Increased dependence on nitrogen-fixation of a native legume in response to alien plant invasion

Abstract Background and aims Inhibition of nutrient acquisition by roots or their symbiotic microorganisms, such as mycorrhizal fungi and rhizobium, is an important for the success of alien plants invasion. However, we know little about how the invasive and native plants interact regarding the ‘quantity’ and ‘activity’ of the strategies, both of which are essential for plant nutrient acquisition. Methods Here we conducted a pot experiment by monoculture and mixed plantings of an invasive plant, Xanthium strumarium, and a widespread native legume, Glycine max, and a suite of traits of fine roots and nodules as well as mycorrhizal colonization were measured. Results Compared with monoculture, fine root ‘quantity’ (biomass, and surface area) and ‘activity’ (root nitrogen (N) concentration and acid phosphatase activity) of G. max both decreased in mixture plantings; nodule ‘quantity’ (biomass) decreased by 45%, while nodule N-fixing ‘activity’ (nifH gene copies) increased by 106%; mycorrhizal colonization rate remained unchanged. Contribution of N-fixation to leaf N content (%Ndfa) in G. max increased after mixed plantings; this arised from lowered rhizosphere soil N of G. max in mixed planting. Increased root ‘quantity’ and ‘activity’ together with higher mycorrhizal colonization rate of X. strumarium was observed in the mixed planting. Conclusion Although the legume plant can produce plenty of N resource through N-fixing of nodules, the invasive plant does not scavenge nutrients directly from N-fixing nodules, but rather deplete rhizosphere soil N of the legume crop through root strategy of the invasive plant, thereby inhibiting native root strategy while enhancing N-fixation ‘activity’.