2.1 Study site and sampling
The study site is located in Dajigou Forest Park of Wuqi County (107°38′57″~108°32′49″E, 36°33′33″~37°24′27″N), Yan’an City, Shaanxi Province, China, and at the southern margin of Maowusu Desert, with little vegetation cover and few people. The elevation of Wuqi County ranges from 1350 m to 1525 m. From 1957 to 2014, the average precipitation was 469.68 mm, and 87% of this rainfall fell during 5–10 months of the year. The average annual temperature is 8.04 °C (Zhao et al., 2018). The vegetation types in the area are characterized by the transition from forest shrub steppe vegetation to steppe forest shrub steppe vegetation. The vegetation composition is dominated by the flora of North China. Since the project of returning farmland to forestry was implemented, there vegetation coverage has increased significantly. We selected different H. rhamnoides forests as the research objects. We selected typical sample plots were in stands and set up 20 m × 20 m arbor sample plots to observe and record the position (coordinate information, altitude) and site condition (such as slope and canopy closure) of the sample site in August 2017. Table 1 shows the specific situation. At each sampling site, we collected soil from three locations ca. 20m apart from one another by pooling five soil cores from 1m×1m area at each location. The surface soil samples were collected from each quadrat at random and mixed immediately to form a single soil sample. Visible matter, such as loose gravel and plant debris, was removed and divided into two parts. One part was passed through a 2 mm sieve into a 15 mL centrifuge tube, placed into a liquid N canister, transferred to the laboratory, and stored in a freezer at −80 °C for microbial analyses. The other part was subjected to air-drying treatment and used to determine soil nutrients.