Plant‒Soil-Microbial Interactions Mediate Vegetation Succession in Retreating Glacial Forefields

Global warming is accelerating glacial retreat and vegetation succession in young developing soils. Soil microbial communities interact with plants, and they are key to vegetation succession, but the specific microbial groups that control plant‒soil feedback (PSF) are unclear. Vegetation and microbial successions were investigated in young soils after a glacial retreat in the Gongga Mountains. The direction and intensity of PSF were quantified by comparing the biomass of pioneer plants in early- (5-10 years), mid- (30-40 years) and late- (80-100 years) succession soils. The plant‒soil interactions shifted from negative feedback between plants and microorganisms in the early stages to positive feedback in the middle and late stages of soil development in the glacial retreat area. In early-stage soils, microorganisms nearly impaired the growth of pioneer plants present in all successional stages, showing negative biotic legacies. PSF and biotic legacies facilitated plant competitiveness. Poplar trees, which have strong competitive abilities, grow well in the soils of all succession stages because their rapid growth and rapid retrieval of soil nutrients prevents the invasion of other pioneer plants. The strongest drivers of soil legacy changed i) from saprophytic fungal specialists during the early stage, ii) to generalists and specialists of bacteria and arbuscular mycorrhizal fungi (AMF) in the middle stage, iii) to ectomycorrhizal fungal specialists in the late stage. The turnover of AMF and fungal pathogens across successional stages contributed to PSF. The effects of biotic legacies and microbial turnover accelerated primary succession and intensified PSF in the young soils of the glacial retreat area.