Interpretation of anthropogenic impacts (agriculture and urbanization) on tropical deltaic river network through the spatio-temporal variation of stable (N, O) isotopes of NO⁻ ₃ ^*

For the first time, the dual isotope approach was applied to trace the sources of impacts and to identify the governing biogeochemical processes in a river network in the tropical deltaic region of the Red River (Vietnam). Our long term surveys concluded that water in this river network was severely impacted by anthropogenic activities. Analysis has shown strong spatio-temporal variation of nitrate isotopes; ranges of δ¹⁵N– (Formula presented.) and δ¹⁸O– (Formula presented.) were from −5 to 15 ‰ and from −10 to 10 ‰, respectively. Average values of δ¹⁵N– (Formula presented.) and δ¹⁸O– (Formula presented.) in the dry season, when fertilizer is applied, were 3.54 and 3.15 ‰, respectively. In the rainy season, the values changed to 6.41 and −2.23 ‰, respectively. Denitrification and biological assimilation were active throughout the year, but were especially enhanced during fertilization time. Mineralization of domestic organic matter and consequent nitrification of mineralized (Formula presented.) were the dominant processes, particularly during the rainy period.