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

Zhenwei Yan1, Zhou Liang2, Xiaosong Zhong1, Maojun Yan3 and Yu Xin1, (1)Ocean University of China, Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Qingdao, China, (2)Florida State University, department of earth, ocean & atmospheric science, Tallahassee, FL, United States, (3)Shanghai Jiao Tong University, Qingdao, 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 δ15NLMDON. 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 δ15NDON 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.