GASEOUS PRODUCTS OF THE DENITRIFICATION PROCESS AS AFFECTED BY THE ANTECEDENT WATER REGIME OF THE SOIL

We have shown in previous studies that the antecedent redox regime in soil, and the associated dynamics of the reduction enzymes, were impor tant determinants of the rate and the timing of denitrification in per manent pasture. The assays, however, involved the use of soil slurries to optimise conditions for denitrification. We have now determined wh ether prior conditioning could also influence the proximate controls o f denitrification in structurally-intact soil. Cores of a clay loam so il From permanent pasture were conditioned by submerging in distilled water for 6 h (''Short-wet'' soil) or 96 h (''Long-wet'' soil) and dra ined for 2 or 48 h, respectively, to obtain comparable water contents in both treatments. The activity of the reduction enzymes involved in the denitrification process was assessed in the ''Short-wet'' and ''Lo ng-wet'' soil, and the cores were then amended with 100 mg NO3--N kg(- 1) and incubated aerobically with or without C2H2 for 96 h at 25 degre es C. The production of CO2 and N2O was monitored daily. The cores wer e then cut into four 2.5 cm sections and NO3- and NO2- concentrations and the gravimetric water content were determined in each subsample. T he assay for enzyme activity indicated that the initial concentrations of reduction enzymes, or the potential for their synthesis, was great er in soils which had been conditioned under the ''Long-wet'' treatmen t than in the ''Short-wet'' one. Concentrations of NO3- and NO2- decre ased faster in the ''Long-wet'' than in the ''Short-wet'' soil and N2O -to-N-2 ratios were smaller. in the intact soil cores, concentrations of NO3- and N2O production rates in the C2H2-amended soil were not sig nificantly different between the two treatments. Water contents showed statistically significant differences although the values for the tre atments were comparable (43.6% vs 40.4%). Hence differences in N2O-to- N-2 ratios (1.18 in the ''Long-wet'' vs 4.15 in the ''Short-wet'' soil ) were attributed to differences in concentrations of reduction enzyme s induced by the conditioning of the cores. It was concluded that the antecedent water regime of a soil could, independently of its current water content and NO3- concentration, affect N2O-to-N-2 ratios, thereb y contributing to temporal and spatial variability of the denitrificat ion process. The results of the assay systems using intact cores were similar to those for soil slurries.