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.