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

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 muM 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 h. With 1,250 muM 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 h. 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 muM. 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.