Cycling of Dissolved Organic Nitrogen in a High Turbidity and High Dissolved Inorganic Nitrogen Content River-a case study in Yellow River,China

Yellow River is well-known for its high turbidity and its high dissolved inorganic nitrogen content that dominates the nitrogen inventory by more than 90%. Such an overwhelming ratio of dissolved inorganic nitrogen (DIN, 181.9-575.8μmol/L) to organic nitrogen (DON, 10.5-29.6μmol/L) significantly restrains our knowledge on the cycling of DON in Yellow River, since the accurate measurement of δ¹⁵N of DON under DIN replete conditions is quite challenging. However, with the assistance of tangential flow filtration (TFF) and a modified SPE-based DON enrichment method, we successfully examined the molecular-size conduit of DON pool along the main stream of Yellow River and measured the δ¹⁵N of DON by molecular weight cut-offs (< 1 kDa as Low Molecular DON, LMDON, > 10 kDa as High Molecular DON, HMDON) with satisfying quality. The modified SPE-based DON enrichment treatment achieved a recovery rate of 65.8±16.0% for the component of LMDON, thus the δ¹⁵N of SPE-enriched LMDON could truthfully trace the cycling processes of LMDON. LMDON dominates the DON pool by an average of 92.6%, while the HMDON and 1-10 kDa components accounts for 3.8% and 3.5%, respectively. A G-model regression is found out to well-fit the co-variation between DIN and LMDON (R=0.94, n=17,P< 0.001), which suggests that the degradation of LMDON into nitrate is an important sink for DON in Yellow River. This is further validated by the isotope Rayleigh fractionation in δ¹⁵N_LMDON. Based on a fractionation factor of 2.0 ‰, 8.6±5.7μmol/L of DON is re-mineralized in the downstream of Yellow river, and contributes 3.1% of the local DIN pool. A box model on DON cycling shows that the DON contributed by branch river Weihe in the mid-stream is completely degraded to nitrate in the downstream; of the 14.8μmol/L DON that flows into Bohai Sea, the recalcitrant DON is 8.9μmol/L (60.0% ). Our study presents the first dataset on δ¹⁵N_DON by molecular weight cut-offs in a unique river watershed with high turbidity and high DIN/DON ratio, and highlights the importance of DON degradation under DIN-replete conditions. Further study is needed on the composition, isotopic signature and fate of the refractory component of Yellow River DON.