Global Patterns of Oceanic Nitrogen Fixation Over the Past Two Decades

Abstract

Oceanic nitrogen fixation is a major external source of bioavailable nitrogen, sustaining marine primary production and regulating the global carbon cycle. However, its long‐term variability and climate‐driven mechanisms remain poorly constrained because observations are sparse. Here, we develop a global, high‐resolution (0.25° × 0.25°), long‐term (1997–2024) data set of marine N ₂ fixation rates by establishing an ensemble machine‐learning model with in situ measurements and other multi‐source data. The reconstructed data set reveals pronounced spatial heterogeneity, with elevated rates concentrated in oligotrophic tropical and subtropical oceans (30°S–30°N), particularly in the northern Indian Ocean and the western Pacific‐Maritime Continent region, where rates frequently exceed 160 μmol N m ⁻²  d ⁻¹ . Globally, N ₂ fixation shows a weak but persistent increasing trend (∼2.8 × 10 ⁻²  μmol N m ⁻²  d ⁻¹  month ⁻¹ ), with weak background changes superimposed by localized hotspots of increases and decreases. Basin‐scale analysis reveals heterogeneous trends, with the strongest increases in the equatorial Atlantic and North Atlantic, and weak declines in the northern Indian Ocean and South Atlantic. Climate‐mode attribution indicates regionally heterogeneous controls, with Western Hemisphere Warm Pool (WHWP), North Pacific Index (NP), and Niño 3.4 Index (Niño 3.4) emerging as dominant drivers across major oceanic subregions. WHWP dominates most Northern Hemisphere basins and the equatorial Atlantic, Niño 3.4 dominates the equatorial Pacific, and NP dominates the South Pacific and South Atlantic. This study provides a long‐term global N ₂ fixation data set and quantitative evidence for its sensitivity to major climate modes, improving understanding of climate‐driven regulation of the oceanic nitrogen cycle.