DENITRIFICATION IN PERMANENT PASTURE SOIL AS AFFECTED BY DIFFERENT FORMS OF C SUBSTRATE

The effect of the addition of different forms of C substrate on gaseou s products of the denitrification process and the dynamics of the redu ction enzymes were investigated in a soil from permanent pasture. Soil was amended with 100 mg NO3--N kg(-1) plus glucose, cellulose, grass litter or cattle manure. Treatments were with or without C2H2 and with or without chloramphenicol to inhibit de novo synthesis of reduction enzymes. Samples were anaerobically incubated for 48 h while CO2 and N 2O production and NO3- and NO2- concentrations were monitored. Data we re used to calculate characteristic parameters of the reduction enzyme s, as stipulated in the DETRAN model. Chloramphenicol increased the pr oduction of CO2 significantly in each of the treatments but the additi onal application of C2H2 to the chloramphenicol-amended soil negated t hat increase significantly. Concentrations of NO3- dropped sharply wit hin the first 2 h, increased again after 5 h and showed a more gradual decrease thereafter. Concentrations of NO2- increased sharply within the first 5 h and continued to increase in the chloramphenicol-amended soil but gradually decreased in the unamended soil. The application o f glucose increased the N2O-to-N-2 ratio from 2.1 to 3.2 but the addit ion of other forms of organic material had no detectable effect. The N 2O-to-CO2 ratio increased from 0.46 in the control soil to 0.49 in the cellulose, to 0.50 and to 0.51 in the glucose-amended soil, Cattle ma nure decreased the ratio to 0.44. Dynamics of NO3-, NO2-, N2O and N-2 could be described with the DETRAN model assuming a ratio of NO3- redu ced to CO2 produced ranging from 0.4 to 0.5. Chloramphenicol and gluco se decreased the affinity for NO2- approximately 8-fold. This could im ply that chloramphenicol which increased the CO2 production did not in hibit the NO2- reductase activity as was postulated before but favoure d a reduction of NO3- to NO2- increasing the concentration of NO2-. It was concluded that the addition of glucose increased the N2O-to-N2O a nd the CO2-to-N2O ratio but the effect of more resistant material was less pronounced. The increase appeared to be related to a greater redu ction of NO3- which was not matched by a greater reduction of N2O.