Diffusion of substrates and products in soil may be thought of as a problem in the optimization of specific microbial processes. However, these same diffusional constraints promote the physical establishment of a great diversity of microorganisms that would not otherwise occur in close spatial and temporal proximity. This heterogeneity effects the occurrence of sequential rate processes that would not occur in a homogeneous system. One such example is the anaerobic-aerobic mineralization of aerobically recalcitrant environmental contaminants which undergo limited transformation, under anaerobic conditions, to products that are mineralized aerobically. Another example is the production of H2 from root nodules and its subsequent consumption by H2-oxidizing bacteria. Since H2 is the fastest moving energy source, the distance of this rhizosphere is larger than is classically envisioned. Hence, “rhizosphere” can also be viewed as an operational term governed by metabolism and not by an arbitrary distance.
