The eastern Indian Ocean (EIO) is a major hotspot of oceanic nitrous oxide (N₂O) emissions. However, owing to sparse observations in time and space, its spatiotemporal variability and the mechanisms driven by the monsoon remain unclear. Based on three inter-monsoon observations conducted in 2019, 2023, and 2024, this study integrates Random Forest (RF) regression, generalized additive models (GAMs), and Optimum Multi-Parameter (OMP) mixing analysis to elucidate the distribution patterns, driving mechanisms, and air–sea fluxes of N₂O in the EIO. Our results reveal that the interannual variability of N₂O concentrations substantially outweighs the differences between monsoon transition periods, with apparent oxygen utilization (AOU) emerging as the most robust nitrification-related driver across all surveyed years. N₂O exhibits a pronounced vertical structure: the upper layer is dominated by nitrification (ΔN₂O/AOU = 0.09–0.10 nmol/μmol), whereas intense denitrification within the Bay of Bengal oxygen minimum zone (OMZ) drives ΔN₂O sharply upward to 40–80 nmol/L. In contrast, deep-layer concentrations are primarily governed by the historical accumulation of old water masses. The OMP analysis reveals that water mass transport governs the broader spatial distribution of concentrations. Meanwhile, in situ processes provide distinct regional modifications within specific water masses. The Indian Ocean Equatorial Water (IEW) emerges as a core hotspot for in situ production (5.6 ± 0.7 nM). Within its OMZ, measured concentrations peak at two to four times the values anticipated from conservative transport alone. After isolating the physical transport contributions, analysis indicates that approximately 53%–58% of the apparent nitrification efficiency signal originates from exogenous N₂O accumulated along water mass trajectories. Only 42%–47% of this signal reflects genuine in situ nitrification Surface waters remained persistently supersaturated with N₂O (116%–124%), yielding an area-weighted mean air–sea flux of 2.2 ± 2.5 µmol/m²•d, with annual emissions from this region accounting for approximately 2.9% ± 3.4% of the global marine total. This study provides systematic multi-cruise, multi-method observational constraints on the N₂O source-sink dynamics and climate-driven mechanisms in the EIO, offering an important quantitative basis for assessing the N₂O budget of monsoon-influenced marine regions and their contribution to the global nitrogen cycle.
 

暂无