Fig. 7. (a) Structure equation modeling exploring direct and indirect effects of fraction(grazing and grazing exculsion), grassland type, plant index(coverage and patrick index), and soil chemical properties in fungal diversity and (b) standardized effects of fraction, grassland type, plant index, and soil chemical properties in fungal diversity. Line thickness indicates the correlation strength: the thicker the line, the stronger is the correlation. The red and blue arrows indicate negative and positive relationships, respectively. The width of the arrow is proportional to the strength of the relationship. The numbers adjacent to the arrows represent standardized path coefficients. The proportion of variance explained (R2) appeared alongside each response variable in the model.
Discussion
4.1. Effect of grazing exclusion on soil physicochemical properties in different grassland types
Whether grazing exclusion can change the diversity and composition of soil fungal communities, soil chemistry is the dominant factor (Li et al., 2019; Qin et al., 2021). There is growing evidence that fungal communities are more sensitive to carbon and nitrogen sources than bacterial communities (Zhang et al., 2018). Our study found that C:N did not differ significantly among the three grassland types in either 0-5 cm or 5-10 cm soils before grazing exclusion, probably because grazing can promote root secretions and fallen leaves decomposition through trampling, which can increase the amount of C and N entering the soil and keep soil C and N in dynamic equilibrium (Bi et al., 2018). Whereas C:N was significantly higher in temperate deserts than in temperate steppes and mountain meadows after grazing exclusion, the interaction of exclusion and grassland type significantly altered C:N, indicating that grazing exclusion reduces soil C and N pools (Sun et al., 2020), and is prominent in steppes and meadows. In addition, we found that grazing exclusion significantly increased SOC in all three grassland types and SWC in both temperate steppes and mountain meadow, due to higher soil fertility associated with good plant cover and greater apoplastic accumulation (Marzaioli et al., 2010; Jing et al., 2014). Above-ground biomass has a direct effect on apoplastic accumulation, which increases soil moisture. It explains the increase in SWC in both temperate steppesand mountain meadow. At the same time, apoplastic decomposes more rapidly in moist soils, and large amounts of decomposing apoplastic can increase soil nutrients (Jing et al., 2014).
Secondly, soil TP showed a close correlation with temperate desert, and grazing exclusion significantly increased the content of temperate desert TP, which is inconsistent with previous studies (Yang et al., 2018), it may be due to the fact that soil nutrients still had not returned to their original levels after 9 years of grazing exclusion, but were in an unstable and fluctuating state, this may be related to the change in the main vegetation types on the surface, the rate of decomposition and the amount of nutrient uptake and return by plant roots. The effects of grazing exclusion on pH, C:N, C:P and N:P in the three grasslands were not significant regardless of the soil stratum. The reasons for this phenomenon may be: (1) the removal of grazing pressure after grazing exclusion in degraded grasslands, the disappearance of conditions that maintain the spatial pattern of soil nutrients or vegetation, the increase in spatial heterogeneity of soil nutrients, and the increase in the proportion of stochastic factors, making the research results more stochastic and the study results appearing differences (David, 1988); (2) the grazing areas have a higher soil nutrient content due to the trampling of livestock, which leads to the crushing of apoplastic and its entry into the soil, thus accelerating the circulation of organic matter, as well as the excrement of grazing livestock; and (3) the regional differences in the soil itself may also be responsible for this difference.
4.2. Effects of grazing exclusion on the composition and diversity of soil fungi
Our results go further than the commonly reported differences in fungal composition between grassland types (Zhang et al., 2010; Yang et al., 2022; Wang et al., 2021). Consistent with previous results (Maestre et al., 2015; Ren et al., 2017), we found that at the phylum level, the pre-grazing exclusion Ascomycota and Basidiomycota were the dominant fungi in all three grassland types. After the grazing exclusion Ascomycota decreased by 14.72% and 9.74% in temperate deserts and mountain meadows, respectively, while it increased by 4.82% in temperate steppes. In contrast to the Ascomycota , theBasidiomycota increased by 238.8% and 47.68% in temperate deserts and mountain meadows, respectively, while it decreased by 23.12% in temperate steppes. This may be due to the higher soil pH in temperate desert and the soils of temperate steppes and mountain meadows is acidic, and the positive response of Ascomycota andBasidiomycota fungi to changes in soil pH, the higher abundance of Ascomycota in soils with higher pH (Tedersoo et al., 2014), which also explains the higher relative abundance of Ascomycotain temperate deserts. Additionally, studies of soil fungi have found that the highest diversity in soils is found in Ascomycota andBasidiomycota fungi in recent years, while later studies analysing the occurrence patterns of globally dominant soil fungal taxa have found a strong spatial structure of soil fungal communities along ecological gradients, but also signs of mass dispersal and the ability of some fungi to dominate many environments, with wind-dispersedAscomycota taxa dominating the soil fungal communities, followed by Basidiomycota and other fungi (Bucbe et al., 2009; Egidi et al., 2019). Next, significant differences were found between the three grassland types for the Glomeromycota and Mucoromycota , and it was found that the Mortierellomycota was not influenced by either grazing exclusion or grassland type, but the interaction between the two had a significant effect on the Mortierellomycota . It has been shown that the activity of Glomeromycota is reduced in high acidity environments and is not efficient at taking up nitrogen or phosphorus from organic matter and converting it to inorganic components for plant use, eventually plants gradually form new symbiotic relationships with other organic matter mycorrhizal fungi and replace the Glomeromycota (Smith et al., 2008). In this study, the low organic matter content of the temperate desert soil and the lower relative abundance of Glomeromycota compared to the other two types of grassland. Validating the findings of most studies on the relationship between Glomeromycota and soil physicochemical properties, and support previous studies suggesting a suppressive effect of soil organic matter content on Glomeromycota (Pellissier et al., 2015; Smith et al., 2008). Furthermore, at the phyla level, it was found that grazing exclusion, grassland type and the interaction between the two significantly altered Sordariomycetes andPezizomycetes , with grassland type having a highly significant effect on Ustilaginomycetes and a significant effect onEurotiomycetes and Orbiliomycetes . This represents the different responses of various fungal phylas to environmental change, reflecting the role of environmental niche separation in selecting fungal communities (Pellissier et al., 2014).
Unlike the effect of grazing exclusion on soil fungal composition, grazing exclusion, grassland type and the interaction between the two did not significantly alter soil fungal α-diversity in either 0-5 cm or 5-10 cm soils, similar to previous findings (Hao et al., 2020; Yang et al., 2019; Ding et al., 2020). The insignificant response of fungal alpha diversity to grazing exclusion and grassland type may be due to that fungi are less sensitive than bacteria (Cheng et al., 2016; Zhang et al., 2018). These results are consistent with previous findings that soil fungi are more stable than bacteria (Hamonts et al., 2017; Wang et al., 2019). Although there was no effect on fungal α-diversity, we found that the interaction of grazing exclusion and grassland type significantly altered fungal β-diversity. This may be the ability of many soil microorganisms to colonise the inter-rhizosphere, where root biomass and plant C:N ratios may influence the utilisation of inter-rhizosphere resources by altering root turnover or root secretions, which may in turn strongly alter the composition of the microbial community (Chase, 2010; Philippot et al., 2013). Moreover, other studies have detected significant effects of grazing exclusion on soil fungal β-diversity (Chen et al., 2020). In the current study, the strong link between soil microbial diversity and grazing exclusion and grassland type can be explained by the fact that microbial communities are often directly influenced by above-ground plant biomass and community structure (de Vries et al., 2012). For example, grazing exclusion increases above-ground plant biomass and apoplastic content, thus increased fungal diversity (Hamonts et al., 2017). The increase in species richness induced by grazing exclusion suggests that high plant diversity results in a greater variety of organic matter ultimately flowing into the subsurface, thereby generating more ecological niches for use by different species of microorganisms and altering the diversity of microbial communities (Lodge, 1997; Brodie et al., 2003). The results indicated that there was no significant change in soil fungal alpha diversity but significant differences in fungal beta diversity under the influence of both grazing exclusion and grassland type.
4.3. Potential drivers of soil fungal community diversity
SEM analysis showed that grazing exclusion had a significant effect on fungal community diversity by directly affecting total soil phosphorus, and that grassland type had a significant effect on fungal community diversity by directly affecting soil organic carbon, which in turn affected total nitrogen and phosphorus. In line with our findings, other studies have also shown that soil total phosphorus and nitrogen are closely related to soil fungal diversity (Lindahl et al., 2015; Lauber et al., 2008). In contrast, a recent experiment showed that soil fungal abundance and diversity differed between grassland habitats, but that this difference did not correlate with geographical distance, and that certain environmental factors, including climate, soil pH, nitrogen and phosphorus, were the main influences on the distribution, abundance and diversity of soil fungal communities (Pellissier et al., 2014). In our study, combining direct and indirect effects reveals that soil total nitrogen is the most important predictor of changes in fungal community diversity, the strong association between soil total nitrogen and fungal diversity is justified because total nitrogen is the main source of energy for soil microorganisms (Lindahl et al., 2015). For example, nitrogen provides an adequate source of nitrogen for fungal symbioses, and plants no longer import too much carbon into the subsurface for nitrogen acquisition, which can have an impact on fungal composition and biomass (Norris et al., 2013). However, it has been shown that fungal diversity does not correlate with total soil nitrogen and phosphorus, suggesting that environmental factors may not be a limiting factor in the structural diversity of soil fungal communities at the larger scale studied (Li., 2015). In the present study, soil organic carbon had an indirect effect on fungal community diversity, similar to the results of previous studies (Li., 2015). Soil organic carbon provides nutrients and energy for plant growth and soil microbial life (Li et al., 2006), and its content reflects the effectiveness of limiting resources in the soil and is the ‘source’ of nutrients for soil fungal community growth, the level of which affects soil fungal community structure and diversity. In addition, the natural environment in different regions can also affect soil fungal diversity, harsh environments such as drought can reduce fungal diversity and abundance (Maestre et al., 2015). In summary, soil nutrient content is the main factor influencing fungal diversity, especially total soil nitrogen, total phosphorus and organic carbon.
Conclusions
As revealed by the data from our study, the composition of soil fungal communities differed between grassland types. Under the influence of both grazing exclusion and grassland type, there was no significant change in soil fungal alpha diversity, but significant differences in fungal beta diversity. The changes in soil total nitrogen and phosphorus caused by grazing exclusion were closely related to soil fungal diversity. These results suggest that soil nutrient content is the main factor influencing fungal diversity, especially soil total nitrogen, total phosphorus and organic carbon.