Soil Organic Carbon Depletion and Microbial Community Restructuring Under Chronic Nitrogen Deposition in Temperate Deciduous Forest Ecosystems: Evidence from a 12-Year Mesocosm Experiment

Abstract

Chronic atmospheric nitrogen (N) deposition exerts profound yet heterogeneous effects on soil organic carbon (SOC) dynamics and belowground microbial assemblages in temperate deciduous forests. This study integrates 12 years of manipulative mesocosm data from a mixed Quercus–Fagus stand to quantify SOC mineralization rates, fungal-to-bacterial biomass ratios, and extracellular enzyme stoichiometry under simulated N loading gradients (0–60 kg N ha⁻¹ yr⁻¹). Phospholipid fatty acid (PLFA) profiling and metagenomic sequencing revealed a significant suppression of ectomycorrhizal fungal guilds and a concomitant shift toward copiotrophic bacterial dominance at deposition rates exceeding 30 kg N ha⁻¹ yr⁻¹. SOC stocks declined by 18.4% in high-N treatments, driven primarily by enhanced oxidative enzyme activity. These findings underscore critical thresholds for N-mediated destabilization of forest carbon sequestration and inform deposition-sensitive ecosystem management frameworks.