Abstract：
Estuaries are hotspots where terrestrially originated dissolved organic matter (DOM) is modified in molecular composition before entering marine environments. However, very few research has considered nitrogen (N) modifications of DOM molecules in estuaries, limiting our understanding of dissolved organic nitrogen (DON) cycling and the associated carbon cycling in estuaries. This study integrated optical, stable isotopes (δ15N and δ13C) and molecular composition (FT-ICR MS) to characterize the transformation of DOM in the Yangtze River Estuary. Both concentration of dissolved organic carbon (DOC) and DON decreased with increasing salinity, while their δ13C and δ15N increased with the increasing salinity. A significant positive correlation was found between δ15N and δ13C during the transportation of DOM to marginal seas, indicating that the behavior of both DOC and DON are primarily controlled by the mixing of freshwater and the seawater in the YRE. During the mixing process, the DON addition was observed using the conservative mixing curves. In the view of molecular composition, DOM molecules became more aromatic as the number of N atoms increased. Spearman correlations reveal that DOM molecules with fewer N atoms exhibited a higher enrichment in protein-like components, while those with more N atoms were more enriched in humic-like components. In addition, the δ15N and δ13C tended to increase as the N content of DOM decreased. Therefore, DON molecules with fewer N atoms were likely to be transformed into those with more N atoms based on the isotopic fractionation theory. This study establishes a linkage between the molecular composition and the δ15N of DOM, and discovers the N transformation pattern within DOM molecules during the transportation to marginal seas.
摘要：
河口是热点地区，陆地来源的溶解有机物（DOM）在进入海洋环境之前会改变分子组成。然而,很少有研究考虑河口DOM分子的氮（N）修饰，限制了我们对河口溶解有机氮（DON）循环和相关碳循环的理解。这项研究集成了光学，稳定同位素（δ15N和δ13C）和分子组成（FT-ICRMS）表征了长江口DOM的转化。溶解有机碳（DOC）和DON的浓度均随盐度的增加而降低，而它们的δ13C和δ15N随着盐度的增加而增加。在DOM向边缘海的运输过程中，δ15N和δ13C之间存在显着的正相关。表明DOC和DON的行为主要受淡水和海水在YRE中的混合控制。在混合过程中,使用保守混合曲线观察到DON添加。从分子组成的角度来看，随着N原子数的增加，DOM分子变得更加芳香。Spearman相关性表明，具有较少N原子的DOM分子在蛋白质样成分中表现出更高的富集，而那些具有更多N原子的则富含腐殖质样成分。此外,随着DOM中N含量的降低，δ15N和δ13C趋于增加。因此,根据同位素分馏理论，具有较少N原子的DON分子可能会转化为具有较多N原子的DON分子。这项研究建立了分子组成与DOM的δ15N之间的联系，并在运输到边缘海的过程中发现DOM分子内的N转化模式。
