Nitrogen gas flux from sediments: insights from simulation modelling

Direct measurements of dinitrogen efflux from continental shelf sediments ~ndicated that denltnflcation (mean 3.2 mm01 N m-2 d-l) was very important ~n nitrogen cycling. Most dinitrogen came from sediment-nitrate. All ammonium produced in these sediments was probably nitrified and then denitrified. In a closed incubation, the linear production of dinitrogen, a s oxygen decreased, was unexpected as was the low rat10 of oxygen consumption to denitrification (3.6: l ) . Sin~ulation modelling suggests the following explanation: Most carbon IS oxidised anoxically, but mtrogen (ammonium) diffuses to the oxygen zone relatively deep in the sediment, where hlgh rates of coupled nltrif~cat~on-denitrification result. As oxygen decreases, the zones of nltrification and denitrification move upward. The nitrate initially present In the enclosed overlying water decreases, but due to the decreasing diffusional path to the zone of denitrification, ~ t s rate of denitrification remains constant. The concentration of n~tra te from sediment nitrification increases in the overlylng water, but due to the decreasing rates of sediment nitrification, its rate of denitrlficatlon is also constant.