Temporal shifts in microbial life-history strategies control ecosystem functioning under sustained nitrogen enrichment

Soil microorganisms play a central role in regulating terrestrial ecosystem functioning, yet their long-term responses to sustained atmospheric nitrogen (N) enrichment remain unclear. Here, we compiled a global dataset of 6,255 paired observations from 308 field-based N addition experiments to assess how ecosystem functionality and microbial properties respond across gradients of N input rate and experimental duration. Across ecosystems, N enrichment increased ecosystem functionality by 17.6% but reduced microbial biomass by 4.2%, with both effects strengthening under higher N inputs and longer exposure times. Spatially explicit meta-forest modelling revealed that long-term N enrichment elicited stronger ecosystem and microbial responses globally, with grid-scale variation primarily controlled by soil properties—especially soil pH, sand content, and bulk density—rather than by N deposition rates alone. Notably, we detected a temporal reversal in the relationship between microbial biomass and ecosystem functionality: positive under short-term N enrichment but increasingly negative over time. This shift likely reflects a transition in microbial life-history strategies, characterized by the replacement of oligotrophic (K-selected) taxa with copiotrophic (r-selected) taxa, leading to altered resource-use efficiency and declining microbial biomass. Our findings demonstrate that microbial biomass and life-history strategy shifts are critical determinants of long-term ecosystem functioning under sustained N enrichment, highlighting the dominant role of soil constraints in shaping ecosystem responses at the global scale.