MAINTENANCE OF DENITRIFICATION POTENTIAL IN PASTURE SOIL FOLLOWING ANAEROBIC EVENTS

Reduction enzymes involved in the denitrification process which are cr itical for the kinetics of NO3-, NO2-, N2O and N-2 are synthesized und er anaerobic conditions and repressed by subsequent aerobic conditions . The enzymes are progressively degraded in aerobic soil but are de-re pressed 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 w as then aerobically stored at 25 degrees 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 degrees C and the n 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, 2 4, 36 and 48 h in an anaerobic incubation at 25 degrees C. Nitrate red uctase concentration appeared to be high relative to the availability of C substrate at the onset of each anaerobic incubation. The reductio n of NO3- and the production of N2O increased when glucose was added t o the soil although de novo synthesis of the enzyme was inhibited by c hloramphenicol. Concentrations of nitrite and nitrous oxide reductase decreased over storage time under aerobic conditions. The proportion o f N-2 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 N-2 was formed on day 70. The ratio between the rate of N2O produced and t he rate of CO2 produced decreased sharply from 1.01 to 0.78 between da y 0 and 3 but only gradually declined thereafter. The anaerobic prior incubation of 3 days decreased the concentrations of NO2- and the N2O- to-N-2 ratio, but increased the N2O-to-CO2 ratio as compared to the un conditioned soil. Chloramphenicol not only increased CO2 and N2O produ ction, but also decreased nitrite reductase and nitrous oxide reductas e activity. It was concluded that the duration of aerobiosis affected the concentrations of reduction enzymes involved in the denitrificatio n process and thus the N2O-to-N-2 ratio when anaerobic conditions were induced. The link between the C mineralization and the reduction of N O3- not only depends on the composition of the C substrate but also on the period of preceding anaerobiosis.