Dissolved organic nitrogen cycling revealed at the molecular level in the Bohai and Yellow Sea

Marginal seas play a crucial role in the cycling of dissolved organic nitrogen (DON) between the terrestrial and marine environments. However, very few studies have considered the molecular transformation of DON in marginal seas, leaving the DON molecular modifications in its cycling largely unknown. Therefore, this study examined DON cycling in the Bohai Sea and Yellow Sea, two semi-closed marginal seas in northern China, using stable isotopes (& delta;15N and & delta;13C), optical characteristics, and molecular compositions. Compared to the Yellow Sea, the Bohai Sea had a weaker exchange with the open ocean, resulting in higher concentrations, lower & delta;15N, and more recalcitrant properties in DON. The DON cycling showed significant differences inside and outside the Yellow Sea Cold Water (YSCW). Degradation was the major sink of DON in the YSCW, during which more highly unsaturated compounds and carboxyl-rich alicyclic molecules were produced. Nitrogen atoms were found to be removed from the molecules with more N atoms to those with fewer ones during the DON degradation. This study discovered the molecular modifications in DON cycling and highlighted the intrinsic mechanisms in the cycling of DON in marginal seas.

Automated summary

This study examined the cycling of dissolved organic nitrogen (DON) in the Bohai Sea and Yellow Sea in northern China. The results showed that the Yellow Sea had a stronger exchange with the open ocean, resulting in lower DON concentrations, while the Bohai Sea had a weaker exchange, leading to higher DON concentrations. Furthermore, this study discovered the molecular modifications in DON cycling.