Nitrogen loss from a turbid river network based on N2 and N2O fluxes: Importance of suspended sediment

Riverine nitrogen loss makes a large contribution to the global nitrogen budget. However, little research has focused on nitrogen loss from large turbid rivers with high suspended sediment (SPS) concentrations. In this work, nitrogen loss amounts and related drivers were studied across fluvial networks of the Yellow River, the largest turbid river in the world, based on in situ measurement of nitrogen gas (N2) and nitrous oxide (N2O) fluxes at the water-air interface via the diffusion model and floating chamber methods, respectively. The results showed that N2 and N2O fluxes from the Yellow River ranged from −2.93 to 48.54 mmol m−2 d−1 and from 2.42 to 712.23 μmol m−2 d−1, respectively, with the nitrogen loss amount estimated to be 5.56 × 107 kg N yr−1 for the Yellow River, including the mainstem and main tributaries. Other than nitrogen compounds and water temperature, nitrogen loss from the Yellow River was also affected by SPS. Both N2 flux: DIN and N2O flux: DIN ratios increased remarkably in the middle reaches, probably due to a sharp increase of SPS concentration in this section. Furthermore, greater SPS concentrations were a main cause for the higher N2O flux in the middle reaches than those in the other reaches of the Yellow River, and the possible effect of SPS was stronger on N2O flux than on N2 flux. This study demonstrates the importance of SPS in nitrogen loss from large turbid rivers, and more research is demanded to further clarify the role of SPS in riverine nitrogen cycle.