DISSIMILATORY REDUCTION OF 15N-LABELED NITRATE IN THE PRESENCE OF NONLABELED NO OR N2O

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Abstract
Dissimilatory reduction of labeled nitrate in the presence of nonlabeled N2O, or NO, was studied in forest mor humus during anaerobic incubation. Data on the isotopic composition of the N2O and N2 pools suggested that all of the N2 formed had passed the N2O pool. This finding supported the earlier suggestion that N2O is an obligatory intermediate in the dissimilatory reduction of nitrate to N2. Due to a likely occurrence of nonenzymatic isotopic exchange between NO and nitrate, the technique used was considered unreliable to study the nature of NO as a possible intermediate in the sequence of nitrate reduction. However, qualitative observations indicated that NO was not a free, obligatory intermediate. Reduction of nitrate was strongly inhibited by the presence of NO. Reduction of N2O to N2 was significantly retarded by the presence of nitrate. The length of the lag period increased as the initial nitrate concentration was increased. Key words: Dissimilatory nitrate reduction, denitrification, nitrous oxide, nitric oxide