*3.4. The Relative Abundance and Occurrence Frequency of AMF Genus in Mt. Taibai*

It was found that the relative abundance ranged from 0.76% to 61.29% and from 0.24% to 53.58% based on species and OTU levels, respectively (Table 1). The fungi in the genus of *Glomus* were the most dominant, with the highest relative abundance of 61.29% and 53.58% based on species and OTU level, which was significantly higher than other genera. Meanwhile, the occurrence frequency of *Glomus* also was higher at 91.67%. *Unclassified in Glomeromycetes* was found in all altitudes with the highest occurrence frequency of 100%. The second abundant genus is *Acaulospora*, with a relative abundance of 9.46% and 5.32% at species and OTU levels, and the occurrence frequency was 66.67%. At the same time, the relative abundance of *Pacispora* was the same as *Ambispora* based on species level with 2.18%. In addition, the occurrence frequency of *Pacispora* was the same

as *Scutellospora* with 33.33%, and the occurrence frequency of *Diversispora*, *Acaulospora*, and *Paraglomus* were the same with 66.67%.


**Table 1.** Relative abundance and occurrence frequency of AMF genus in Mt. Taibai.

### *3.5. The Drive Factors of AMF Community and Diversity*

Among different genera, *Glomus* was most affected by soil and root nutrient factors (Figure 5). Both soil factors (pH and C/N) and root nutrients (C, N, and C/N) had a significant effect on *Glomus* through correlation analysis by heatmap at the genus level (Figure 5). *Unclassified in Diversisporaceae* and *unclassified in Archaeosporales* were both affected by root nutrient of N (*p* < 0.01) and ecological stoichiometry of C/N (*p* < 0.01). As for *unclassified in Glomeromycetes*, it was influenced by C/N (*p* < 0.01) and available phosphorus (*p* < 0.05) in soil. And soil and root nutrients had no significant effect on other AMF genera.

**Figure 5.** Influence of environmental factors on AMF genus. **Note:** \*, \*\*, \*\*\* indicate significant correlation at *p* < 0.05, *p* < 0.01, *p* < 0.001 confidence level, respectively.

Elevation had a positive and prominent effect on AMF Shannon diversity (*r* = 0.493) based on OTU level (Figure 6). Meanwhile, elevation had a positive effect on soil factors of pH (*r* = 0.651) and ecological stoichiometry of C/N (*r* = 0.605), plant factors, such as C (*r* = 0.364), N (*r* = 0.379) and C/N (*r* = 0.345). Besides, the AMF diversity index of Shannon

was greatly affected by soil and root nutrients, while the Sobs index was affected by soil and root ecological stoichiometry C/N and root N content.

**Figure 6.** The relationships among elevation, soil factors (C, N, P, C/N, and pH), plant factors (C, N, P, and C/N), and AMF diversity indices are based on OTU level. Note: The effect of altitude on soil and root is expressed as a correlation coefficient, while the effects of soil and root factors on diversity indices are represented by standard regression coefficients. Blue solid represents significant positive or negative effects. Blue dashed represent nonsignificant paths. \*\* means *p* < 0.01; \* means *p* < 0.05, respectively.

### **4. Discussion**

The changes and laws of biodiversity along different environmental gradients are the important topic of biodiversity research [26,27]. Many environmental factors vary with altitude in mountain systems, so altitude is often used as an integrated factor to study plant and animal distribution patterns in mountain systems. In recent years, people have become more and more interested in knowing how microbes respond to the changes in environmental conditions because of their critical role in ecosystem functions [28–30]. Luo et al. also suggested that understanding the diversity of fungi in ecosystems may have predictive implications for biodiversity and ecosystem evolution processes [31]. Therefore, AMF diversity and community distribution along different altitudes were studied to explore the role of AMF in the mountain ecosystem and the responses to clime change.

In this study, the colonization rate and colonization density of AMF showed a trend of first increasing, then decreasing trends with the elevation. However, Gai et al. and Kotilinek et al. believed that AMF colonization showed a downward trend with increasing altitudes [32,33]. There are even studies that there were no significant differences in AMF colonization between high-altitude and low-altitude areas in the southeast of the Qinghai-Tibet Plateau [34]. The different results may be due to the differences in research sites or environmental factors, such as plant species, soil types, and so on. Liu showed that the arbuscular abundance of AMF was significantly influenced by altitude gradients [35], which was consistent with the results of our research. These different results also suggest that AMF could form a good symbiotic relationship with plant roots in the mountain ecosystem.

In the present study, the 287 OTUs and 62 species of AMF were identified and represented 8 identified genera, 5 unidentified genera in Mt. Taibai, which supported that AMF had a wide ecological range and was an important part of the ecosystem. Our research showed that 39 species belonged to the genus of *Glomus*, followed by *Acaulospora* with 5 species. Whether at the species or OTU levels, the relative abundance and the occurrence frequency of *Glomus* were the highest, which was consistent with the conclusions of most previous studies on the molecular diversity of AMF that *Glomus* was the dominant genus in the AMF community [19,36,37]. This may be due to its wide ecological range and the certain resistance in complex environments [38]. Besides, *Glomus* can usually produce large

numbers of spores and hypha fragments, which can extensively spread and colonize the roots of plants [37,39]. In terms of different altitudes, *Glomus* dominated at lower altitudes, whereas *Acaulospora* were more abundant at the higher altitudes of 3250–3511 m. This result was consistent with Oehl et al., who suggested that the genus of *Acaulospora* was more abundant in the highlands than in the lowlands in Switzerland [40]. Moreover, Haug et al. and Yang et al. also came to similar conclusions [8,41]. The different distribution at low and high altitudes in AMF explains the correlation of AMF species with altitude and suggests that there may be potential niche differentiation along the altitudinal gradient.

In addition to different distribution, the diversity of AMF was also different with the altitude change. It was discovered that AMF diversity indices of Sobs and Shannon showed the trends of quadratic function increasing first and then decreasing, whether on the level of species or OTU. However, Guo et al. and Egan et al. studied the AMF diversity and suggested that the alpha diversity decreased monotonically with the increase in altitude [21,42]. The reason for this phenomenon may be that this study was conducted in a large-scale altitude range of 663–3511 m, while Guo et al. and Egan et al. explored the AMF diversity in a relatively small altitude range. Therefore, in this study, the AMF diversity varies in different climatic environments. Moreover, some studies have proved that AMF diversity is closely related to plant richness [43], and plant richness is also different on different altitude gradients in Mt. Taibai (Table S1). This may also be the reason why AMF diversity shows different trends with increasing altitude in Mt. Taibai.

In this study, it was also found that the higher Shannon and Sobs indices appeared at mid-altitudes, whether based on species or OTU level. The highest diversity indices occurred at 2460 m, and altitude has a significant effect on them. Bonfim et al. showed that AMF diversity at higher altitudes was higher than at lower altitudes in the Atlantic forest system [44]. The reason for this phenomenon may be that the mid-altitudes have less human disturbance than the low altitude region and a less extreme climate environment than the high altitude region [45]. Therefore, the mid-elevation area is favorable for AMF sporulation and growth. Gai et al. and Shi et al. supported that altitude has no significant influence on AMF diversity [32,39]. Because altitude is a comprehensive factor, the effects of altitude on AMF diversity may be caused by differences in geographic location and environmental factors [15,46,47]. Besides, previous studies have shown that environmental factors, especially the geographical environment and soil factors, have an important impact on AMF diversity. Different ecological factors would affect the growth, development, colonization, and reproduction of AMF, which would cause differences in AMF diversity in different ecosystems [48–51]. Therefore, it is necessary to study the environmental factors of different altitudes further.

Determination of soil and plant nutrients found a significant impact on AMF diversity indices of Sob and Shannon. Our results were consistent with previous research conclusions that altitudes and soil variables had a significant impact on AMF diversity and richness [36]. Besides, Montiel-Rozas et al. [52] showed that AMF diversity and richness were only affected by soil properties, and soil factors were the main driving force for AM fungal communities. Our research found that whether it was in soil or plant roots, P concentration significantly affects the AMF Shannon index. This result was consistent with Maitra et al., who confirmed that the AMF Shannon diversity index showed a positive response to P [53]. Ceulemans et al. showed that AMF diversity decreased with the increase in soil P utilization [54]. Therefore, the change of P concentration is an important predictor of the response of AMF diversity to soil nutrients [55]. Previous studies have also shown that the addition of N increased AMF diversity in N-deficient soil [56–58], which was consistent with our study that N content of plant roots has a significant effect on AMF diversity, but N content in the soil had no effect on it. This result showed that plant roots had a greater impact on AMF diversity than soil. This also suggests that AMF tends to be symbiotic with plants to absorb nutrients, thereby increasing the diversity of mycorrhiza to increase the plant's own competitive advantage [59]. In addition, studies have suggested that the identity of the host plant has been considered to be one of the most important factors in

shaping AMF community composition [60–62]. It was speculated that the vital effects of the host plant on the AMF community might be related to the C/N in plant roots.

Moreover, it was found that the AMF community have different affinities with soil and root nutrient. And glomus is most affected by soil and root nutrient factors through the analysis of Heatmap. The genus of *unclassified in Diversisporales* and *unclassified in Archaeosporales* were significantly affected by root N concentration and C/N. However, the genus of unclassified *Glomeromycetes* were correlated with soil P concentration and C/N. The results revealed that there were different relationships between soil characteristics and the AMF genus, which is consistent with Kim et al. [63]. These also suggested that AMF taxa have different environmental preferences in tropical montane rainforests. Therefore, this also explained the different distribution of AMF communities at different altitudes. Besides, soil nutrient concentration also is an important factor in the AMF community. Zhao et al. [37] confirmed that soil nutrients have an impact on AMF communities, as a lack of nutrients inhibits spore germination and dissociation. Therefore, soil factors play an important role in AMF diversity and community in the mountain ecosystem.
