Loss of negative plant-soil feedback through fine-tuning of rhizosphere stoichiometry as a cause of invasiveness

Feedback with soil biota is an important determinant of species dominance and diversity in terrestrial plant communities. However, the factors regulating within-species variation in PSF, which varies from positive to negative among plant genotypes, remain uncertain. Here, we tested whether rhizosphere-mediated effects on nitrogen dynamics by invasive alligator weed could contribute to a positive (or less negative) PSF compared to native genotypes by conducting a conventional soil conditioning (8 weeks) and feedback (4 weeks) two-phase experiments. In the common garden, native genotypes showed negative feedback, whereas invasive genotypes displayed neutral feedback. Soil conditioned by the invasive genotypes had higher microbial C:N ratio and greater net N mineralization rate relative to the native genotypes. These differences have a strong direct effect on PSF, and also affect PSF indirectly via a significant cascade effect on N availability and rhizosphere stoichiometry at the feedback phase. These results suggest that invasive alligator weed has evolved more acquisitive nitrogen (N) economics strategies to release from negative plant-soil feedback regulation by enhanced rhizosphere effects on N cycling, emphasizing the importance of disentangling how modifications of the environment affect species interactions. This experimental approach can shed new light on how interspecies interactions scale up to community level processes in natural environments.