Speciation and dynamics of dissolved inorganic nitrogen export in the Danshui River, Taiwan

[摘要] Human-induced excess nitrogen outflowing from land through rivers to oceanshas resulted in serious impacts on terrestrial and coastal ecosystems.Oceania, which occupies < 2.5% of the global land surface, delivers12% of the freshwater and dissolved materials to the ocean on a globalscale. However, there are few empirical data sets on riverine dissolvedinorganic nitrogen (DIN) fluxes in the region, and their dynamics are poorlyunderstood. In this study, a river monitoring network covering differenttypes of land uses and population densities was implemented to investigatethe mechanism of DIN export. The results show that DIN concentration/yieldvaried from∼20 μM/∼300 kg-N km⁻² yr⁻¹ to∼378 μM/∼10 000 kg-N km⁻² yr⁻¹from the relatively pristine headwaters to the populous estuary. Agricultureand population density control DIN export in less densely populated regionsand urban areas, respectively, and runoff controls DIN at the watershedscale. Compared to documented estimates from global models, the observed DINexport from the Danshui River is 2.3 times larger, which results from theregion-specific response of DIN yield to dense population and abundantrunoff. The dominating DIN species change gradually from NO₃⁻ in theheadwaters (∼97%) to NH₄⁺ in the estuary(∼60%) following the urbanization gradient. The prominentexistence of NH₄⁺ is probably the result of the anaerobic water bodyand short residence time, unlike in large river basins. Given the analogouswatershed characteristics of the Danshui River to the rivers in Oceania, ourstudy could serve as a first example to examine riverine DIN fluxes inOceania.

[学科分类] 地球化学与岩石