Sampling sites
We set up a north-east to south-west grassland transect across Inner Mongolia, Jiling, and Heilongjiang provinces in northern China (Fig. S1). The transect was approximately 2,200 km long and covered latitudes from 36.1 to 50.9° N and longitudes from 105.6 to 126.4° E. The climate was predominantly arid and semi-arid continental. Mean annual temperature along the transect ranged from -4.5 to 9.5°C, and mean annual precipitation ranged from 119 to 540 mm. The main vegetation types that were distributed from the west to east across this transect were desert steppe, typical steppe, and meadow steppe. Soil types were predominantly arid, sandy, brown loess rich in calcium, and belonged to Kastanozem soil group in the Food and Agriculture Organization (FAO) classification system.