Microzonation of Denitrification Activity in Stream Sediments as Studied with a Combined Oxygen and Nitrous Oxide Microsensor

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Abstract
Microzonation of denitrification was studied in stream sediments by a combined O2 and N2O microsensor technique. O2 and N2O concentration profiles were recorded simultaneously in intact sediment cores in which C2H2 was added to inhibit N2O reduction in denitrification. The N2O profiles were used to obtain high-resolution profiles of denitrification activity and NO3− distribution in the sediments. O2 penetrated about 1 mm into the dark-incubated sediments, and denitrification was largely restricted to a thin anoxic layer immediately below that. With 115 μM NO3− in the water phase, denitrification was limited to a narrow zone from 0.7 to 1.4 mm in depth, and total activity was 34 nmol of N cm−2 h−1. With 1,250 μM NO3− in the water, the denitrification zone was extended to a layer from 0.9 to 4.8 mm in depth, and total activity increased to 124 nmol of N cm−2 h−1. Within most of the activity zone, denitrification was not dependent on the NO3− concentration and the apparent Km for NO3− was less than 10 μM. Denitrification was the only NO3−-consuming process in the dark-incubated stream sediment. Even in the presence of C2H2, a significant N2O reduction (up to 30% of the total N2O production) occurred in the reduced, NO3−-free layers below the denitrification zone. This effect must be corrected for during use of the conventional C2H2 inhibition technique.

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