Nitrogen Enrichment Reverses Microbial Biomass-Function Relationships Over Time in a Global Meta-Analysis.

Soil microbial communities are key regulators of terrestrial ecosystem functions, yet their long-term responses to atmospheric nitrogen (N) enrichment remain poorly understood. Here, we synthesized a global dataset of 6255 paired observations from 308 field-based N addition experiments to evaluate how ecosystem functionality and microbial attributes respond to N enrichment across varying addition rates and durations. Multivariate linear mixed-effects models revealed that N enrichment enhanced ecosystem functionality by 17.6%, while reducing microbial biomass by 4.2%, with both effects intensifying under higher N addition rates and longer experimental durations. Meta-forest modelling generated high-resolution global maps that further demonstrated stronger ecosystem and microbial responses under long-term N enrichment. At the grid scale, spatial variation in these responses was primarily governed by edaphic factors, particularly soil pH, sand content, and bulk density, rather than by N deposition rates. Notably, we identified a temporal reversal in the relationship between microbial biomass and ecosystem functionality, which was positive in the short term but became increasingly negative in the long term. This transition likely reflects a shift in microbial life-history strategies, where oligotrophic taxa (K-strategists) are progressively replaced by copiotrophic taxa (r-strategists), with consequences for resource-use efficiency and microbial biomass. Together, our findings highlight microbial biomass as a pivotal determinant of long-term ecosystem functionality and underscore the importance of microbial life-history strategy shifts and soil constraints in shaping ecological responses to sustained N enrichment.