The elemental stoichiometry of particulate organic matter (POM) remineralization determines the magnitude and composition of regenerated nutrients in the ocean interior, thereby exerting a fundamental control on regional biogeochemical cycling. However, the stoichiometry of degradable POM remains poorly constrained because conventional estimates based on apparent oxygen utilization (AOU)-nutrient relationships are strongly biased by physical mixing and differential molecular diffusion.

Here we combine dissolved oxygen isotope (δ¹⁸O-DO) observations with an inverse biogeochemical model to quantify the stoichiometry of degradable POM in the 200–500 m twilight zone of the northern South China Sea (NSCS). The isotope approach constrains maximum apparent oxygen utilization (AOUmax), while the inverse model reconstructs the elemental composition of the degradable POM pool. We infer a degradable POM composition of approximately C104HxOyN14.5P1, substantially enriched in nitrogen and phosphorus relative to the global mean degradable POM. Consequently, remineralization of one unit of organic carbon releases proportionally more bioavailable nitrogen and phosphorus, indicating enhanced nutrient turnover in the NSCS twilight zone.

More importantly, the inferred degradable POM exhibits an N ratio of 14.5:1, lower than the canonical Redfield ratio (16:1). This stoichiometry agrees remarkably well with the observed nitrate-to-phosphate ratio (~15:1) in intermediate waters (200–1000 m) of the NSCS, suggesting that remineralization of exported POM is a key process shaping the characteristic low-N nutrient reservoir of the region. The relatively nitrogen-depleted degradable fraction (relative to P) likely reflects the elemental composition of surface-produced organic matter under persistent nitrogen limitation, linking upper-ocean ecological conditions with nutrient regeneration in the twilight zone.

Our results reveal that the northern South China Sea possesses a distinct remineralization stoichiometry compared with the global ocean, characterized by enhanced nitrogen and phosphorus turnover together with a lower regenerated N ratio. These findings provide new insights into how regional organic matter composition regulates nutrient recycling and contributes to the unique biogeochemical characteristics of the South China Sea.
 

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