*2.4. Measurement of AM Colonization and Parameters of Soil and Plant Roots*

The colonization of AMF was determined according to the method of Phillips et al. [23]. First, selected fresh fine roots were cleaned and wiped dry, then immersed in a test tube with a mass fraction of 10% KOH solution, and heated in a water bath at 90 ◦C for 30 min until the roots became relatively transparent. When the roots were relatively transparent, the lye on the roots was cleaned and the roots were soaked in a 5% CH3COOH for 5 min. They were dyed with 5% volume fraction acid acetic ink and heated in a water bath at 90 ◦C for about 30 min. As the roots were fully dyed by the acetic acid ink, they were cleaned and then put in lactic acid for color separation for approximately 30 min. Finally, the roots were cut at about 1 cm and placed on a glass slide (each glass slide has 15 roots). The glass slide was observed under a Motic BA310 microscope at 100–400 times magnification to survey the colonization status.

The concentration of total carbon and nitrogen were measured by an elemental analyzer (GC IsolinkFlash 2000; Thermo Scientific, Waltham, MA, USA) analyzer. The phosphorus content in plant roots is determined by the molybdenum-antimony colorimetric method [24].

### *2.5. Calculation of AM Colonization, Relative Abundance, and Occurrence Frequency*

The percentage of root length colonized by AM fungal structures was estimated and calculated according to the grading criteria of Trouvelot et al. [25].

The relative abundance (species/OTU) of AM fungal genus was calculated as the percentage of the number of species/OTUs in each genus divided by the total number of species/OTUs in all genera, then take the average of the three samples.

The occurrence frequency of AM fungal genus was defined as the percentage of the number of samples where this genus was observed to the number of all samples in this genus.

Shannon–Weiner index: the *Pi* of AMF species or OTUs was defined as the percentage of the sequences for each species or OTUs detected to total species or OTUs sequences in a sample;

$$\mathcal{H} = -\sum \left[ P\_i \log\_2 \left( P\_i \right) \right]$$

The Sobs index of AMF species or OTUs was defined as the numbers of species or OTUs in a sample.

### *2.6. Statistics and Analysis of Data*

The colonization ratio, colonization density, AMF diversity indies including Sobs and Shannon index, stoichiometric characteristics of plant roots, and edaphic factors were all statistically analyzed and curved estimated by SPSS 25.0. Then made the scatter charts through Origin 21.0. AMF relative abundance and occurrence frequency were analyzed based on genus level by Excel 2019. The heat map was presented to explore the relationships between the AMF community and environmental variables based on correlation analyses. And it is conducted on MajorBio Cloud's bioinformatics analysis cloud platform. Based on multiple linear regression analysis of the relationship between altitude, environmental factors, and AMF diversity.

### **3. Results**
