Using dual isotopes to evaluate sources and transformation of nitrogen in the Liao River, northeast China

• 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.