Sensitivity of Global Terrestrial Gross Primary Productivity to Nitrogen Deposition Changes

Global nitrogen deposition has escalated steadily since the 1980s, peaking around 2015 before stabilizing. However, global atmospheric chemical transport models often underestimate their magnitude, limiting the accurate assessment of their impact on terrestrial gross primary productivity (GPP). In this study, we elucidated the drivers of interannual GPP variability and quantified the contribution of nitrogen deposition from 1980 to 2020 using the latest global nitrogen deposition dataset, the TRENDY GPP products, and an interpretable machine learning framework (SHAP). Our findings revealed a consistent expansion in global GPP over the past four decades, averaging 156.95 ± 6.4 Pg C yr⁻¹. Intriguingly, although nitrogen deposition has recently plateaued, its relative influence on GPP has increased. Although climatic factors, primarily temperature and precipitation, dominate interannual GPP fluctuations across plant functional types (PFTs), nitrogen deposition explains 6.5% ± 3.6% of global variability. Notably, its impact is disproportionately pronounced in shrublands, savannas, grasslands, and croplands. Specifically, nitrogen enrichment stimulated GPP in grasslands and croplands but had an inhibitory effect in tropical forests. We identified a non-linear, hump-shaped response of vegetation to nitrogen loading, with an ecological threshold of 13.4 kg N ha⁻¹ yr⁻¹, beyond which the stimulatory effects diminished. Furthermore, the direct effect of nitrogen deposition on GPP outweighed its synergistic interactions with climate and CO₂, suggesting that nitrogen availability independently modulates terrestrial carbon sinks. This study underscores the biome-specific sensitivities to nitrogen loading and highlights the necessity of incorporating nitrogen saturation thresholds into the predictions of ecosystem feedbacks to global change.