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 gaseous products of the denitrification process and the dynamics of the reduction 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 N2O production and NO3− and NO2− concentrations were monitored. Data were used to calculate characteristic parameters of the reduction enzymes, as stipulated in the DETRAN model. Chloramphenicol increased the production of CO2 significantly in each of the treatments but the additional application of C2H2 to the chloramphenicol-amended soil negated that increase significantly. Concentrations of NO3− dropped sharply within 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 of glucose increased the N2O-to-N2 ration from 2.1 to 3.2 but the addition of other forms of organic material had no detectable effect. The N2O-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 manure decreased the ratio to 0.44. Dynamics of NO3−, NO2−, N2O and N2 could be described with the DETRAN model assuming a ratio of NO3− reduced to CO2 produced ranging from 0.4 to 0.5. Chloramphenicol and glucose decreased the affinity for NO2− approximately 8-fold. This could imply that chloramphenicol which increased the CO2 production did not inhibit the NO2− reductase activity as was postulated before but favoured a reduction of NO3− to NO2− increasing the concentration of NO2−. It was concluded that the addition of glucose increased the N2O-to-N2 and the CO2-to-N2O ratio but the effect of more resistant material was less pronounced. The increase appeared to be related to a greater reduction of NO3− which was not matched by a greater reduction of N2O.