Nitrogen composition and stable isotope ratios of surface sediment in the artificial lakes at west coast of Korean Peninsula

Abstract

Stable isotopes of sedimentary nitrogen, carbon and the C to N ratio have been used as the available proxies for origins of organic matter and other biogeochemical parameters. In this research, organic nitrogen and labile organic carbon was separated from total nitrogen and total organic carbon by KOBr-KOH treatment. Organic nitrogen concentration of sediment was calculated by difference between total nitrogen and inorganic nitrogen (Ntot-Nbnd=Norg) in two artificial lakes (Lake Shi-hwa and Hwa-ong). The average organic nitrogen fractions of total nitrogen contents in surface sediment were 42% in Lake Shi-hwa and 62% in Lake Hwa-ong, respectively. As a result, Corg / Ntot ratios(mean=12) are much higher than Corg / Norg ratios(mean=24), showing the significant presences of inorganic nitrogen in surface sediments of both lakes. In addition, stable isotope compositions of each fractionated carbon and nitrogen indicate different tendency, suggesting their different origin. The stable isotope compositions of organic nitrogen are heavier than those of inorganic nitrogen (approximately 0.18 in Hwa-ong and 2.15 in Shi-hwa) except a few stations in Lake Hwa-ong. Stable isotope of organic labile carbon, which is removed by KOBr-KOH solution, is heavier (approximately 0.1 to 2.4 per-mil, average: 1.4) than residual organic carbon.