2.2 Field sampling
All the three sampling sites were located on flat ground with representative native steppe communities. The sites were fenced to exclude animal grazing at the beginning of the plant growing season in 2016, and plant and soil samples were collected from these sites at the end of August. At each site, five quadrates of 1m ×1m were set up at the center and four corners of a delineated area of 20 m×20 m, and all standing live and dead vascular plants (that was obviously produced during the current season) in these quadrats were harvested at ground level species by species, dried to a constant weight at 65℃ and weighed. The dry mass of all plant species per quadrat averaged over five replicates was used to determine the aboveground plant biomass at peak plant biomass time, and this was also used to approximate ANPP of the grassland (Scurlock et al., 2002). The belowground biomass and its distribution profile (0-70 cm) were measured using the soil coring method, and BNPP (during the plant growing season from May to October) of the studied grassland was obtained from previous studies (Chai et al., 2014; Hou et al., 2014).
Whole plants of dominant species at each site were collected by digging up each individual to a depth of 20 cm below ground level, and then each individual was cut into two parts: aboveground part (shoots) and belowground part (noted as roots, but it includes plant roots and rhizomes as well as shoot stumps buried below the soil surface). A sample of about 300g dry matter of the shoots and the roots of each plant species was collected in each of the three plots at each site. The samples were washed with deionized water, dried at 65℃ and then cut into pieces (<5mm) for phytolith analysis.
The soil bulk density and moisture content were obtained by the cutting-ring method and the oven drying method (at 105℃). The soil samples were collected using soil cores (diameter =7cm) and air dried at ambient temperature in laboratory.