Maintenance of denitrification potential in pasture soil following anaerobic events

Reduction enzymes involved in the denitrification process which are critical for the kinetics of NO3−, NO2−, N2O and N2 are synthesized under anaerobic conditions and repressed by subsequent aerobic conditions. The enzymes are progressively degraded in aerobic soil but are de-repressed by re-introduction of anaerobiosis. Dynamics of these enzymes after different periods of aerobic incubation were investigated for a soil from permanent pasture. Soil anaerobically incubated for 3 days was then aerobically stored at 25°C for 70 days. On days 0, 3, 7, 14, 28, 49 and 70 of storage, soil was amended with a KNO3 solution and combinations of C2H2, glucose and chloramphenicol (to prevent de novo enzyme synthesis). Additionally, on days 28, 49 and 70, soil was first pre-conditioned anaerobically for 3 days at 25°C and then assayed using the same treatments. The production of CO2 and N2O and the concentrations of NO3− and NO2− were then measured at 2, 5, 12, 24, 36 and 48 h in an anaerobic incubation at 25°C. Nitrate reductase concentration appeared to be high relative to the availability of C substrate at the onset of each anaerobic incubation. The reduction of NO3− and the production of N2O increased when glucose was added to the soil although de novo synthesis of the enzyme was inhibited by chloramphenicol. Concentrations of nitrite and nitrous oxide reductase decreased over storage time under aerobic conditions. The proportion of N2 relative to N2O formed decreased from 49% on day 0 to 28% on day 3 but was still 21% of gaseous N losses on day 49. No detectable N2 was formed on day 70. The ratio between the rate of N2O produced and the rate of CO2 produced decreased sharply from 1.01 to 0.78 between day 0 and 3 but only gradually declined thereafter. The anaerobic prior incubation of 3 days decreased the concentrations of NO2− and the N2O-to-N2 ratio, but increased the N2O-to-CO2 ratio as compared to the unconditioned soil. Chloramphenicol not only increased CO2 and N2O production, but also decreased nitrite reductase and nitrous oxide reductase activity. It was concluded that the duration of aerobiosis affected the concentrations of reduction enzymes involved in the denitrification process and thus the N2O-to-N2 ratio when anaerobic conditions were induced. The link between the C mineralization and the reduction of NO3− not only depends on the composition of the C substrate but also on the period of preceding anaerobiosis.