Response of bacterial communities to shrub encroachment and forage planting in alpine grassland of the Qinghai-Tibetan Plateau

Abstract

Shrub encroachment and conversion of grazing land to forage cultivation are the main disturbance factors at alpine grasslands in the Qinghai-Tibetan plateau. Yet our understanding of soil bacterial communities impacts of these disturbances is limited. Using shrub (Potentilla fruticose) encroachment systems and forage lands (Medicago sativa lands, and Elymus sibiricus lands) as a model, we investigated the effects of shrub encroachment and land use on bacterial communities in alpine grass soil at the Eastern edge of the Qinghai-Tibetan plateau of China. Soil bacterial communities were examined using high-throughput sequencing of the 16S rRNA gene (V4-V5 region, Illumina MiSeq). Soil properties of TN and SOC were tested using an Elementar Variomax CNS Analyser (Elementar Corp., Germany). WEOC concentrations were determined following Ma's procedures, to answer the following hypotheses: i) shrub encroachment and forage cultivation would reduce soil organic carbon content, ii) the bacterial communities in shrub encroachment and forage lands are less diverse than those of alpine grassland, and iii) the occurrence of extensive mutualism or commensalism in alpine grassland, and competition or mutual exclusion in shrub encroachment and forage land. In this study, we found that SOC and TN decreased on shrub encroached and forage lands, supporting our first hypothesis. In total, 56 bacterial phyla were identified across the sampling sites. The dominant phyla at the four plots, Proteobacteria, Actinobacteria, Acidobacteria, Bacteroidetes and Chloroflexi, accounted for 27–28%, 13–30%, 13–26%, 4–8% and 4–9% of the total population, respectively. Contrary to our expectation, alpha-diversities (phylogenetic diversity, Operational Taxonomic Unit (OTU) richness, Shannon, and Chao1) were consistently higher in disturbed soils than in alpine grassland, and the highest appeared in Medicago sativa lands compared to other sites. Soil texture and soil organic carbon together explained 30% of the total variations of bacterial communities, indicating that bacterial habitat (e.g. soil texture) and substrate (soil organic carbon) were affected in shrub encroachment and forage lands. Copiotrophs (Actinobacteria and Proteobacteria) and Nitrospirae were enriched in alpine grassland, while oligotrophs (Acidobacteria) were concentrated in shrub encroached and forage lands, indicating that shrub encroachment and conversion of alpine grassland to forage land may alter bacterial life history strategies. Through the phylogenetic molecular ecological network approach, we found that the bacterial network of alpine grassland was more complex than that of shrub and forage lands. Furthermore, the bacterial network of alpine grassland was more modular, and the taxa tended to co-occur, with positive associations accounting for 95% of all potential interactions. By contrast, negative interactions appeared more in shrub encroached (34%) and forage land (33%) compared to alpine grassland. This indicated that bacterial species have greater niche-sharing and more collaboration in alpine grassland than in shrub and forage lands. In conclusion, our findings revealed that shrub encroachment and conversion of alpine grassland to forage increased competition of bacterial species for substrates, with potential for serious consequences such as elevating losses of soil carbon.