Biological Nitrogen Fixation and Denitrification in Lake Kasumiga-ura

The biological nitrogen fixation and denitrification were studied in Lake Kasumiga-ura to evaluate the significance of the biological processes for nitrogen enrichment and the self-purification in aquatic environments. The water and sediment samples were collected from several sites and analyzed for the amount of inorganic nitrogen, the nitrogen-fixing activity by acetylene reduction method, the denitrifying activity by nitrous oxide formation and the numbers of total heterotrophic bacteria and denitrifying bacteria. No significant activities of nitrogen fixation and denitrification were found in the lake water in this study. The rate of denitrification in sediments was about two orders of magnitude higher than that of nitrogen fixation. The average microbial activities of nitrogen fixation in sediments sampled in July, August and December were 0.02, 0.11, and 0.14 μg N/g/day, respectively ; those of denitrification are 4.1, 8.1, 93.4 μg N/g/day, respectively. These results suggest that the surface sediments in Lake Kasumiga-ura have a high potential for self-purification of nitrogenous compounds in water and sediments.