1.2.2 Sample collection
The sampling time of this study was from July to August 2019, when the plants were in full bloom. The central area of the typical distribution area of the above 3 types of grassland (TS, TSD and TD) were selected for the sampling site. We established 6 random sampling quadrates (1 m × 1 m) in each site. In each quadrate, plant species, coverage, height, density of the respective species and aboveground biomass were measured and recorded. Aboveground parts of the green plants of the respective species were harvested by clipping to the soil surface, and collecting all litter in each quadrate. All the aboveground plant samples were placed into envelopes and then tagged, respectively. All the green plant samples were immediately dried at 105℃ for 30 min, then oven-dried at 60℃for 48 h and weighed. Meanwhile, a 60-meter sample line was set for each sample site and the sample spots were set at a 20-meter interval. Four soil samples were taken around each sample spot using soil drill at the soil layers with a depth of 0-10 cm, 10-20 cm and 20-30 cm respectively, and mixed four soil samples from each layer as one sample respectively. The samples were put into a sample bag and taken back to indoor for air-drying, which were measured for soil organic matter, Total N, Total P, pH and soil enzyme activity.