Distinct soil bacterial patterns along narrow and broad elevational gradients in the grassland of Mt. Tianshan, China

Environmental variable quantification along an altitudinal gradient

This study area included 22 sampling sites, and 66 samples, classified into three transects, namely Transect 1 (1047–1587 m), Transect 2 (876–3070 m), and Transect 3 (1602–2110 m). Significant differences in soil properties and plant parameters were observed along the three studied altitudinal transects (P < 0.05) (Table 1). The soil pH ranged from 7.11 to 8.45, 5.54 to 7.82, and 5.02 to 8.83 in Transects 1, 2, and 3, respectively. In contrast to altitudinal Transects 1 and 3, the broad-scale altitudinal gradients of Transect 2(876–3070 m) exhibited more substantial changes in soil moisture (SM), soil organic carbon (SOC), total nitrogen (TN), C:N ratio (C/N), plant species richness (PSR), and belowground biomass (BGB) between sampling sites. Compared to sites situated at 876 m, 920 m, and 3070 m, the intermediate altitude site samples at 1744 m and 2513 m tended towards increased SM, TN, and SOC. In contrast, PSR and BGB were the highest in the high-altitude alpine meadow environments at 2903 m and 2981 m.

Bacterial community composition across the altitudinal gradients

Over the study area, the Actinobacteria constituted the most abundant phylum in all 22 sampling sites on Mt. Tianshan, accounting for 41.76% of total sequences, followed by the Proteobacteria (24.13%), Acidobacteria (10.85%), Chloroflexi (8.73%), Gemmatimonadetes (3.65%), Verrucomicrobia (3.23%), Bacteroidetes (1.64%) and Planctomycetes (1.53%) (Fig. 1A). Although there were consistent trends in soil bacterial phylum composition across samples the average relative abundance of phyla varied across elevations. For example, in Transect 2 (876–3070 m), the relative Actinobacteria abundance was lower in the high elevation sites (2903 m and 2981 m) relative to both the low (876 m and 920 m) and the intermediate elevation sites (1744 m and 2513 m). At the class level, a total of 11 classes were detected, with 10 having a relative abundance > 0.05%, while the remaining bacteria were merged into an “others” class. As shown in Fig. 1B, the proportion of Actinobacteria, Alphaproteobacteria and Gammaproteobacteria at each elevation was 45%, whereas Deltaproteabacteria, Acidobacteria_Subgroup_6, and Gemmatimonadetes were prevalent at low levels in most soil samples. At the genus level (Supplementary Fig. 1), 76 genera were detected in the research areas, with the dominant genera including norank_f_67-14_ o_Solirubrobacterales (5.72%), Rubrobacter (4.35%), Solirubrobacter (2.83%), Pseudonocardia (2.26%) and Bradyrhizobium (2.19%) and less than 0.01% of the bacterial genera were classified into others.

Bacterial community composition varies along elevation gradients

We next sought to analyze the differences in relative bacterial abundance at the phylum level among Transects 1–3 (Fig. 2). Significant differences in the relative abundance of Actinobacteria, Proteobacteria, Acidobacteria, Verrucomicrobia, Firmicutes, and Rokubacteria were detected in samples from the different transects (Fig. 2A). The relative abundance of Actinobacteria and Firmicutes in Transect 1 (48.64% and 1.89%, respectively) was significantly higher than in Transect 2 (38.43% and 1.49%, respectively) and Transect 3 (39.63% and 0.98%, respectively) (P < 0.001 for all). In contrast, relative abundance of Acidobacteria and Verrucomicrobia in Transect 1 was lower (8.54% and 1.51%, respectively) than in Transect 2 (11.48% and 4.38%, respectively) and Transect 3 (12.06% and 3.37%, respectively) (P < 0.001 for all).

The relative abundance of the dominant phyla in the bacterial communities did not differ significantly among the six elevation sites (S1047, S1071, S1277, S1423, S1580, and S1587) in Transect 1, whereas significant changes were observed in the Transect 2 and Transect 3 elevation sites. The relative Proteobacteria, Acidobacteria and Verrucomicrobia abundance at the high elevation sites in Transect 2 and 3 (i.e., S2903 and S2981 in Transect 2, S2075 and S2110 in Transect 3) were significantly higher than those at low elevation sites in both transects (S876 and S920 in Transect 2 and S1602 and S1661 in Transect 3), whereas the opposite trend was observed for Actinobacteria and Gemmatimonadetes.

Bacterial alpha diversity elevation patterns

A total of 3,335,681 quality sequences across all 66 soil samples were grouped into 5314 operational taxonomic units (OTUs) at a 97% similarity level. We identified significant relationships between soil bacterial richness, diversity, and elevation (Fig. 3). No dramatic changes in the elevational bacterial diversity patterns were observed between Transects 1, 2 and 3 (Fig. 3A–C,E–G). However, across the overall elevational gradient, the Chao1 and Shannon indices exhibited pronounced hump–shaped patterns (Fig. 3D,H).

The “peak” value in Chao1 richness was detected at lower elevations at around 1423 m, while a “hollow” pattern was evident at an intermediate elevation of 2513 m (Fig. 3D). Shannon diversity decreased from 876 to 2513 m (Fig. 3H), but increased from 2513 to 3070 m. These data suggest that soil bacterial community diversity conformed to a hump-shaped profile in the Mt. Tianshan grasslands.

Bacterial community beta diversity along elevational gradients

A nonmetric multidimensional scaling analysis (NMDS) was next conducted to evaluate overall bacterial community structure composition at the OTU level. As shown in Fig. 4A, two small-scale elevational samples (1047–1587 m and 1602–2110 m) were significantly separated from one another (ANOSIM test, stress = 0.0104, R = 0.19, P = 0.001), and the elliptical coverage of the broader-scale elevation samples (Transect 2, 876–3070 m) partially overlapped with that of these two small-scale elevation samples (Transect 1, 1047–1587 m and Transect 3, 1602–2110 m). Overall, there were significant changes in the bacterial community structure in the three analyzed transects (Fig. 4B–D), except for several neighboring sites in the 1602–2110 m elevation range (S1703, S1739, S2045, S2075, and S2110). This suggests that elevation had the most significant impact on bacterial community structure in the Mt. Tianshan grassland.

Relationships between bacterial community structures and environmental variables

Lastly, we evaluated correlations between environmental variables and the top 30 soil bacterial phyla, classes and genera using heatmaps, revealing that the soil bacterial community composition was simultaneously affected by climatic, plant, and soil physicochemical factors (Fig. 5; Supplementary Fig. 2). The relationships between the environmental variables and relative abundance varied as a function of bacterial phylum and class which are affected differently by environmental factors. As shown in Fig. 5, Spearman correlation analysis revealed that soil pH and mean annual temperature (MAT) were the key factors influencing on the relative abundances of the main bacterial phyla and classes. The abundance of Actinomycetes, Gemmatimonadetes and Chloroflexi phyla, as well as the diversity variables, were significantly and positively correlated with both soil pH and PET (Fig. 5A; Table 2; Supplementary Fig. 2; P < 0.001), while Acidobacteria, Proteobacteria and Verrucomicrobia showed significant negative correlations with soil pH and (potential evapotranspiration) PET. Additionally, at the genus level, MAT, PET, and pH were also significantly correlated with bacterial genera (Supplementary Fig. 2).

Source: Ecology - nature.com