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

Abstract

Sedimentary nitrogen composition and their stable isotope ratios were investigated in artificial Lake Shihwa and Lake Hwaong which were originated by a large dam in semi-enclosed bay. In this research, organic nitrogen was removed from total nitrogen by KOBr-KOH treatment. The average organic nitrogen fractions of total nitrogen contents in surface sediment were 42% in Lake Shihwa and 62% in Lake Hwaong, respectively. The organic carbon/organic nitrogen ratios (Corg/Norg)(mean: 24) are much higher than organic carbon/total nitrogen ratios (Corg/Ntot)(mean: 12), showing the significant presences of inorganic nitrogen in surface sediment of both lakes. The stable isotope ratios of organic nitrogen are 5.2±1.3‰ (SD=3.3) heavier than those of inorganic nitrogen in Lake Shihwa. However, in Lake Hwaong, it is shown that the stable isotope ratios of organic and inorganic nitrogen are comparably close,suggesting that the origin of nitrogen is different in both lakes. In addition, Corg/Norg ratio is a useful proxy to understand the degree of degradation of organic matter.