Tracking Nitrogen Sources, Transformation, and Transport at a Basin Scale with Complex Plain River Networks

Abstract

This research developed an innovative approach to reveal nitrogen sources, transformation, and transport in large and complex river networks in the Taihu Lake basin using measurement of dual stable isotopes of nitrate. The spatial patterns of δN corresponded to the urbanization level, and the nitrogen cycle was associated with the hydrological regime at the basin level. During the high flow season of summer, nonpoint sources from fertilizer/soils and atmospheric deposition constituted the highest proportion of the total nitrogen load. The point sources from sewage/manure, with high ammonium concentrations and high δN and δO contents in the form of nitrate, accounted for the largest inputs among all sources during the low flow season of winter. Hot spot areas with heavy point source pollution were identified, and the pollutant transport routes were revealed. Nitrification occurred widely during the warm seasons, with decreased δO values; whereas great potential for denitrification existed during the low flow seasons of autumn and spring. The study showed that point source reduction could have effects over the short-term; however, long-term efforts to substantially control agriculture nonpoint sources are essential to eutrophication alleviation for the receiving lake, which clarifies the relationship between point and nonpoint source control.

Coupling stable isotopes and water chemistry to assess the role of hydrological and biogeochemical processes on riverine nitrogen sources

“…With respect to land-use, NO 3 À and NH 4 þ concentrations were significantly (p < 0.001) higher in tributaries dominated by residential and agricultural land use than those dominated by forests ( Fig. 3c), which is consistent with previous studies in eastern and northern China (Sun et al, 2013;Yi et al, 2017). There were no significant differences (p > 0.05) in NO 3 À concentrations among seasons in the mainstem; however, NH 4 þ concentrations were lower (p < 0.05) in summer (Fig.…”

“…6d). These results are similar to previous studies and emphasize human impacts on N dynamics (Xia et al, 2017;Yi et al, 2017). Greater than 80% of river water (including mainstem and tributaries) d 18 O values for NO 3 À ( Fig.…”

“…The dual NO 3 À stable isotope approach (usually coupled with Bayesian statistics, Parnell et al, 2010) has been successfully applied to assess N transformation processes and sources (e.g., atmospheric deposition, soil pools, chemical fertilizers, manure/sewage) in surface waters and groundwater. For example, the denitrification process in groundwater (Xue et al, 2009;Ji et al, 2017;Minet et al, 2017) and river water (Kaushal et al, 2011;Xia et al, 2017;Yi et al, 2017) was quantified using dual NO 3 À stable isotope data. Furthermore, the dual stable isotope approach was applied to quantify the major contributors of riverine NO 3 À in forest (Ohte, 2013), agricultural, sub-urban (Yang et al, 2013;Ji et al, 2017;Xia et al, 2017) and urban (Yang and Toor, 2016) catchments.…”

“…Furthermore, the dual stable isotope approach was applied to quantify the major contributors of riverine NO 3 À in forest (Ohte, 2013), agricultural, sub-urban (Yang et al, 2013;Ji et al, 2017;Xia et al, 2017) and urban (Yang and Toor, 2016) catchments. In the majority of these studies, dual isotope records were collected for a short-term period (<1 yr), which hinders rigorous assessment of watershed N dynamics and sources given the large temporal variability in watershed processes (Yi et al, 2017;Yang et al, 2013). Additionally, few studies have attempted to verify dual NO 3 À stable isotope results by comparing to hydrological and biogeochemical information from other isotopes and hydrochemistry (Xue et al, 2009;Abbott et al, 2016;Minet et al, 2017;Yi et al, 2017).…”

“…In the majority of these studies, dual isotope records were collected for a short-term period (<1 yr), which hinders rigorous assessment of watershed N dynamics and sources given the large temporal variability in watershed processes (Yi et al, 2017;Yang et al, 2013). Additionally, few studies have attempted to verify dual NO 3 À stable isotope results by comparing to hydrological and biogeochemical information from other isotopes and hydrochemistry (Xue et al, 2009;Abbott et al, 2016;Minet et al, 2017;Yi et al, 2017). Increasing evidence indicates that N leaching from legacy sources in soils and groundwater is a major cause for increasing riverine N exports despite significant declines in anthropogenic N inputs (Meals et al, 2010;Hamilton, 2012;Van Meter et al, 2017;Chen et al, 2018).…”