Nitrate reduction to ammonium and organic nitrogen in an estuarine sediment

Nitrate reduction to NH₄⁺ and incorporation into organic matter was investigated in sediment-water systems composed of a 2.4-cm layer of estuarine sediment covered by 2 cm of water. Between 15 and 28% of the ¹⁵N-labelled NO₃^? added to the sediment or floodwater of the systems was recovered as NH₄⁺ and organic N. The results indicated that NO₃^? was reduced to NH₄⁺ by a dissimilatory mechanism. A separate experiment examined the influence of redox potential (+300, 0 and ?200 mV) on NO₃^? reduction in sediment suspensions maintained at pH 7.5. Conversion of NO₃^? to NH₄⁺ and organic N increased markedly with decreasing redox potential. The results suggested that although the reaction accounted for 35–42% of the NO₃^? reduced under intensely-reduced conditions (?200 mV), the significance of the reaction in nature was dependent upon NO₃^? movement into zones sufficiently reduced for reduction of NO₃^? to NH₄⁺ instead of denitrification. Under conditions in which NO₃^? moved downward through a sediment-water column into a reduced sediment zone approximately 15% of the NO₃^? was reduced to NH₄⁺ and recycled in the sediment.