Increased dependence on nitrogen-fixation of a native legume in competition with an invasive plant

Suppression of roots and/or their symbiotic microorganisms, such as mycorrhizal fungi and rhizobia, is an effective way for alien plants to outcompete native plants. However, little is known about how invasive and native plants interact with the quantity and activity of nutrient-acquisition agents. Here a pot experiment was conducted with monoculture and mixed plantings of an invasive plant, Xanthium strumarium, and a common native legume, Glycine max. We measured traits related to root and nodule quantity and activity and mycorrhizal colonization. Compared to the monoculture, fine root quantity (biomass, surface area) and activity (root nitrogen (N) concentration, acid phosphatase activity) of G. max decreased in mixed plantings; nodule quantity (biomass) decreased by 45%, while nodule activity in N-fixing via rhizobium increased by 106%; mycorrhizal colonization was unaffected. Contribution of N fixation to leaf N content in G. max increased in the mixed plantings, and this increase was attributed to a decrease in the rhizosphere soil N of G. max in the mixed plantings. Increased root quantity and activity, along with a higher mycorrhizal association was observed in X. strumarium in the mixed compared to monoculture. Together, the invasive plant did not directly scavenge N from nodule-fixed N, but rather depleted the rhizosphere soil N of the legume, thereby stimulating the activity of N-fixation and increasing the dependence of the native legume on this N source. The quantity-activity framework holds promise for future studies on how native legumes respond to alien plant invasions.