Production of NO, N2O and N2 by extracted soil bacteria, regulation by NO2(-) and O2 concentrations.

The oxygen control of denitrification and its emission of NO/N2O/N2 was investigated by incubation of Nycodenz-extracted soil bacteria in an incubation robot which monitors O2, NO, N2O and N2 concentrations (in He+O2 atmosphere). Two consecutive incubations were undertaken to determine (1) the regulation of denitrification by O2 and NO2(-) during respiratory O2 depletion and (2) the effects of re-exposure to O2 of cultures with fully expressed denitrification proteome. Early denitrification was only detected (as NO and N2O) at <or=80 microM O2 in treatments with NO2(-), and the rates were three orders of magnitude lower than the rates observed after oxygen depletion (with N2 as the primary product). When re-exposed to O2, the cultures continued to denitrify (8-55% of the rates during the foregoing anoxic phase), but its main product was N2O. The N2O reductase activity recovered as oxygen was being depleted. The results suggest that expression of the denitrifying proteome may result in significant subsequent aerobic denitrification, and this has profound implications for the understanding and modelling of denitrification and N2O emission. Short anoxic spells caused by transient flooding during rainfall, could lead to subsequent unbalanced aerobic denitrification, in which N2O is a major end product.