Elevation rather than season determines the assembly and co-occurrence patterns of soil bacterial communities in forest ecosystems of Mount Gongga.

Seasonal dynamics of soil microbial communities may influence ecosystem functions and services. However, few observations have been conducted on the dynamics of a bacterial community assembly across seasons in different elevations in mountain forest ecosystems. In this study, the diversity, compositions, community assembly processes, and co-occurrence interactions of soil bacterial communities were investigated using Illumina sequencing of 16S rRNA genes across different seasons during two consecutive years (2016 and 2017) at two elevational sites in Mount Gongga, China. These two sites included an evergreen broad-leaved forest (EBF, 2100 m a.s.l.) and a dark coniferous forest (DCF, 3000 m a.s.l.). The results showed that bacterial diversity and structure varied considerably between the two elevational sites with only limited seasonal variations. Interannuality had a significant effect on the diversity and structure of soil bacterial communities. The bacterial alpha diversity was significantly higher at site EBF(e.g., OTUs richness, 2207 ± 276) than at site DCF(e.g., OTUs richness, 1826 ± 315). Soil pH, temperature, elevation, and water content were identified as important factors shaping soil bacterial communities in the mountain forests. Bacterial community assembly was primarily governed by deterministic processes regardless of elevation and season. Deterministic processes were stronger at site DCF than at EBF. The soil bacterial community at site EBF harbored a more complex and connected network with less resistance to environmental changes. Overall, this study showed that seasonal dynamics of bacterial communities were much weaker than those along elevations, implying that a single-season survey on a bacterial community along an elevational gradient can represent overall changes in the bacterial community. KEY POINTS: • Seasonal dynamics of soil bacterial communities were studied in Mount Gongga. • The bacterial community was mainly affected by elevation rather than season. • Deterministic processes dominated bacterial community assembly. • The bacterial network was more complex but less stable at EBF than at DCF.