Phytoplankton dominates the suspended particulate nitrogen source in the Yangtze River

Globally, the large rivers export amounts of particulate nitrogen (PN) to the ocean, and studying the source and biogeochemical processes of the PN in the large river is important to regional/global N budget and cycle. However, the source apportionments and transformations of the PN along the Yangtze River remain unclear. Here, the elemental and isotopic compositions (N%, δ15N and δ13C value) of suspended particulate N (SPN) and surface sediment N (SSN) were determined in the wet and dry seasons along the Yangtze River, to investigate the seasonal and spatial variation in sources apportionments and transformations of the PN. In the wet season, the δ15N and PN content (%) of SPN increased from the headwater stream (2.3 ± 3.9 ‰ and 0.12 ± 0.04 %, respectively) to the lower stream (6.8 ± 2.4 ‰ and 0.47 ± 0.41 %, respectively) due to the imported anthropogenic N into the downriver section, while there was no significant change in the dry season. There were no distinctly spatial and seasonal variations in the source apportionment of SPN. Phytoplankton was identified as the dominant source of the SPN (42.3–56.9 %), which is probably the result of reducing soil erosion, reservoir construction, rapid urbanization and development of freshwater aquaculture. Terrestrial soil was the major source of SSN (34.5–49.8 %). Furthermore, the significant relationship between the δ15N-SPN values and NO3− concentrations, as well as the lower δ15N-SPN value compared to that of δ15N-NO3−, indicated a strong assimilation process of nitrate during phytoplankton growth. Meanwhile, most of the SSN showed lower C and N concentrations and higher δ13C and δ15N values, compared to those of the SPN samples at the corresponding sampling sites, which might be due to the SSN degradation. Our findings provided valuable information for the N cycle in the Yangtze River and underlined the importance of phytoplankton in the PN inputs.