Effects of oxygen on denitrification inhibition, repression, and derepression in soil columns

Although it is known that O-2 inhibits and represses denitrification, few studies have examined the effect of O-2 on NO production. Our objectives were to measure O-2 inhibition, repression, and derepression of NO and N2O production by denitrifying microorganisms in soil columns continuously purged by N-2 or various constant (0.074-15%) O-2-N-2 mixtures. Net rates of NO and N2O production were measured under successive anaerobic, partially aerobic, and anaerobic conditions. Oxygen inhibition was rapid and reversible. Within 5 min after exposure to >5% O-2, NO production was reduced to approximate to 50 to 58% and N2O rates to approximate to 29 to 32% of their maximum anaerobic rates. Maintaining O-2 at greater than or equal to5% in soil without added C or at > 10% O-2 in C-amended soil decreased (repressed) NO production rates by a factor of approximate to1.5 to 1.8 d(-1). Rates of N2O repression remained constant at approximate to0.07 d(-1) for all C and O-2 treatments. Restoration of anoxic conditions following the aerobic phase reversed inhibition; within 5 min, NO production rates by the nonrepressed denitrifiers increased to 55 to 101% of their respective anaerobic rates and N2O production rates increased to 26 to 62%. The rates of NO and N2O production then increased more slowly (derepression) during this anaerobic period. This research supports previous observations for O-2 effects on N2O production and apparently is the first systematic study of O-2 inhibition, repression, and derepression of NO production.