Helan mountain is situated between the Alashan Plateau and the Yinchuan Plain, spanning coordinates 38°27′-39°30′N and 105°41′-106°41′E. The highest altitude of the sampling point reaches 2,580 m. The Helan mountains have a typical continental climate, with an average annual temperature of −1 to 8°C, an average annual precipitation of 200–400 mm, and a large average annual evaporation of more than 2,000 mm. The sampling area is situated on the sunny slope of the eastern side of the Helan mountains. Common plant species in this region include Roegneria alashanica, Aristida adscensionis, Stipa breviflora, Prunus mongolica, Ulmus glaucescens, Picea crassifolia, and so on. The soil is mostly sandy loam.

The sample plots were established at altitudes ranging from 1,349 to 2,580 m, covering typical plant communities on the eastern slope of the Helan mountains, including grassland (GR), shrubs (SH), U. glaucescens forests (UL), Pinus tabuliformis forests (PF), coniferous and broad-leaved mixed forests (CB), P. crassifolia forests (PI), and subalpine meadow (SU), as shown in Table 1. Within each sampling area, three 20 m × 20 m quadrats were established. The five-point sampling method was used to collect plant inter-root soil from 0 to 20 cm depth with a 4 cm diameter soil sampler, which was mixed and sealed in a sterilized bag, transferred to the laboratory in a refrigerated box, and a part of the soil was air-dried and stored at room temperature, while the other part was stored in a refrigerator at −80°C to be preserved and used for soil microbial sequencing (Schleuss and Muller, 2001).

The data were organized using Excel 2021, and SPSS 26 was utilized for data processing. To analyze the differences in soil physicochemical properties among vegetation zones (P < 0.05), one-way ANOVA and Tukey’s honest Significant Difference test were employed. Pearson’s correlation analysis (PCA) with a two-tailed significance test was used to explore the correlations between different variables. Non-metric multidimensional scaling analysis (NMDS) and analysis of similarity test (ANOSIM) were selected to examine the differences among AMF communities. Bar scale plots were generated using Origin 2022. The computation platform provided by Shanghai Meiji Biomedical Technology Co. was used for the analysis and mapping of soil AMF community diversity and its correlation with environmental factors.

Soil AMF community α-diversity analysis was conducted using the Shannon-Wiener diversity index (H), Simpson dominance index (D), Chao1 index (S_obs), and ACE index (S_ACE).

① Shannon-Wiener index (H):

② Simpson dominance index (D):

Where: S_obs is the number of OTUs actually observed; n_i is the number of sequences contained in the i-th OTU and N is the number of all sequences.

③ Chao1 index (S_obs):

Where: S_chao1 is the estimated number of OTUs; F₁ is the number of OTUs containing only one sequence; and F₂ is the number of OTUs containing only two sequences.

④ ACE index (S_ACE):

Where, γA⁢C⁢E2=max⁡[Sr⁢a⁢r⁢e⁢∑i=110i⁢(i-1)⁢FiCA⁢C⁢E⁢Nr⁢a⁢r⁢e⁢(Nr⁢a⁢r⁢e-1)-1,0]

S_rare is the number of OTUs with no more than “abund”” sequences; S_abund is the number of OTUs with more than “abund” sequences; abund is the threshold for “dominance” OTUs. abund is the threshold of “dominance” OTUs, and the default is 10.

The physicochemical properties of soils at different altitudes on the eastern slope of Helan mountain exhibited significant variations (P < 0.05), as presented in Table 2. WC and TP displayed an increasing trend with altitude, reaching their highest values in the high-altitude area (13.07% and 161.84 mg/kg, respectively). On the other hand, OM, TN, AN, AP, and AK demonstrated a pattern of initially increasing and then decreasing along the altitude gradient. The middle altitude area had the highest levels of OM, AN, and TN (9.25%, 137.26 mg/kg, and 165.77 mg/kg, respectively), while AP reached its peak in the high-altitude area (12.43 mg/kg). AK, on the other hand, exhibited its highest concentration in the low-altitude region (49.83 mg/kg). Soil pH decreased with altitude, ranging from 7.95 to 8.13, with minor fluctuations, indicating a slightly alkaline nature of the soil on the eastern slope of Helan mountain. In summary, the nutrient content of soils on the eastern slope of Helan mountain increased with altitude, in agreement with the findings of previous studies.

As shown in Figure 1, when the end of the dilution curve of the Shannon index tends to flatten toward flat, it indicates that the amount of sequencing data in the sample is reasonable enough. That is, the biodiversity in the sample is high, and there exists a relative abundance of different species or gene expression levels that can be reasonably analyzed for subsequent data.

Table 3 displays significant differences (P < 0.05) in the α-diversity of AMF communities among different vegetation types and altitude gradients. Specifically, the soil AMF community in GR exhibited the highest Shannon index and the lowest Simpson index, while the soil AMF community in CB showed the lowest Shannon index and the highest Simpson index. This indicates that the α-diversity of soil AMF communities was highest in desert grasslands and lowest in mixed coniferous forests. Regarding the ACE index and Chao1 index, UL had the highest values, whereas CB had the lowest. No significant differences were observed among vegetation types at different altitudes.

In summary, the α-diversity of soil AMF communities on the eastern slope of the Helan mountains exhibited significant variations among different vegetation types. The overall trend indicated a decrease followed by an increase in altitude. Overall, gray elm forests showed the highest alpha diversity among soil AMF communities, while mixed coniferous and broadleaf forests displayed the lowest diversity.

A total of 986 OTUs of AMF were identified through high-throughput sequencing in this study. These OTUs belonged to 1 phylum, 4 classes, 6 orders, 12 families, 14 genera, and 114 species.

Figure 2 illustrates significant variations in the relative abundance of soil AMF at the genus level among samples at different altitudes. Among all the samples, Glomus exhibited the highest relative abundance, with an average percentage of 85.13%. The highest relative abundance (98.27%) was observed in sample SH at a low altitude, indicating that Glomus was the dominant species in the soil AMF community on the eastern slope of the Helan mountains. In addition, the 2nd to 5th relative abundances were unclassified__p__Glomeromycota, Claroideoglomus, Archaeospora, and unclassified__o_Archaeosporales in the order of relative abundance. As shown in Figure 3, further analysis of the species differences at the genus level across different samples revealed that Glomus, unclassified_p_Glomeromycota, Claroideoglomus, and Diversispora were present in all altitudinal gradient samples, indicating their wide distribution on the eastern slopes of Mount Helan. Sclerocystis was found exclusively in sample UL, while unclassified_p_Glomeromycota, Archaeospora, and Paraglomus exhibited significant differences (P < 0.05) among samples.

Comparative analysis of AMF community species diversity among different samples was conducted at the OTU level. The results of NMDS analysis, utilizing the Bray–Curtis distance algorithm, and the ANOSIM test revealed significant differences in the community structure of AMF across the samples (R = 0.5556, P = 0.001). As depicted in Figure 4, the samples exhibited distinct clustering patterns in a two-dimensional space. The PF sample points at the middle altitude appeared more scattered, indicating lower species similarity among those samples. Conversely, the SH and GR sample points at the low altitude were more closely grouped, suggesting higher species similarity. Notably, the NMDS1 axis clearly distinguished the AMF communities between high and low altitudes. The greatest species differences were observed between the SH and GR communities at low altitudes and the PF and CB communities at medium altitudes.

To further investigate the differences between the PF and GR samples, LEfse analysis (Figure 5) was performed. The analysis revealed that the abundance of Glomus was significantly higher in GR compared to PF. Conversely, the abundance of unclassified__p__Glomeromycota, Archaeospora, and Paraglomus was significantly higher in PF compared to GR.

The results of Pearson correlation analysis (Table 4) revealed significant associations between soil AMF community α-diversity indices and various soil factors. The Shannon index exhibited a highly significant positive correlation with soil OM and a significant positive correlation with WC. Conversely, the Simpson index showed a highly significant negative correlation with OM and a significant negative correlation with WC. The ACE index was significantly positively correlated with ALT and OM, while the Chao1 index demonstrated a highly significant positive correlation with WC and OM, as well as a significant positive correlation with ALT.

To explore the relationship between soil AMF community α-diversity and environmental factors, redundancy analysis (RDA) analysis was conducted (Figure 6). The first and second ordination axes explained 86.60% and 3.63% of the variance in AMF community composition, respectively. The Shannon, ACE, and Chao indices exhibited positive correlations with AP, OM, ALT, WC, total TN, AN, and TP, while showing negative correlations with AK and pH. Simpson’s index displayed the opposite pattern. The results of the Monte Carlo significance test indicated that OM (r² = 0.3926, P = 0.003) and WC (r² = 0.2715, P = 0.049) were the primary environmental factors influencing the α-diversity of AMF communities.

The results of the heatmap plot of environmental factors and soil AMF genus level showed (Figure 7) that different environmental factors had different degrees of influence on soil AMF genera. Among them, soil OM showed a highly significant positive correlation with the genera unclassified_ p__Glomeromycota and Archaeospora, a significant positive correlation with the genera Paraglomus and unclassified_o_Glomerales, and a highly significant negative correlation with Glomus. WC was positively correlated with Ambispora and Claroideoglomus, AN was positively correlated with Paraglomus, Scutellospora was most affected by ALT and AK, and Claroideoglomus was most affected by WC and pH. In addition, the effects of environmental factors on unclassified__o_Archaeosporales, Acaulospora, Sclerocystis, unclassified__c__Glomeromycetes, unclassified__f__Glomeraceae, and Diversispora had no significant effect.

Consistent with the results of many studies (Zhang M. G. et al., 2022), Glomus, as the most common genus of AMF, is widely distributed in the natural environment (Schwarzott et al., 2001). In our study, Glomus also emerged as the dominant genus in the AMF community of the soil on the east slope of Helan mountain. This can be attributed to Glomus’ exceptional reproductive capacity and its ability to establish mycorrhizal associations rapidly with host plants. These symbiotic relationships enhance the nutrient and water absorption capabilities of host plants, promote plant growth efficiency, and improve stress resistance (Beimforde et al., 2014; Zhao et al., 2017). Consequently, Glomus contributes to the stability of plant communities and indirectly enriches soil biodiversity. Precipitation enhances vegetation photosynthesis efficiency, leading to increased biomass in host plants and providing more nutrients for AMF (Manzoni et al., 2012). However, as altitude rises, precipitation and soil WC increase and plant roots become less reliant on AMF for water uptake. Furthermore, the decrease in temperature at higher altitudes hinders the growth and development of AMF (Yang et al., 2022). Since Glomus encompasses a greater number of species, the adverse effects of environmental changes associated with increased altitude are more pronounced for this genus compared to others (Zhang M. G. et al., 2022).

Significant differences in soil AMF community α-diversity were observed among different altitudes. Consistent with previous studies (Guo et al., 2020), the α-diversity of soil AMF communities on the eastern slopes of the Helan mountains exhibited a positive correlation with ALT. This suggests that altitude and topography influence variations in soil properties and vegetation types, which, in turn, impact the AMF and mycorrhizal environments. The influence of host plants on soil AMF community α-diversity is substantial (Li et al., 2014; Davison et al., 2015). The Helan mountains exhibit distinct vertical distribution patterns in vegetation zones, giving rise to significant differences in soil AMF across these zones (Li et al., 2015). Research has established a direct correlation between soil AMF community diversity and plant community diversity and stability (Stover et al., 2018; Zhao et al., 2022). The region with the highest soil AMF diversity on the eastern slopes of the Helan mountains is situated in the lower altitude GR. Although the vegetation diversity and richness in the GR are not particularly high, the presence of dense shrubs and grasses, coupled with lower altitudes and favorable temperatures, fosters increased wildlife activity and appropriate disturbances. These factors accelerate material cycling, and energy flow, and increase the diversity of soil AMF. UL located at mid-altitude showed the highest AMF abundance among the seven experimental sample sites. This may be because the diversity and richness of vegetation in this region is significantly higher than that at lower altitudes, providing a wider range of hosting options for AMF. The PI also showed relatively higher abundance and diversity at higher altitudes. This may be because the PI is a relatively mature forest area with a more stable plant community. It may also be that in arid and semi-arid regions, mycorrhizal hyphae are more important to plants due to low soil WC (Bahadur et al., 2019). The increase in soil WC at high altitudes promotes the reproduction and growth of plant-AMF symbionts.

The composition and distribution of AMF communities on the eastern slopes of the Helan mountains are significantly influenced by differences in soil physical and chemical properties, as indicated by the findings of this study. Correlation analysis revealed that soil OM and WC exerted a stronger impact on the α-diversity of AMF communities compared to other soil physical and chemical properties. This can be attributed to WC promoting the reproductive development of AMF and influencing the selection and colonization of AMF by host plants (Shukla et al., 2013). OM serves as a carbon source necessary for microbial reproduction and positively affects AMF spore density and colonization, thereby enhancing AMF reproductive capacity to a certain extent (Bending et al., 2002; Shao et al., 2019; Sadeghi et al., 2023). Interestingly, the relative abundance of Glomus, the dominant genus in the study area, exhibited a pattern of decreasing and then increasing along different altitudinal gradients, which contradicted the trend observed for OM. This suggests that factors such as temperature at lower altitudes and soil moisture content at higher altitudes positively promote the growth and reproduction of Glomus as the dominant genus. It indicates that Glomus is not solely reliant on soil OM and possesses adaptability to nutrient-poor soil environments. Alternatively, the growth of Glomus may be limited by chemical inhibition or nutrient competition from plants (Singh et al., 2007).

This study conducted Person correlation analysis and found a highly significant positive correlation between altitude and soil OM, demonstrating the significant influence of altitude on the α-diversity of AMF communities. As a bridge for nutrient conversion between plant roots and the soil environment, the diversity of AMF is inevitably affected by inter-root soil OM (Zinger et al., 2011). Excessively high or low nutrient levels can inhibit AMF colonization and growth, with soil nitrogen and OM content playing a key role in determining the abundance and diversity of AMF (Xiao et al., 2018). Although soil nitrogen content was not the primary factor influencing AMF diversity and richness in this study, TN showed a significant positive correlation with altitude in Person’s correlation analysis, indicating that altitude indirectly influenced the α-diversity of AMF communities by affecting soil nitrogen content. Contrary to the results of previous studies by Shen et al. (2013), Guo and Gong (2014), and Li et al. (2022), this study found that pH was not the main factor influencing the AMF community structure on the east slope of Helan mountain. This discrepancy may be attributed to the limited range of soil pH variation in the samples of the present experiment.

Among the seven sample sites in this study, the middle altitude area, represented by the UL, exhibited medium levels of soil physicochemical indices, and the richness and diversity of the AMF community were relatively high across all sample sites. In the high-altitude region, represented by the PI, all soil physicochemical properties were at the upper range among the experimental sample plots, but the AMF community diversity was not high. In the low-altitude regions (SH and GR), the levels of soil physical and chemical properties were relatively lower, yet the AMF community diversity was high. This may be attributed to the fact that low and middle altitudes have more suitable temperatures that positively impact AMF compared to soil nutrients and moisture. Additionally, the timing of sampling may have influenced the results (Liu et al., 2021).