Uneven decline and role of long-range atmospheric transport of global wet nitrogen deposition

Atmospheric wet nitrogen deposition (NDep_wet) represents the key pathway through which anthropogenic nitrogen emissions are transferred to terrestrial ecosystems. However, global assessments remain limited by sparse observations and strong regional variability in emissions, atmospheric chemistry, and transport. To overcome these constraints, we developed a global ensemble machine-learning framework to generate annual NDep_wet estimates for all terrestrial regions from 2005 to 2019 by integrating satellite-derived reactive nitrogen concentrations, meteorological fields, and data from major ground-based monitoring networks. The model achieved strong predictive performance (R² > 0.8), enabling a consistent reconstruction of global deposition trends. 

Globally, NDep_wet declined from 61.24 Tg N yr⁻¹ in 2005 to 52.31 Tg N yr⁻¹ in 2019 (−14.6%), driven mainly by reductions in NOₓ emissions. Yet this decline was highly uneven. Developed regions reduced NOₓ emissions by 26% and NH₃ by 5%, but achieved only ~15% reductions in NDep_wet , revealing a clear decoupling between emission controls and deposition outcomes. In contrast, developing regions exhibited minimal declines (−3.4% in Africa; −0.6% in India) or slight increases (+0.8% in South America), reflecting continued emission growth and shifts in atmospheric circulation that enhanced cross-boundary nitrogen transport. 

Trajectory-derived backward/forward ratios further revealed changes in each region’s role as a net importer or exporter of reactive nitrogen. Africa and India showed sharp decreases in these ratios (Africa: 0.94→0.36; India: 1.16→0.88), indicating a transition toward export-dominated regimes and reduced sensitivity of NDep_wet to domestic emissions. Across most regions, only 18–35% of deposition originated from local emissions, implying that long-range transport is the dominant driver of NDep_wet. 

These findings demonstrate that regional emission controls alone cannot effectively reduce nitrogen deposition when transboundary imports remain high. Effective mitigation will require internationally coordinated emission reductions and targeted support for developing regions where emissions continue to rise.