Plant traits explain variation in symbiotic nitrogen fixation responses to global nitrogen enrichment: a meta-analysis

Anthropogenic nitrogen enrichment is widely expected to suppress symbiotic nitrogen fixation in terrestrial ecosystems. Nevertheless, observed symbiotic nitrogen fixation responses remain incompletely explained by exogenous nitrogen inputs, climate, and edaphic factors. In this meta-analysis, we integrate 908 globally distributed field measurements to identify the key predictors that improve simulation of symbiotic nitrogen fixation responses to nitrogen enrichment. On average, symbiotic nitrogen fixation declines by 33.0% upon nitrogen enrichment, with the reduction being more pronounced in non-croplands than croplands. Models considering only environmental factors overestimate symbiotic nitrogen fixation decline relative to observations. The better performance of plant traits like plant growth and biomass allocation (shoot:root ratio) partially buffer symbiotic nitrogen fixation suppression under nitrogen enrichment. Integrating both environmental factors and plant performance traits improves predictive accuracy of symbiotic nitrogen fixation responses by 42.7% and brings the simulated symbiotic nitrogen fixation reductions into close agreement with observations. The alterations in plant performance traits are thus critical for explaining variability in terrestrial symbiotic nitrogen fixation responses, and incorporating plant trait dynamics in Earth System Models can quantitatively partition the compensatory symbiotic nitrogen fixation supported by nitrogen-fixing plant growth from the direct negative impact of nitrogen inputs. Anthropogenic nitrogen enrichment reduces symbiotic nitrogen fixation, but environmental factors alone cannot explain this response. This meta-analysis shows that integrating both environmental factors and plant performance traits improves the predictive accuracy of symbiotic nitrogen fixation responses by 42.7%.