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.