Dissimilatory Nitrate Reduction To Ammonium In Submerged Soils

Dissimilatory reduction of nitrate in submerged soils is mediated by two groups of bacteria. One is respiratory denitrifiers, which re-oxidize NADH through energy-generating electron transport chain with N20 and N2 as the main reduced products. The other is the fennentative reducers, which reduce nitrate to mainly ammonium. Assimilatory reduction and chemical reduction are believed to be little importance in agricultural soils. The prevailing concept that denitrification accounts for essentially all of nitrate reduction in submerged soils has been challenged since 1970抯. Many papers indicated that dissimilatory nitrate reduction to amrnonium (DNRA) could occur in soils, sediments and digested sludge. DNRA in submerged soils is important for its potential contribution to N reservation and minimizing N losses. Since nitrate is the substrate for both DNRA and denitrification, any measures that could enhance DNR.A would lead to minimizing denitrification, which is agriculturally un-wanted. In addition, DNRA is reported to be one of the sources of nitrous oxide production and therefore one of the subjects environmentalists concern. Purpose of present paper is to see what extend to which DNRA could occur in submerged soils, environmental factors that affect DNRA, relationship between DNRA and denitrification and the dominant bacterial flora responsible DNRA. In addition, DNRA and denitrfication in inter-tidal marine sediments as well as Taihu Lake sediments are also investigated. Under anaerobic incubation conditions, 1.?% of ?N-labelled nitrate was converted into ammonium in three soils from China. In an Australia soils, however, 12.5% of nitrate was converted into ammonium and 2% into organic form. There is sufficient amount of non-labeled ammonium present in the soils before the experiments started, nitrate assimilation should be inhibited and therefore the ?N-labelled aminonium and organic N should come from DNRA. The population of bacteria responsible for DNRA is about per gram of soil, with dominant genus Bacillus. DNRA occurred at the C/NO3TN above 12. Intensively reduced conditions favored DNRA, as was indicated by adjusting soil Eh at ?00 mY and ?40 mY. Addition of glucose increased the proportion of organic N to ammonium as compared to the check soil (without glucose). DNRA could occur at pH ranging from 5 to 9. Higher pH seemed to favor DNRA. One DNRA bacteria isolated from soil was closely examined. When supplied with 50 mmol glucose and 10 mmol nitrate as sole source of nitrogen, this bacterium converted 20% of nitrate into ammon.ium during logarithmic growth phase. Ammonium produced as such was in excess of cetl growth and cell death during this phase was minimal, these data suggested that nitrate assimilation and re-mineralization of cell materials were not important processes contributing to the ammonium formed. Using ?N-Iabelled ?NH4N03 as the nitrogen sources, 15 abundance of cell-N increased nearly to the starting ?NH4 abundance of the medium, indicating that the bacterium synthesized cell materials by using ammonium rather than nitrate. Resting cells reduced nitrate to ammomum as quickly as within 20 mm of incubation. Cell yield was not proportional to the amount of nitrate added. There was 1 ?% of nitrous oxide produced from nitrate by this bacterium. Inter-tidal marine sediment sample showed DNRA process while Taihu lake sediment...