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 important determinants of the rate and the timing of denitrification in permanent pasture. The assays, however, involved the use of soil slurries to optimise conditions for denitrification. We have now determined whether prior conditioning could also influence the proximate controls of denitrification in structurally-intact soil. Cores of a clay loam soil from permanent pasture were conditioned by submerging in distilled water for 6 h(“Short-wet” soil) or 96 h (“Long-wet” soil) and drained 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 “Long-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°C. The production of CO2 and N2O was monitored daily. The cores were then cut into four 2.5 cm sections and NO3− and NO2− concentrations and the gravimetric water content were determined in each subsample. The assay for enzyme activity indicated that the initial concentrations of reduction enzymes, or the potential for their synthesis, was greater in soils which had been conditioned under the “Long-wet” treatment than in the “Short-wet” one. Concentrations of NO3− and NO2− decreased faster in the “Long-wet” than in the “Short-wet” soil and N2O-to-N2 ratios were smaller. In the intact soil cores, concentrations of NO3− and N2O production rates in the C2H2-amended soil were not significantly different between the two treatments. Water contents showed statistically significant differences although the values for the treatments were comparable (43.6% vs 40.4%). Hence differences in N2O-to-N2 ratios (1.18 in the “Long-wet” vs 4.15 in the “Short-wet” soil) were attributed to differences in concentrations of reduction enzymes 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-N2 ratios, thereby contributing to temporal and spatial variability of the denitrification process. The results of the assay systems using intact cores were similar to those for soil slurries.