A THEORETICAL STUDY RELATING THE CONCENTRATION AND DIFFUSION OF OXYGEN TO THE BIOLOGY OF ORGANISMS IN SOIL

Abstract

Some components necessary for the eventual construction of a general model relating the concentration and diffusion of oxygen to the ecology of soil organisms are elaborated from biochemical and physical data and concepts. From the analyses and discussions presented, tentative conclusions can be drawn concerning some of the factors involved in the relationship between oxygen and soil ecology. For roots, bacteria and fungi, most oxygen is taken up through the cytochrome system. Little error is introduced by neglecting the rate of uptake by other terminal oxidases. The rate of diffusion to cells is thus a direct consequence of uptake by the cytochrome system. Oxygen uptake by individual cells will not be affected until the concentration at the sites of the terminal oxidases is of the order of 1 × 10−6 to 1 × 10−7 M. In many species, the rate of respiration of single fungal hyphae or single bacterial cells will be limited at low external oxygen concentrations by the rate of diffusion of oxygen to the respiratory sites through membranes, walls and sheaths external to those sites. An increase in concentration of oxidase at limiting oxygen concentrations may be ecologically significant in micro‐organisms. The rate at which oxygen diffuses radially through roots with a high respiration rate will often restrict the total rate of oxygen uptake. The centres of such roots will tend to be anaerobic under some field conditions and the situation is likely to be aggravated by root infection and by the presence of some organisms in the rhizosphere. Most roots will be adversely affected at oxygen concentrations higher than those required to adversely affect micro‐organisms on the root surface. The oxygen status of roots will be greatly modified if longitudinal diffusion of oxygen through intercellular spaces that are gas‐filled is considerable. The evaluation of the critical oxygen characteristics in field studies is fraught with difliculties largely associated with specifying the diffusion path of oxygen through the soil to the organ and with obtaining accurate measurements of either oxygen concentration or oxygen flux at the surface of the organ. Use of the platinum micro‐electrode to measure flux can probably give only empirical results but appears to be of practical use where a statistical approach can be used. In some organisms, the major effect of reduced oxygen concentration may well be on the synthesis, or absorption, of some metabolite and not be linked directly to the cytochrome system. Such an organism may be adversely affected by a relatively small reduction in the oxygen concentration and would thus be an exception to the general arguments presented in the article. Copyright © 1968, Wiley Blackwell. All rights reserved