Sulfide-induced dissimilatory nitrate reduction to ammonia in anaerobic freshwater sediments

Different reduced sulfur compounds (H2S, FeS, S2O32−) were tested as electron donors for dissimilatory nitrate reduction in nitrate-amended sediment slurries. Only in the free sulfide-enriched slurries was nitrate appreciably reduced to ammonia (), with concomitant oxidation of sulfide to S0 (). The initial concentration of free sulfide appears as a factor determining the type of nitrate reduction. At extremely low concentrations of free S2− (metal sulfides) nitrate was reduced via denitrification whereas at higher S2− concentrations, dissimilatory nitrate reduction to ammonia (DNRA) and incomplete denitrification to gaseous nitrogen oxides took place. Sulfide inhibition of NO- and N2O- reductases is proposed as being responsible for the driving part of the electron flow from S2− to NH4+.