MICROSCALE DISTRIBUTION OF NITRIFICATION ACTIVITY IN SEDIMENT DETERMINED WITH A SHIELDED MICROSENSOR FOR NITRATE

Microprofiles of O2 and NO3- were measured simultaneously in freshwate r sediment with microsensors which were completely free from electrica l interference because of coaxial designs. Depth profiles of nitrifica tion (NO3- production) and denitrification (NO3- consumption) were sub sequently determined by computer simulation of the measured microprofi les. The nitrifying bacterial community responded very quickly to chan ges in environmental conditions, and new steady-state microprofiles of O2 and NO3- were usually approached within a few hours after perturba tion. Nitrification started quickly after introduction of O2 in previo usly anoxic layers, suggesting prolonged survival of the nitrifiers du ring anaerobiosis. Changes in the availability of O2 and NH4+ greatly affected the nitrification profile, and there was a high rate of coupl ed nitrification-denitrification under conditions in which nitrificati on occurred right above the oxic-anoxic interface. Addition of C2H2 ra pidly removed the NO3- peaks, indicating that NO3- production was due mainly to autotrophic nitrification.