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

Abstract

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 δ(15)N-[Formula: see text] and δ(18)O-[Formula: see text] were from -5 to 15 ‰ and from -10 to 10 ‰, respectively. Average values of δ(15)N-[Formula: see text] and δ(18)O-[Formula: see text] 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: see text] were the dominant processes, particularly during the rainy period.

Stable isotopes as an effective tool for N nutrient source identification in a heavily urbanized and agriculturally intensive tropical lowland basin

“…There are, on the other hand, some distinctive differences between the DRB and eutrophic river systems elsewhere in the world. For instance, compared to other studies (Battaglin et al, 2001, Chen et al, 2009, Lin et al, 2019, the present study (building on earlier work of Ta et al, 2016) is among the few reporting 15 N-NO3 values < 0 (Popescu et al, 2015;Vrzel et al, 2016). A rational explanation is that urea characterized with depleted 15 N is extensively used in the DRB and agriculture is the dominant source of NO3 in many parts of this system.…”

“…10 m from river banks and divided into sub-samples for analysis of N nutrient concentrations, Dissolved Organic Carbon (DOC), and stable isotope analyses. These parameters were chosen as it is acknowledged that to assess the sources of NO3 and to identify the governing processes of NO3 in stream water, stable isotopes of water (δ 18 O-H2O), N species and carbon availability (represented as DOC) should also be analysed (Baker and Vervier, 2004;Zarnetske et al, 2011;Ta et al, 2016).…”

“…We considered the following four main sources of nitrate based on the geographical and natural conditions of the DRB (e.g. Thibodeau et al 2013;Ta et al 2016): (1) inputs from soil and groundwater sources (representing natural, background input levels), (2) inputs from the Red River (also considered as natural sources), (3) inputs from regional, excessive application rates of chemical fertilizers (urea), and (4) inputs from organic matter deriving from urban regions and livestock farming (sewage and manure). In order to derive the proportion that these sources derive in the DRB, we used the following partition equations of NO3, which are based on our stable isotope data:…”

“…A rational explanation is that urea characterized with depleted 15 N is extensively used in the DRB and agriculture is the dominant source of NO3 in many parts of this system. In fact, synthetic urea fertilizers of low (negative) 15 N were reportedly used in paddy fields of the RRD (Ta et al, 2016;Saiki et al, 2019). In all surveys conducted in this study, 15 N at site D9 was always lower than the preceding upstream site (D8).…”

“…In order to address these issues, the application of nitrate N isotopes ( 15 N-NO3), and more recently nitrate O isotopes ( 18 O-NO3) provides an excellent means by which to trace the sources of N pollution in rivers. Furthermore, by applying these analyses in tandem permits an evaluation of the governing biogeochemical processes altering nitrate species in natural environments (Popescu et al, 2015;Vrzel et al, 2016;Ta et al, 2016). The dual isotope approach is based on the principle that nitrate from different origins has distinct 15 N-NO3 and 18 O-NO3 isotopic signatures (see Kendall et al, 2007 for a review).…”