Invasive plant soil legacies shape microbial function and community organization under short-term carbon and nitrogen amendments

Plant invasion can modify soil microbial communities and ecosystem processes through plant-soil feedbacks, yet it remains unclear whether these effects are expressed mainly through taxonomic turnover or through shifts in microbial function and interaction structure. We tested how soil legacy generated by the invasive Conyza bonariensis, the native Helminthotheca echioides, or unplanted control soil influenced short-term microbial responses to standardized amendments and plant-derived inputs. In Experiment 1, conditioned soils were amended with water, cellulose, or ammonium and analyzed for extracellular enzyme activity, qPCR-based gene abundance, bacterial community composition, and family-level co-occurrence networks. In Experiment 2, the same soil legacies were exposed to water, glucose, or sterile root exudates from native or invasive plants. Native- and invasive-conditioned soils differed significantly in composition, but they were not consistently distinguished by strong indicator taxa, indicating that legacy effects were expressed mainly through redistribution of shared taxa rather than community turnover. In contrast, functional responses were clearer: enzyme activity and nirS abundance showed strong soil-legacy dependence, and network analysis revealed that invasive-conditioned soil supported a denser, more positive, and more compact family-level association structure than native-conditioned soil. In Experiment 2, invasive root exudates produced stronger short-term functional-based differentiation among soil legacies than native exudates, especially for extracellular enzymes. Together, the two experiments show that plant invasion can leave a persistent belowground legacy that is expressed primarily through functional filtering and network rewiring of a broadly shared microbiome, rather than through major taxonomic turnover alone.