Effects of Oxygen on Denitrification Inhibition, Repression, and Derepression in Soil Columns

Abstract

Although it is known that O2 inhibits and represses denitrification, few studies have examined the effect of O2 on NO production. Our objectives were to measure O2 inhibition, repression, and derepression of NO and N2O production by denitrifying microorganisms in soil columns continuously purged by N2 or various constant (0.074–15%) O2–N2 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% O2, NO production was reduced to ≈50 to 58% and N2O rates to ≈29 to 32% of their maximum anaerobic rates. Maintaining O2 at ≥5% in soil without added C or at >10% O2 in C-amended soil decreased (repressed) NO production rates by a factor of ≈1.5 to 1.8 d−1 Rates of N2O repression remained constant at ≈0.07 d−1 for all C and O2 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 O2 effects on N2O production and apparently is the first systematic study of O2 inhibition, repression, and derepression of NO production.