Isotopic evidence for impacts of urbanization on active nitrogen turnover in the Dongjiang River Basin

WEN Jing; HUANG Bang-jie; FANG Huai-yang; TAN Xiu-qin; WU Chen-long; HUANG Zhi-wei; ZHANG Zong-yao; LIN Shu; DU Hong-wei

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China Environmental Science ›› 2025, Vol. 45 ›› Issue (5) : 2757-2766.

Environmental Ecology

Isotopic evidence for impacts of urbanization on active nitrogen turnover in the Dongjiang River Basin

WEN Jing, HUANG Bang-jie, FANG Huai-yang, TAN Xiu-qin, WU Chen-long, HUANG Zhi-wei, ZHANG Zong-yao, LIN Shu, DU Hong-wei

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Abstract

The Dongjiang River Basin was chosen as the study area to compare the nitrogen concentrations and isotopes in the river with different land uses, so as to provide a better understanding and evidence for impacts of urbanization on active nitrogen turnover. In this study, the distinct characteristics of nitrogen concentrations and isotopes were found in urbanized rivers compared to rivers with other land uses. Firstly, the nitrogen concentrations in urbanized rivers were gradually increased due to urbanization and nitrate has become a main nitrogen speciation. Secondly, based on the isotope Rayleigh fractionation model, it was found that the nitrification potential in urbanized river channels was enhanced and about 25.8% of nitrate nitrogen came from nitrification in situ. In addition, a positive correlation between ln(NO₃^--N) and δ¹⁵N-NO₃^- was recorded in urbanized rivers. Moreover, Δδ¹⁵N/Δδ¹⁸O in the dry and wet seasons were –2 and -6, respectively, indicating that the denitrification potential was weakened to provide evidence for nitrate accumulation in urbanized river channels. Likewise, a weak correlation between δ¹⁵N-PN and δ¹⁵N-NH⁴⁺ was also recorded in urbanized rivers, and the ¹⁵N enrichment factor of nitrate assimilation also differed from the theoretical value, indicating a weak assimilation of ammonia and nitrate nitrogen. Finally, more input pathways and less sink processes of nitrogen in urbanized rivers have become a key mechanism for elevated riverine nitrogen concentrations in the lower reaches of Dongjiang river.

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nitrogen isotope / nitrogen load / nitrogen transformation / the urbanized river

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WEN Jing, HUANG Bang-jie, FANG Huai-yang, TAN Xiu-qin, WU Chen-long, HUANG Zhi-wei, ZHANG Zong-yao, LIN Shu, DU Hong-wei. Isotopic evidence for impacts of urbanization on active nitrogen turnover in the Dongjiang River Basin[J]. China Environmental Science. 2025, 45(5): 2757-2766

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