2.1 Sample collection
Soil samples were collected from Yudaokou prairie in Chengde city, Hebei province, China. The prairie has a total area of 1,000 km2, with an altitude of 1200–1800 meters. It is located in a region with a cold temperate continental monsoon climate with an average annual temperature of 3°C. Our sampling was carried out in August, and the daytime temperature was 15–30 degrees. Some areas displayed obvious degradation due to overgrazing. To ensure the accuracy and repeatability of the research results, two grasslands in which vegetation cover was reduced significantly to different degrees were employed (Figure S1) . Grassland 1 (G1) was dominated by a perennial herb species, Bothriochloa ischaemum (L.) Keng, whereas grassland 2 (G2) was dominated by an annual herb species,Polygonum aviculare L. Based on the different vegetation landforms, G1 (ca. 1400 m, 42°21’ N; 117°7’ E) was classified into six levels of degradation with each plot area being more than 200 m2), whereas G2 (~1200m, 42°7’ N; 116°57’ E) was classified into five levels of degradation (> 200 m2 each) (Figure S1) . Soil erosion and soil nutrient loss are major phenomena observed under grassland degradation (Dong, Wang, et al., 2021). Hence degradation levels were classified according to the levels of major soil nutrients (organic matter [OM], total N [TN], total phosphorus [TP]) in combination with physical appearance (Table S1) . A lower index value indicated higher soil nutrient contents and lower degradation. The deserted regions had the highest degradation levels (“6” at G1 and “5” at G2) because of desertification and low vegetation cover. Six soil samples were collected from the top 20-cm soil layer in each degraded grassland region using the checkerboard sampling method. Each soil sample was sieved through a 2-mm mesh to remove plant debris and stones, and was then stored at -80°C for DNA extraction and at 4°C for chemical analyses. In addition, soil bulk density was determined by in situ sampling using the volumetric ring method, with a volume of 100 cm3.