Shift in soil microbial communities along ∼160 years of natural vegetation restoration on the Loess Plateau of China

Natural restoration of vegetation has been widely implemented as an effective strategy for recovery of degraded ecosystems. However, how soil microbial communities vary with natural restoration of vegetation and associated drivers remains unclear. Here, we investigated the changes in soil microbial communities at 0-60 cm soil depths along-160 years of natural restoration of vegetation from farmland to pioneer weeds, to herbs, to shrublands and early forests, and finally to climax forests on the Loess Plateau of China. Our phospholipid fatty acids (PLFA) analysis showed that natural restoration of vegetation largely enhanced the abundances of the total PLFA, the bacterial, fungal, Gram-positive (G(+)) bacterial, Gram-negative (G(-)) bacterial, arbuscular mycorrhizal fungal (AMF), actinomycete, monounsaturated, branched, and saturated straight-chain (SSC) PLFA in the surface soil layer (0-20 cm). However, its effect was negligible on the deep soil layer (20-60 cm). The biomass of these microbial communities declined sharply along with soil depth at each of the restoration stage. Natural restoration of vegetation significantly changed composition proportion of soil microbial communities. In particular, the ratio of G(+): G(-), proportions of G(+) bacterial PLFA, and AMF PLFA in the total PLFA gradually decreased, while the proportions of bacterial PLFA and G(-) bacterial PLFA in the total PLFA gradually increased in the surface soil layer along vegetation restoration stages. Our finding suggested that natural restoration of vegetation altered the biomass and composition proportion of soil microbial communities, which was strongly driven by variations in soil nutrient substrates and physiochemical properties (e.g., soil moisture and pH), as well as litter and root biomass along the long gradient of vegetation. This study revealed that natural restoration of vegetation was an effective strategy for reestablishment of soil microbial communities. Additionally, the impacts of natural restoration of vegetation on soil microbial communities were primarily concentrated in the surface soil layer rather than deep soil layer, and soil microbial biomass reached maximum in the climax forests.