Dissolved organic nitrogen structural and component changes in overlying water along urban river at molecular and material levels — Beiyun basin case study

The properties of dissolved organic nitrogen (DON) affect its stability in river ecosystems. This study is the first to evaluate DON by assessing its stability in the overlying water of an urban river. The results showed that CHON was the largest contributor (87.5%) to the DON, with the largest portion being molecules with one nitrogen atom. The DON in the overlying water of the Beiyun River is mainly composed of lignin (70.43%), following by protein (14.02%), carbohydrates (5.28%), and condensed aromatic (3.74%). Compared with other urban rivers, the higher protein content and lower lignin content of the Beiyun River increase the lability of the DON. From upstream to downstream, 53% of molecules are always stable in the river, most of them are lignin with highly oxidized functional groups. The comparison of stable and unstable components shows lipids and condensed aromatics are unstable and susceptible to degradation as compared with lignin. Molecules with a high H/C, low O/C, and double bond equivalent (DBE) were removed the fastest, while those with higher O/C and O/N were relatively stable. Flowing along the river, biological processes have contributed to the transformation of DON molecules from high-N into low-N molecules. Correlation analysis suggests that cutting off external pollutants can be helpful to control the nitrogen pollution of the Beiyun River.