Application of a soil core method to investigate the effect of oxygen concentration on denitrification

The effect of O₂ concentration on denitrification rate was investigated in sandy loam and clay loam soils. Oxygen at different concentrations and acetylene were recirculated through freshly-collected soil cores. Denitrification rates under anaerobic conditions with argon as the recirculating gas were also determined. The effect of O₂ on denitrification rate among cores could then be normalized by expressing the data as percent of the anaerobic rate. Denitrification rates were less than 2% of the anaerobic rate for O₂ concentrations greater than 3%, but greatly increased at concentrations below 0.5% O₂. For individual cores the denitrification rate increased 2- and 4-fold as the O₂ content of the recirculating gas was decreased from 20 to 5%. The pattern of denitrification rate versus O₂ concentration was very similar to that generated from the theoretical model of K. A. Smith, which describes the proportional anaerobic volume of soil aggregates as a function of O₂ concentration. A soil gas sampling probe is described which uses porous Gortcx tubing buried in horizontal soil layers. The recirculating gas in the denitrificating assay 4system could then be adjusted to match the soil O₂ content measured at the time of sampling.