Turncoat? Native Nitrogen-Fixing Legumes Facilitate Plant Invasion via Soil Microbes and Nitrogen Transfer

Nitrogen deposition often promotes plant invasions and can arise from multiple sources. However, it remains unclear whether native legumes facilitate invasive plants directly through biotic nitrogen transfer or indirectly via soil microbial mediation. It is also unknown how exogenous nitrogen addition influences these facilitative effects. To address these questions, we combined evidence from a greenhouse experiment and global meta-analysis. The greenhouse experiment tested the effects of neighboring plants (a native nitrogen-fixing legume Albizia julibrissin; a native non-nitrogen-fixing legume Styphnolobium japonicum; or a conspecific) on the growth of invasive Rhus typhina and its native congener R. chinensis. Within-pot membrane treatments were used to obstruct microbial and nitrogen movement between focal and neighboring plants, with and without added nitrogen. The total biomass of invasive R. typhina increased when grown with the nitrogen-fixing legume compared with conspecific neighbors, but not when grown with the non-nitrogen-fixing legume. Positive effects of the nitrogen-fixing legume on R. typhina were suppressed when nitrogen transfer or soil microbial movement was obstructed by membranes. Exogenous nitrogen addition did not alter the growth of invasive R. typhina or its interactions with legumes. The biomass of native R. chinensis did not differ among neighboring plant treatments, indicating that facilitation by legumes favored the invasive species. The global meta-analysis of 268 effect sizes from 46 studies of the effects of legumes on neighboring plants also revealed that native legumes enhanced invasive plants' performance. Together, our findings recognize nitrogen-fixing legumes as overlooked facilitators of plant invasion, operating through both nutrient and microbial mediated pathways. These findings highlight a paradox in which native species can act as "turncoats," accelerating invasion and reshaping plant communities under global change.