Reassessing Reactive Nitrogen Saturation Thresholds in Temperate Forest Soils: Decoupling Nitrification Kinetics from Mycorrhizal Nitrogen Retention Under Chronic Atmospheric Deposition

Abstract

Chronic atmospheric nitrogen (N) deposition has conventionally been assessed through the lens of critical load exceedance, yet emerging biogeochemical evidence challenges the predictive validity of soil N saturation thresholds in temperate forest ecosystems. This study integrates high-resolution 15N isotope tracing, enzyme activity assays (leucine aminopeptidase, urease), and metatranscriptomic profiling of ectomycorrhizal communities across twelve long-term deposition gradient plots in Central Europe and East Asia. Our findings demonstrate that gross nitrification rates decouple systematically from net nitrate leaching when ectomycorrhizal biomass exceeds a critical hyphal density index of 0.74 g m⁻². These results invalidate unilinear N saturation models and demand recalibration of critical load frameworks embedded in UNECE and EU Air Quality Directives. Policy-relevant implications for ecosystem nitrogen retention modeling are discussed.