Structures and determinants of soil microbiomes along a steep elevation gradient in Southwest China

Abstract

Soil microbial communities play a vital role in accelerating nutrient cycling and stabilizing ecosystem functions in forests. However, the diversity of soil microbiome and the mechanisms driving their distribution patterns along elevational gradients in montane areas remain largely unknown. In this study, we investigated the soil microbial diversity along an elevational gradient from 650 m to 3,800 m above sea level in southeast Tibet, China, through DNA metabarcode sequencing of both the bacterial and fungal communities. Our results showed that the dominant bacterial phyla across elevations were Proteobacteria, Acidobacteriota and Actinobacteriota, and the dominant fungal phyla were Ascomycota and Basidiomycota. The Simpson indices of both soil bacteria and fungi demonstrated a hollow trend along the elevational gradient, with an abrupt decrease in bacterial and fungal diversity at 2,600 m a.s.l. in coniferous and broad-leaved mixed forests (CBM). Soil bacterial chemoheterotrophy was the dominant lifestyle and was predicted to decrease with increasing elevation. In terms of fungal lifestyles, saprophytic and symbiotic fungi were the dominant functional communities but their relative abundance was negatively correlated with increasing elevation. Environmental factors including vegetation type (VEG), altitude (ALT), soil pH, total phosphorus (TP), nitrate nitrogen (NO₃ ^--N), and polyphenol oxidase (ppo) all exhibited significant influence on the bacterial community structure, whereas VEG, ALT, and the carbon to nitrogen ratio (C/N) were significantly associated with the fungal community structure. The VPA results indicated that edaphic factors explained 37% of the bacterial community variations, while C/N, ALT, and VEG explained 49% of the total fungal community variations. Our study contributes significantly to our understanding of forest ecosystems in mountainous regions with large elevation changes, highlighting the crucial role of soil environmental factors in shaping soil microbial communities and their variations in specific forest ecosystems.

Keywords: Qinghai-Tibet Plateau; altitudinal gradient; microbial communities; soil ecology; subtropical forests.

Figure 1

Geographic location of sampling sites in the montane range of Medog county.

Figure 2

Soil bacterial (A) and fungal (B) communities at the phylum level at different elevations and vegetation zones in Medog county along an elevation gradient. From left to right on the X-axis with increasing elevation, TR, EBL, CBM, DCF, S, M, and IEV represent tropical rainforest, evergreen broad-leaved forest, coniferous and broad-leaved mixed forest, dark coniferous forest, shrub, meadow and ice-edge vegetation, respectively.

Figure 3

Alpha-diversity (Simpson index) of soil bacteria (A) and fungi (B) at different vegetation zones along an elevation gradient. From left to right on the X-axis with increasing elevation, TR, EBL, CBM, DCF, S, M and IEV represent tropical rainforest, evergreen broad-leaved forest, coniferous and broad-leaved mixed forest, dark coniferous forest, shrub, meadow and ice-edge vegetation, respectively.

Figure 4

Beta-diversity (NMDS) of soil bacteria (A) and fungi (B). TR, EBL, CBM, DCF, S, M, and IEV represent tropical rainforest, evergreen broad-leaved forest, coniferous and broad-leaved mixed forest, dark coniferous forest, shrub, meadow and ice-edge vegetation, respectively.

Figure 5

Redundancy analysis (RDA) demonstrating the effect of soil properties on community composition of soil bacteria (A) and fungi (B). Abbreviations: TR, tropical rainforest; EBL, evergreen broad-leaved forest; CBM, coniferous and broad-leaved mixed forest; DCF, dark coniferous forest; S, shrub; M, meadow; IEV, ice-edge vegetation; pH, pH value; TC, total carbon; TN, total nitrogen; C/N, carbon nitrogen ratio; TP, total phosphorus; TK, total potassium; NH₄⁺-N, ammonium nitrogen; NO₃⁻-N, nitrate nitrogen; ure, urease; pho, polyphenol oxidase; suc, sucrase; ppo, acid phosphatase; cellu, cellulase; amy, amylase; pro, acid protease; SWC, soil water content; VEG, vegetation type.

Figure 6

Correlation between environmental factors and microbial communities along an elevation gradient in southeast Tibet. Bacterial and fungal communities were related to total of 11 significantly differential environmental factors using Mantel-test analysis with r > 0.15, p < 0.05. Edge width corresponds to the Mantel’s r statistic for the corresponding distance correlations, and edge color denotes the statistical significance. Pairwise comparisons of environmental factors are shown, with a color gradient denoting Pearson’s correlation coefficient. pH, pH value; TC, total carbon; TN, total nitrogen; C/N, carbon nitrogen ratio; TP, total phosphorus; TK, total potassium; NH₄⁺-N, ammonium nitrogen; NO₃⁻-N, nitrate nitrogen; ure, urease; pho, polyphenol oxidase; suc, sucrase; ppo, acid phosphatase; cellu, cellulase; amy, amylase; pro, acid protease; SWC, soil water content; VEG, vegetation type.

Figure 7

Functional prediction of bacteria (A–D) and fungi (E–H) at different vegetation zones along an elevation gradient. From left to right on the X-axis with increasing elevation: TR, EBL, CBM, DCF, S, M and IEV represent tropical rainforest, evergreen broad-leaved forest, coniferous and broad-leaved mixed forest, dark coniferous forest, shrub, meadow and ice-edge vegetation, respectively. Lowercase letters indicate statistically significant differences at p < 0.05.