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.