The coupling of nitrification and denitrification in two estuarine sediments1,2

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Abstract
Nitrification and denitrification in estuarine sediments were measured by 15N isotopic tracer techniques at two stations in the Patuxent River estuary for April and August. A close coupling of the two processes was demonstrated directly from the evolution of labeled N2 following 15NH4+ amendments during spring experiments: >99% of the added 15NH4+ which was oxidized to 15NO3‒ was subsequently reduced to 15N‐labeled N2 during 48‐h incubations, suggesting that the two reactions were occurring in close proximity to one another. In contrast, this coupled nitrification‐denitrification was decreased by two orders of magnitude in the summer, even though the capacity for denitrification (N2 production from NO3‒ enrichment) remained similar to spring levels. This pattern of sharply seasonal nitrification was corroborated with measurements of bacterial relative abundance. Indirect evidence suggests low redox conditions (and reduced O2 concentrations) as the possible cause of decreased summertime nitrification. Ambient rates of nitrification‐denitrification were inferred from 15N experiments in these sediments. Estimated springtime rates of about 77–89 µmol N·m‒2·h‒1 are similar to previously reported values for denitrification supported by NO3‒ diffusion from overlying water to coastal sediments.