DENITRIFICATION, NITRIFICATION AND NITROGEN-FIXATION IN SUBLITTORAL SEDIMENTS OF THE SOUTH CHINA SEA

Measurements of denitrification, nitrification and nitrogen fixation rates were made alongside with measuring of chemical and physical properties in sublittoral sediments of the South China Sea near the coast of Vietnam. Studied sediments were suboxic (Eh was positive as a rule), had 0.18-1.5 % of organic carbon, 0.004-0.135 % of total nitrogen and 3-12 % of total iron. The numbers of denitrifying and nitrate-reducing bacteria were as high as millions and hundreds of millions cells per gram wet weight of sediment matter, respectively. The processes of nitrification and denitrification were not spearated spacely. The nitrification was measured in both superficial layer and in a 10-cm sediment column. There were indirect evidences suggesting possibility of anaerobic ammonium oxidation. Denitrification was detectable in the sediments from two sites of sampling; maximal value was 86.2 mumoles N m-2h-1. The denitrification potential determined at 1 mM nitrate decreased regularly from the upper to lower layers. Its values in the different sediments ranged from 134 to 532 mumoles N m-2h-1. Nitrogen fixation (from 4.8 to 86mumoles N M-2h-1) was close to that found in similar sediments in temperate waters in summer, and was not a significant source of nitrogen. It was comparable with diffusion of ammonium from sedimnts.