2.1 Study site
This field study was carried out in a natural grassland in the Northwest China (37°76 N, 107°28 E). The annual average precipitation of this study site was 289.4 mm from 1980 to 2020, most of which fell between April and September (Fig. S1). The long-term (1980–2010) average annual temperature is 7.8 °C. The soil type in this region is sandy loam and has a pH of 8.27; the soil available N is 4.48 mg kg−1, and soil available P is 1.87 mg kg−1. The study site is a Stipa brevifloradesert steppe. Stipa breviflora (Poaceae), a perennial C3 bunch grass, is a dominant species, in addition to plant species such as Agropyron mongolicum(Poaceae, C3 grass), Chenopodium aristatum (Chenopodiaceae, C4 forb) and Cleistogenes squarrosa (Poaceae, C4 grass). Stipa breviflora , A. mongolicumand C. squarrosa are considered to be mycorrhizal plants (Bao & Yan, 2004; H. Tian, Gai, Zhang, Christie, & Li, 2009; Xin Yang, Liu, Tian, & Shen, 2021). In contrast, C. aristatum cannot be colonised by AM fungi and is considered a non-mycorrhizal plant (H. Tian et al., 2009).