Microscale distribution of nitrification activity in sediment determined with a shielded microsensor for nitrate.

Microprofiles of O(2) and NO(3) were measured simultaneously in freshwater sediment with microsensors which were completely free from electrical interference because of coaxial designs. Depth profiles of nitrification (NO(3) production) and denitrification (NO(3) consumption) were subsequently determined by computer simulation of the measured microprofiles. The nitrifying bacterial community responded very quickly to changes in environmental conditions, and new steady-state microprofiles of O(2) and NO(3) were usually approached within a few hours after perturbation. Nitrification started quickly after introduction of O(2) in previously anoxic layers, suggesting prolonged survival of the nitrifiers during anaerobiosis. Changes in the availability of O(2) and NH(4) greatly affected the nitrification profile, and there was a high rate of coupled nitrification-denitrification under conditions in which nitrification occurred right above the oxic-anoxic interface. Addition of C(2)H(2) rapidly removed the NO(3) peaks, indicating that NO(3) production was due mainly to autotrophic nitrification.