Phylogeny and intraspecific variability dominate nitrogen fixer biomass responses to nitrogen enrichment

Anthropogenic nitrogen (N) enrichment profoundly influences the growth of N-fixing plants, but the drivers underlying their highly variable biomass responses remain poorly quantified. Through a meta-analysis of 2,110 paired observations across 76 N-fixing plant species from 121 experiments, we demonstrate that N enrichment increases N-fixing plant biomass by 14.2% on average, with responses ranging from −99.4% to 1301.3%. Intraspecific variability (26.9%; i.e., variation among individuals within species) and plant phylogeny (21.1%) represent substantial yet frequently overlooked sources of uncertainty in biomass responses of N-fixing plants to N enrichment, even though environmental conditions (climatic and edaphic factors) and experimental design (N enrichment rate and duration) account for a significant portion of biomass response variation. Intraspecific variability, phylogeny, and experimental design collectively explain a larger proportion of variance in root than in shoot biomass responses, whereas environmental conditions show the opposite trend. These results provide quantitative evidence that phylogeny and intraspecific variability are indispensable for accurately predicting the response of N-fixing plants to global change. Nitrogen enrichment alters the growth of nitrogen-fixing plants, but responses vary widely. This meta-analysis shows that phylogeny and intraspecific variation are major contributors to that variability.