Distribution and Diffusive Flux of Endogenous Nitrogen in Eutrophic Lake

Abstract

A field study was performed in an urban eutrophic lake of Tianjin to investigate the distribution and diffusive flux of nitrogen species. Dissolved organic nitrogen (DON), accounting for about 98% of total nitrogen (TN), was the dominant component in sediment. Ammonium (NH4+-N) was the main component of inorganic nitrogen in the sediment and the pore water while nitrate (NO3−-N) slightly exceeded NH4+ -N in the overlying water. NH4+-N/DON ratio had an obvious negative correlation with Eh, temperature and FDA while NO3−-N/NH4+-N had a positive correlation with those factors. Calculated by the Fick’s First Law, the NH4+-N was the major nitrogen component in internal release with apparent diffusive tendency when NO3−-N mainly accumulated in the sediment. Obvious seasonal and spatial variations of nitrogen species and diffusion fluxes were observed. The plant area had lower NH4+-N and higher NO3−-N than the centre area due to weak reduction condition, which revealed that the anoxic condition played an important role in adsorption and immobilization of nitrogen by means of plant roots in internal release and also suggested the guidance mechanism for nitrogen nitrification and ammonification. In general, the surface sediment in eutrophic lakes, enriched by nitrogen, was the most active dimension for the biogeochemical cycling of nitrogen in which NH4+-N was the major and most effective forms.