The Changjiang River is the largest freshwater river in China. Here, the sources and variability in nitrate of the Changjiang River are assessed for the first time using dual isotopic approach. Water samples were collected once in August 2006 from the main channel of the Changjiang and its major tributaries. The concentrations and isotopic composition of nitrate were then analyzed for the waters in the Changjiang River. The δ15N and δ18O of NO3− ranges from 7.3‰ to 12.9‰ and 2.4‰ to 11.2‰ in the Changjiang River waters, respectively. The ranges of isotopic compositions of nitrate suggested that nitrification (including “modified fertilizer”) and urban sewage effluent are the major sources of nitrate in the Changjiang River. The high δ18O-NO3− values were observed in the water of the upper reaches, indicated that the current drought might be one important reason for shifting of isotopes in the special sampling period. In addition, there was a strong positive relationship between δ15N-NO3− and δ18O-NO3−, which indicated that denitrification added to the enrichment of heavy isotopes of nitrate.

•Nitrate was the dominant species of DIN in Liao River.•High fraction of NH4+/DIN observed during low flow season.•Nitrate in rivers originated mainly from nitrification and sewage based on isotopic pattern.•About 7.0 × 104 ton N/yr entered the Bay annually amounting to 5% of N fertilizer.The Liao River Basin is one of seven primary river Basins in China. The concentration of dissolved inorganic N (DIN), dual isotopes of NO3- using the denitrifier method, the N isotopes of NH4+ and the N flux in the basin were determined to identify the sources of N and their transformation. The results show that NO3- ranges from 0.3 μmol/L to 1316 μmol/L. In general, NO3- is the dominant inorganic N species during both flow seasons, but the fraction of NO3-/DIN is variable and high NH4+ is present in some waters. Samples collected from the up-stream portion of the Liao River typically had N isotope values of <+8‰, while those from the middle and lower portions had values of >+8‰ during the high flow season. Most water samples had O isotope values of <+10‰ during the high flow season. During the low flow season, the ranges of N and O isotopic values were limited, with average values of +10.3‰ and +4.9‰, respectively. There is a wider isotopic range of NO3- during the high flow season than the low flow season. The isotopic pattern of NO3- suggests that wastewater and soil organic N are the sources of NO4+ during the high flow season, while wastewater is the main source during low flow season. It appears that no intense denitrification occurs in the river according to the isotopic and chemical data. The N flux of the Liao River system entering the Liao Dong Bay annually is nearly 7.0 × 104 tons, which amounts to 5.0% of the N from chemical fertilizers used in this basin.

The dual isotopic compositions of dissolved SO42- in aquatic systems are commonly used to ascertain SO42- sources and possible biogeochemical processes. In this study, the physical parameters, major anions and isotopic compositions of SO42- in water samples from Changjiang River (Nanjin) to the East Sea in Changjiang Estuary were determined. The salinity ranged from 0‰ to 32.3‰ in the estuary water samples. The Cl-,SO42- concentrations and δ18O–H2O values followed the salinity variations from freshwater to seawater, which indicated that mixing processes might be a major factor involved in the distribution of water and solutes. The contents and isotopic compositions of SO42- suggested that atmospheric deposition, evaporite dissolution and sulphide oxidation were the major sources of dissolved SO42- in the freshwater of Changjiang River. In addition, the mixing model calculated by contents and isotopic compositions of SO42- indicated that the mixing of freshwater and sea water was the major factor involved in SO42- distribution in Changjiang Estuary. However, slightly elevated δ18O–SO4 values were observed in the turbidity maximum zone, which suggested that biological processes might affect the O isotopic compositions of SO42- there.Highlights► Changjiang Estuary plays an important role in transportation of the water and solute. ► The dual isotopic method could be used to understand sulfate biogeochemistry in estuaries. ► Mixing processes should be a major factor involved in the distribution of water and sulphate. ► Sulphate in the Changjiang River mainly derived from atmospheric deposition, evaporite dissolution and sulphide oxidation.

•Water chemistry and isotopes (δ34S, δ18O, δ13C) reveal the weathering process.•72% of sulphate derived from sulphide oxidation in the Yalong River.•Dynamics of DIC affected by carbonate weathering and equilibration processes.•Sulphuric acid dominate carbonate weathering in the cold and dry zone with high elevation.To better understand chemical weathering and controlling processes in the Yalong River of the eastern Tibetan Plateau, this study presents major ion concentrations and stable isotopes of the dissolved loads. The isotopic compositions (δ13C-DIC, δ34S and δ18O-SO4) of the dissolved loads are very useful to quantify solute sources and define the carbon budget related with chemical weathering in riverine systems. The isotopic composition of sulphate demonstrates that most of the sulphate is derived from sulphide oxidation, particularly in the upper reach of the Yalong River. The correlations between δ13C-DIC, water chemistry and isotopes of sulphate, suggest that the carbon dynamics are mainly affected by carbonate weathering by sulphuric acid and equilibration processes. Approximately 13% of the dissolved inorganic carbon in the Yalong River originates from carbonate weathering by strong acid. The CO2 consumption rates are estimated to be 2.8 × 105 mol/km2/yr and 0.9 × 105 mol/km2/yr via carbonate and silicate weathering in the Yalong River, respectively. In this study, the influence of sulphide oxidation and metamorphic CO2 on the carbon budget is estimated for the Yalong River draining the eastern Tibetan Plateau.