Declining Contribution of Nitrogen Deposition to the Interannual Variability of Terrestrial Gross Primary Productivity

Abstract

Global nitrogen deposition has steadily increased since the 1980s, peaking around 2015 before stabilizing. However, atmospheric chemical transport models often underestimate its magnitude, limiting accurate assessments of its impacts 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 data set, 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 . As nitrogen deposition has recently stabilized, its contribution to global GPP has shifted from a stimulatory effect to a source of negative fluctuations. This transition is attributable to the declining sensitivity of GPP to nitrogen deposition, potentially driven by increasing vegetation water stress. Climatic factors, primarily temperature and precipitation, dominate interannual GPP fluctuations across plant functional types (PFTs), and nitrogen deposition explains 8.7 ± 2.9% of global variability. Notably, nitrogen enrichment stimulated GPP in grasslands and croplands but had an inhibitory effect on tropical forests. Moreover, the nonlinear response of GPP to nitrogen deposition exhibited distinct optimal thresholds across PFTs, with the global vegetation optimum identified at 12.5 kg N ha ⁻¹  yr ⁻¹ . Crucially, the direct effect of nitrogen deposition on GPP outweighed its synergistic interactions with climate and CO ₂ concentrations, 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 predictions of ecosystem feedback to global change.