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

Abstract

Microzonation of denitrification was studied in stream sediments by a combined O 2 and N 2 O microsensor technique. O 2 and N 2 O concentration profiles were recorded simultaneously in intact sediment cores in which C 2 H 2 was added to inhibit N 2 O reduction in denitrification. The N 2 O profiles were used to obtain high-resolution profiles of denitrification activity and NO 3 − distribution in the sediments. O 2 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 NO 3 − 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 NO 3 − 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 NO 3 − concentration and the apparent K m for NO 3 − was less than 10 μM. Denitrification was the only NO 3 − -consuming process in the dark-incubated stream sediment. Even in the presence of C 2 H 2 , a significant N 2 O reduction (up to 30% of the total N 2 O production) occurred in the reduced, NO 3 − -free layers below the denitrification zone. This effect must be corrected for during use of the conventional C 2 H 2 inhibition technique.