DYNAMICS OF REDUCTION ENZYMES INVOLVED IN THE DENITRIFICATION PROCESSIN PASTURE SOIL

When oxygen is depleted in soil, reduction enzymes involved in the den itrification process are activated and de novo synthesis of enzymes st arts within a few hours. The dynamics of these enzymes and the effect on the concentration of inorganic N formed were investigated for a soi l from permanent pasture. Soil was incubated aerobically for 5 days an d then amended with 100 mg NO3--N kg-1. Treatments were with or withou t C2H2 and with or without chloramphenicol (found to inhibit de novo s ynthesis of reduction enzymes), purged of all O2, shaken and anaerobic ally incubated for 48 h while CO2 and N2O production and NO3- and NO2- concentrations were monitored. Chloramphenicol was found to have no i nhibitory effects on nitrate reduction indicating that nitrate reducta se activity persisted in the absence of de novo synthesis. The persist ence of nitrite reductase and nitrous oxide reductase was lower as the application of chloramphenicol increased NO2- concentrations and redu ced N2 production. In the absence of chloramphenicol, de novo synthesi s of nitrite reductase started 5 h and that of nitrous oxide 16 h afte r anaerobiosis was imposed. It is concluded that the dynamics of nitri te reductase have only a small effect on the N2O production as NO2- co ncentrations remained below 1 mg N kg-1 but the low persistence of N2O reductase in combination with its retarded de-repression results in a high N2O-to-N2 ratio when anaerobic conditions are rapidly induced.