Nitrogen-fixing plants increase soil nitrogen and neighboring plant biomass, but decrease community diversity: A meta-analysis reveals the mediating role of soil texture

Several recent regional studies have cast doubt on the widespread assumption that nitrogen-fixing plants (N-fixers) act as facilitators of neighboring plant communities. We conducted a meta-analysis to synthesize the effects of N-fixers on plant communities and to understand how ecological context moderates these effects. We analyzed studies that assessed paired effects of N-fixers and non-fixers on soil N, neighboring-plant (non-fixer) biomass, and plant community diversity; ecological moderators included climate, soil texture, and N-fixer growth form and invasive status. N-fixers led to higher soil N and neighboring plant biomass, but lower community diversity compared to non-fixers. The effect of N-fixers on neighboring plant biomass was strongly mediated by soil texture; N-fixer invasive status and growth form were also significant mediators of the facilitative effects of N-fixers. N-fixer effects lie on a continuum between facilitation and suppression that is moderated by intrinsic and extrinsic processes, and this analysis provides insight into how these factors moderate the effects of N-fixers. Overall, N-fixers facilitate neighbor biomass but suppress diversity, though high variation in these effects can be explained in part by ecological context.