Invasive species often achieve growth success and competitive dominance by accessing and using nutrient resources and pools unavailable to native species, fundamentally changing plant community structure and abiotic conditions within their naturalized ranges. Nitrogen-fixing species that colonize new habitats often initially sustain high growth rates by exploiting nitrogen resources unavailable to other plants. Their sustained presence can alter soil chemistry and create artificially high-nitrogen environments that facilitate the success of other invaders. Trifolium repens, T. pratense, and T. arvense are non-native, nitrogen fixing forbs ubiquitous in old fields and meadows in the Northeast, a region with few native nitrogen-fixing species. Between 2008 and 2010, we performed three controlled field experiments to assess the effect of Trifolium spp. on naturally occurring plant communities. We also conducted a greenhouse experiment in which we factorially crossed the presence or absence of Trifolium conditioned soil, additional T. repens litter, and the presence of T. repens plants within the pots in order to elucidate the mechanisms by which Trifolium increases soil nitrogen concentrations and impacts plant competition. We used the growth of invasive Rumex acetosella and native Schizachyrium scoparium as response variables.
Results/Conclusions
Field plots from which we removed all present Trifolium cover maintained a larger proportion of native cover and biomass than plots from which we removed a corresponding cover of non-nitrogen fixing non-natives. Pots containing either Trifolium-conditioned soil or T. repens litter contained more R. acetosella stems and a larger R. acetosella/S. scoparium biomass ratio. R. acetosella plants within pots that received conditioned soil or T. repens plant treatments maintained a higher density of vesicular-arbuscular mychorrizal nodules on their roots. The results reinforce trends in the literature that suggest that the presence of nitrogen fixing species can facilitate the competitive dominance of non-native and invasive groups at the expense of their native counterparts. The increased density of VAM nodules in the presence of actively fixing plants or treated soil indicate a relationship between active nitrogen fixation and the recruitment of mychorrizal associations on R. acetosella. The extra nodules on the invader may provide a phosphorus source for the nitrogen fixer, which returns an elevated nutrient supply for the invader, forming a positive feedback loop that facilitates the growth success of both non-natives.