As ecosystem engineers, Nitrogen(N)-fixers can increase the amount of available N in soils and are the primary source of natural N in terrestrial ecosystems. Most studies assume that N-fixers facilitate the growth of non-fixing plants, but the magnitude of this interaction can vary across ecosystems. Several recent studies have found that this interaction can become a competitive interaction. Ecological factors such as nutrient availability, N-fixer growth habit, and if the N-fixer is native or invasive are key to understanding the direction and magnitude that N-fixers have on the plant community. To understand the effects of N-fixers across ecological contexts, we conducted a weighted random effects meta-analysis to (1) determine if the data support our current understanding that N-fixers are generally facilitators and (2) understand how this dynamic can change in different ecological contexts using three response variables: species-level biomass, soil N, and plant community diversity. Effect sizes were calculated using the natural log response ratio to compare the means of a control group in which N-fixers were absent to the treatment in which N-fixers were present. Moderators such as soil type, N-fixer growth habit, and invasive status were included to understand the importance of certain abiotic and biotic factors.
Results/Conclusions
The overall model had a significant effect size (ln(RR) = 0.2934 , SE = ±0.0592, p < 0.0001) suggesting that in general, N-fixers have a positive influence on the plant community. Breaking this down by response type, introducing N-fixers into the plant community had a positive effect on species-level biomass (ln(RR) = 0.5116, SE = ±0.0744, p < 0.0001) and soil N (ln(RR) = 0.2068, SE = ±0.1178, p = 0.0097), but a negative effect on plant community diversity (ln(RR) = -0.4058, SE = ±0.1686, p < 0.0001). Soil type was only a significant moderator for biomass and soil N. In loamy soils, N-fixers had a positive effect on biomass, but a negative effect on soil N. In sandy soils, N-fixers had a negative effect on biomass, but a positive effect on soil N. N-fixer growth habit and invasive status were not significant moderators for any response types. The results of this meta analysis suggest that there are ecological controls that influence the magnitude and direction of how N-fixers effect the plant community. N-fixers largely promote the growth of the plant community, confirming the dominant theory, but this is contingent upon the ecological context in which these interactions are take place.