Effects of soil bacterial community composition and function on tree seedling establishment during subalpine forest succession

Plant‒soil microbe interactions play a significant role in community dynamics. While bacteria play important roles in driving plant‒soil feedback (PSF), which regulates grassland succession, it remains unclear how bacteria-mediated PSF influences tree seedling establishment during forest succession. A pot experiment of four tree seedlings (fast-growing broadleaf Betula platyphylla and Betula albosinensis, and slow-growing coniferous Picea asperata and Abies faxoniana), using sterilized soil inoculated with rooting zone soils from dominant plant species at the early, mid- and late-successional stages in a subalpine forest, was conducted in a greenhouse. Plant biomass, soil bacterial communities, and soil chemical properties were measured. Broadleaf experienced stronger negative biotic feedbacks, while coniferous generated more positive biotic feedbacks, especially in mid- and late-successional inoculated soils. Controlled pot experiments demonstrate that broadleaf biomass was correlated positively with the relative abundance of oligotrophic bacteria but negatively with copiotrophic bacteria, while this trend was opposite in coniferous. Oligotrophs, namely, Alphaproteobacteria (nitrogen-fixing bacteria) and Chloroflexi (nitrite oxidizing bacteria), were the dominant influential factors of the variations in broadleaf and coniferous biomass, respectively. These results indicate that the benefit of bacteria on tree seedling establishment is species specific. It can be inferred that plant nutritional requirements and pathogen resistance among plants with different life-history strategies may alter the composition and function of bacteria, thus alleviating soil nutrient limitation and plant‒soil microbe nutrient competition. Our findings provide new insights into the potential role of bacterial functional groups in shaping the PSFs of late-successional tree seedlings.