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Abstract Based on the bimonthly investigation at the Datong station in the Yangtze River during 2012 to 2013, the concentrations and compositions of total dissolved nitrogen (TDN) including dissolved inorganic nitrogen (DIN) and dissolved organic nitrogen (DON) and its important compositions urea, dissolved free amino acids (DFAA) and dissolved combined amino acids (DCAA) were analysed. On the other hand, the bioavailability of DON was evaluated and the fluxes of different nitrogen forms were calculated. The concentration of DON in the Yangtze River showed a seasonal variation tendency, which were higher in spring and winter and were lower in summer and autumn, due to the dilution of Yangtze River and the adsorption of suspended particulate matter to DON. The average concentration of DON was (33.28±21.35)μmol/L, accounting for 20.05%±13.53% of TDN; and the concentration of urea, DFAA and DCAA accounted for 21.94%±19.89%、0.84%±0.52% and 3.68%±2.61% of DON, respectively. The concentration of urea presented a positive correlation with the river runoff, which attributed to the seasonal fertilization in the Yangtze River basin. The carbon-normalized of total dissolved amino acids (TDAA (%DOC)) and the degradation index of TDAA both indicated that the DON in the Yangtze River had high bioavailability and presented a seasonal variation, which was higher in spring and summer and was lower in autumn and winter. The flux of DON was 42.04′104t/a, approximately accounted for 17% of TDN. Accordingly, the flux of urea, DFAA and DCAA accounted for 15.50%、0.64% and 2.80% of DON flux, respectively. In conclusion, the DON in the Yangtze River had high bioavailability and constituted the important nutrients for eutrophication in the Yangtze Estuary and its adjacent areas.
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Received: 20 April 2015
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