Temporal shifts in root exudates driven by vegetation restoration alter rhizosphere microbiota in Robinia pseudoacacia plantations.

Plant-soil-microbiota interactions mediated by root exudates regulate plant growth and drive rhizosphere microbial feedbacks. It remains unknown how root exudates affect rhizosphere microbiota and soil functions in the course of forest plantation restoration. The metabolic profiles of tree root exudates are expected to shift with stand age, leading to variation in rhizosphere microbiota structure, and in turn, potentially altering soil functions. To unravel the effects of root exudates, a multi-omics study was conducted using untargeted metabonomic profiling, high-throughput microbiome sequencing and functional gene array. The interactions among root exudates, rhizosphere microbiota and nutrient cycling-related functional genes were explored under 15- to 45-year-old Robinia pseudoacacia plantations in the Loess Plateau region of China. Root exudate metabolic profiles, rather than chemodiversity, markedly changed with an increase in stand age. A total of 138 age-related metabolites were extracted from a key module of root exudates. The relative contents of six biomarker metabolites, such as glucose-1-phosphate, gluconic acid and N-acetylneuraminic acid, increased distinctly over time. The biomarker taxa (16 classes) of rhizosphere microbiota varied in a time-sensitive manner, which played potential roles in nutrient cycling and plant health. Nitrospira, Alphaproteobacteria and Acidobacteria were enriched in the rhizosphere of older stands. Key root exudates influenced functional gene abundances in the rhizosphere via direct effects or indirectly through biomarker microbial taxa (e.g., Nitrososphaeria). Overall, root exudates and rhizosphere microbiota are essential for soil function maintenance in R. pseudoacacia plantation restoration.