<Li> Important in amino acid catabolism . </Li> <P> The conversion of ammonium to nitrate is performed primarily by soil - living bacteria and other nitrifying bacteria . In the primary stage of nitrification, the oxidation of ammonium (NH + 4) is performed by bacteria such as the Nitrosomonas species, which converts ammonia to nitrites (NO − 2). Other bacterial species such as Nitrobacter, are responsible for the oxidation of the nitrites (NO − 2) into nitrates (NO − 3). It is important for the ammonia (NH 3) to be converted to nitrates or nitrites because ammonia gas is toxic to plants . </P> <P> Due to their very high solubility and because soils are highly unable to retain anions, nitrates can enter groundwater . Elevated nitrate in groundwater is a concern for drinking water use because nitrate can interfere with blood - oxygen levels in infants and cause methemoglobinemia or blue - baby syndrome . Where groundwater recharges stream flow, nitrate - enriched groundwater can contribute to eutrophication, a process that leads to high algal population and growth, especially blue - green algal populations . While not directly toxic to fish life, like ammonia, nitrate can have indirect effects on fish if it contributes to this eutrophication . Nitrogen has contributed to severe eutrophication problems in some water bodies . Since 2006, the application of nitrogen fertilizer has been increasingly controlled in Britain and the United States . This is occurring along the same lines as control of phosphorus fertilizer, restriction of which is normally considered essential to the recovery of eutrophied waterbodies . </P> <P> Denitrification is the reduction of nitrates back into nitrogen gas (N), completing the nitrogen cycle . This process is performed by bacterial species such as Pseudomonas and Clostridium in anaerobic conditions . They use the nitrate as an electron acceptor in the place of oxygen during respiration . These facultatively anaerobic bacteria can also live in aerobic conditions . Denitrification happens in anaerobic conditions e.g. waterlogged soils . The denitrifying bacteria use nitrates in the soil to carry out respiration and consequently produce nitrogen gas, which is inert and unavailable to plants . </P>

Where does most of the nitrogen cycle take place