4.2 Plant community composition
Nutrient input can considerably alter plant species composition in native grassland, and favour the dominance of perennial grasses rather than other plant species (Avolio et al., 2014; Q. Tian et al., 2016). Supporting our first hypothesis, the response of plant species composition to both N and P inputs and mycorrhizal suppression in the present study was year-dependent. Specifically, N + P addition drastically favoured the dominance from S . breviflora overC . aristatum , but only in 2019 (a normal year). However, in the dry year (2020), C . aristatum had a lower relative shoot biomass in the plant community compared to that in the normal year (2019). According to the competitive exclusion principle (Gause, 1934), a dominant annual species (C . aristatum in this case) can quickly gain dominance and overtake sub-dominant species in plant communities in arid grassland when sufficient soil water and nutrients are present (Wang et al., 2014).
We also found that P input alone altered plant richness and diversity in both years, perhaps because P addition improved the dominance of sub-dominant species (e.g. A. mongolicum and C. aristatum ). Further, N input alone had a weak impact on the dominance of common species, which is also in accordance with findings of earlier field and greenhouse studies (G. Yang et al., 2016; Xin Yang et al., 2021). However, Avolio et al. (2004) found that N addition alone could contribute to the almost all variance in plant community composition in tall grass prairies. Observed differences among the influences of both N and P inputs may relate to the specific properties of grassland types and study durations (Crowther et al., 2019; Song et al., 2019).