Molecular composition determines the nitrogen isotopic signatures of dissolved organic matter

Marine dissolved organic matter (DOM) represents the largest reduced carbon pool in global carbon cycles. Nitrogen within marine DOM plays a crucial role in linking carbon and nitrogen cycles by serving as both a major nitrogen reservoir and a dynamic source for microbial utilization. Stable nitrogen isotopes (δ¹⁵N) of DOM could provide information about its sources and transformation in marine environments. However, the difficulties and complexities of its measurement leading to a lack of data on DOM's δ¹⁵N, limiting our understandings of marine nitrogen and carbon cycles. In this study, we developed a quick and effective machine learning method for linking δ¹⁵N of solid phase extracted DOM and its molecular composition. 325 samples with molecular composition and δ¹⁵N from China coastal environment are used for modeling, achieving a mean absolute error of 0.32‰. We found that all formulas, even those without nitrogen atoms, have direct or indirect connections with DOM's δ¹⁵N. The developed model not only performs well in China coastal environments, but also show great generalization in other estuaries. Furthermore, our model establishes a connection between δ¹⁵N and molecular composition of marine DOM in coastal ocean, revealing that higher δ¹⁵N values are associated with more saturated and marine-derived DOM, whereas lower δ¹⁵N values tend to be more unsaturated and exhibit terrestrial characteristics. This work elucidates the intrinsic coupling between DOM molecular characteristics and nitrogen isotopic composition, advancing our understanding of nitrogen cycling and organic matter dynamics in marine coastal environments.