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

Abstract

Nitrification and denitrification in estuarine sediments were measured by 15 N 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 N 2 following 15 NH 4+ amendments during spring experiments: >99% of the added 15 NH 4+ which was oxidized to 15 NO 3‒ was subsequently reduced to 15N‐labeled N 2 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 (N 2 production from NO 3‒ 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 O 2 concentrations) as the possible cause of decreased summertime nitrification. Ambient rates of nitrification‐denitrification were inferred from 15 N experiments in these sediments. Estimated springtime rates of about 77–89 µ mol N·m‒2·h‒1 are similar to previously reported values for denitrification supported by NO 3‒ diffusion from overlying water to coastal sediments.