2. Methods
2.1. Experimental site and climatic conditions
Our study was conducted at Zhifanggou watershed of Ansai County (109°14′36″—109°16′03″E, 36°42′42″—36°46′28″N), in the central area of the Loess Hilly Region, in the Shaanxi Province, China, where the terrain was broken and the gully was vertical and horizontal. The experimental site was located in the Semi-arid monsoon climate zone at about 1200 m above sea level and had a slope of 25-35%. Average annual air temperature at the watershed is 8.8℃ with annual evaporation 1463 mm and total annual precipitation 549.1 mm, about 61.1% of which occurs from July to September. The region has an area of 8.27 km2 with thick soil layer, where the soil is mainly composed of loess soil developed on the loess parent material. Due to serious soil erosion, since the 1970s, large-scale ecological restoration projects based on vegetation restoration have been gradually implemented, and it has an important impact on soil carbon flux in the area. The common types of vegetation restoration in this area are: artificial forests dominated by black locust (Robinia pseudoacacia ); artificial shrubs dominated by caragana (Caragana korshinskii ) and sea buckthorn (Hippophae rhamnoides ), natural shrubs, and abandoned steppe.