Nitrogen addition alters litter chemical traits to regulate decomposition: A meta-analysis.

Litter chemical traits critically regulate decomposition dynamics and are highly responsive to atmospheric nitrogen (N) deposition, yet the magnitude of their responses and their mechanistic roles in decomposition under N enrichment remain uncertain. Here, we synthesized data from 80 studies to quantify the effects of N addition on litter chemistry and decomposition rates. Our analysis reveals that N addition significantly increases litter N (+34.6 %) and phosphorus (P, +18.5 %) concentrations, while reducing lignin (-2.2 %), cellulose (-2.2 %), and hemicellulose (-2.7 %). Moreover, key stoichiometric ratios-C/N (-23.8 %) and lignin/N (-25.4 %)-decreased, thereby enhancing litter quality. These shifts were more pronounced in herbaceous plants and grassland ecosystems compared to woody plants and forests. Nitrogen enrichment accelerated decomposition of herbaceous plant litter in both common-site and in-situ experiments but had no significant effect on woody plant litter. Decomposition rates (k) in herbaceous plant litter correlated strongly with initial traits-including N, phosphorus (P), calcium (Ca), cellulose, and stoichiometric ratios-whereas woody litter decomposition depended primarily on Ca. Our findings reveal that plant functional types and ecosystems govern decomposition responses to N deposition: grasslands exhibit accelerated decomposition via improved litter quality, while forests exhibit attenuated or even negative decomposition responses due to microbial suppression and inhibition of lignin degradation under. Integrating these trait-mediated mechanisms into biogeochemical models will refine predictions of carbon and nutrient cycling under global N enrichment, particularly in contrasting grassland and forest ecosystems.