Global change reshapes native-invasive plant competition through shifts in rhizosphere enzyme investment and soil microbial responses

Biological invasions are increasingly influenced by global change, yet how environmental drivers modify competitive interactions between invasive and native plants is still not well understood. We tested how warming, elevated CO₂, and nitrogen enrichment affect competition between the invasive Conyza bonariensis and the native Helminthotheca echioides , using rhizosphere microbial responses as a lens for interpreting competitive outcomes. Plants were grown alone or in competition under elevated temperature, elevated CO₂, and ammonium-nitrate fertilization. We measured plant growth traits together with rhizosphere extracellular enzyme activities, soil physicochemical properties, and abundances of nitrogen-cycling marker genes. To relate belowground function to plant performance, we calculated a growth-normalized Specific Rhizosphere Index (SRI) and a biomass-normalized Specific Enzyme Activity (SEA). Responses were strongly driver- and context-dependent. Elevated CO₂ most clearly enhanced invasive performance, especially leaf production, whereas warming effects emerged mainly under competition. Fertilization caused comparatively modest changes in plant growth. Belowground responses were strongly shaped by soil conditioning history: native- and invasive-conditioned soils generally showed higher enzyme activities than control and shared competition soils, while elevated CO₂ increased N-acetyl-β-D-glucosaminidase activity mainly in invasive-conditioned soils and increased nirS abundance across soil types. Although raw enzyme activities changed only modestly under some treatments, SRI and SEA revealed shifts in the coupling between rhizosphere function and plant growth across solitary and competitive growth contexts. These findings suggest that soil and rhizosphere responses may contribute to how global change reshapes native-invasive competitive balance.