2.1 Sampling sites
This study was conducted at three sites in the steppe region of central Inner Mongolia along a climatic gradient of increasing annual precipitation, that is, at a desert steppe site (within Sunite Right Banner, at 43°511’N, 113°42’E), a dry typical steppe site (within Maodeng farm of Xilinhot city, at 44°50’N, 116°36’E), and a wet typical steppe site (within West Ujimqin Banner, at 45°43’N, 118°30’E). The region experiences a temperate semiarid climate. The mean annual temperature (MAT) are -0.9, 2.9 and 4.9℃, and mean annual precipitation (MAP) are 182, 278 and 342 mm, respectively at the desert steppe, dry typical steppe and wet typical steppe sites; and 75%-85% of annual precipitation falls in the plant growing seasons from May to September (average of the 1960-2016 period). In the year for field study (2016), the annual precipitation was 189, 309 and 299 mm, and the plant growing-season precipitation was 129, 215 and 237 mm, respectively in the desert steppe, dry typical steppe and wet typical steppe sites. The desert steppe site is on a calcic brown soil, whereas the other two steppe sites are on chestnut soil. The humus layer is 15-30 cm in depth, and the calcic horizon (mostly CaCO3) is 30-60 cm in depth, both increasing from desert steppe to dry typical steppe and wet typical steppe. The dominant species of the vegetation are Stipa klemenzii, Cleistogenes songorica, Allium bidentatum and Salsola collina in the desert steppe, Leymus chinensis, Stipa kryloviiand Cleistogenes squarrosa in the dry typical steppe, andLeymus chinensis, Stipa grandis and Cleistogenes squarrosain the wet typical steppe (Table 2).