Spatiotemporal Decomposition of Reactive Nitrogen Flux Gradients Using Coupled Eddy-Covariance and Isotopic Ratio Mass Spectrometry in Humid Subtropical Agroecosystems

Abstract

Reactive nitrogen (Nr) deposition to humid subtropical agroecosystems represents a critical yet underquantified driver of terrestrial biogeochemical perturbation. This study presents a coupled methodological framework integrating high-frequency eddy-covariance flux partitioning with dual-isotope ratio mass spectrometry (δ¹⁵N, δ¹⁸O) to decompose Nr flux gradients across dry, wet, and occult deposition pathways at four long-term monitoring sites in southeastern China and southern Brazil. Continuous turbulent exchange measurements were post-processed using planar-fit coordinate rotation and Webb–Pearman–Leuning density corrections. Isotopic source apportionment resolved agricultural NH₃ volatilization, vehicular NOₓ, and biomass-burning contributions at fractional precisions below 4.7%. Results demonstrate a statistically significant (p < 0.01) seasonally modulated Nr dry-deposition dominance during monsoonal transition periods, with ecosystem nitrogen saturation indices exceeding critical load thresholds by 2.3–3.8-fold. The framework provides reproducible, transferable protocols for continental-scale nitrogen budget reconciliation.