Oxygen consumption by soil microorganisms as affected by oxygen and carbon dioxide levels

Abstract

Models of soil anaerobiosis generally describe microbial O 2 consumption as independent of 0 2 and CO 2 concentrations. In the present work, the effects of these gases on soil respiration were studied using artificial and natural aggregates without gas transport limitation. The chemical reactions of CO 2 in soil solution were analysed to assess the actual CO 2 levels affecting microorganisms. Both O 2 and CO 2 affected O 2 consumption. For artificial aggregates, respiration was partially inhibited by CO 2 at all tested concentrations (4–12% (v/v) CO 2). Respiration of natural aggregates was stimulated at 4% CO 2 and inhibited at higher concentrations. These different aggregate behaviours could be attributed to changes in microbial populations associated with the preparation of artificial aggregates. A model based on Michaelis-Menten kinetics and including competitive inhibition by CO 2, adequately described the effects of O 2 and CO 2 concentrations on O 2 consumption by artificial aggregates. The model was partially modified to account for CO 2 stimulation effect in natural aggregates. K M values for O 2 consumption were 1.69% O 2 for artificial aggregates and 5.80% 0 2 for natural aggregates. Calculations to estimate soil anaerobiosis showed that the effects of O 2 and CO 2 on respiration may not be ignored to predict the anaerobic regions within aggregates. However, the dissolved CO 2 corresponds to a small fraction of the mineral carbon present in soil solution, and its effect on microorganisms will depend on the position within the soil aggregate or the soil profile.