slope (11.80%), and there was significant difference between the different zones and slope aspects (P<0.05). The SWC continued to increase in the 20-120 cm soil layer after a brief decrease in the 0-20 cm soil layer down the profile. 2) In Y, as the plantation consumed soil water and regulated evaporation, the SWC decreased significantly and its distribution pattern was changed compared with that in D. Specifically, the soils in the lower reaches and eastern slope were more humid than that in the upper reaches and western slope, the SWC of the toe-slope was significantly higher than that of the waist-slope and shoulder-slope (P<0.05), and down the soil profile, it decreased continuously first in the 0-100 cm layer and then increased in the 100-120 cm layer. 3) The spatial variability followed the order of D (29.19%) > Y (23.51%), while the order of time variability was Y (10.99%) > D (8.76%). 4) The upper reaches, eastern slope, waist-slope and 60-120 cm soil layers were the main areas or layers with soil water deficit in Y, and the DRSM between Y and D was bigger. All the above results revealed spatial heterogeneity in soil desiccation caused by the Grain for Green at the small watershed scale and are expected to serve as the basis for the allocation and layout of vegetation in the ecological construction on the Loess Plateau.