The rhizosphere is the soil zone surrounding and influenced by plant roots. To complete the picture of the microbiotic community, the author examines the specific characteristics of the rhizosphere that affect microflora and fauna. The primary influences in this zone are root exudates and the chemical and physical features of the root-soil interface. Root exudates and sloughed root cells in the rhizosphere provide a rich carbon source for microbes. As the microbial population increases, so does the activity of microbe-grazing fauna, leading eventually to the release of inorganic nitrogen and phosphorus.
Root exudates can also have a strong influence on soil pathogens.
The nutrients in exudates release dormant spores and sclerotia from the grip of soil fungistasis, a natural phenomenon imposed by chemical inhibitors and microbial competition for nutrients. Root exudates are also important in the activation of quiescent nematode stages and in the movement of plant parasitic forms to infection sites. This analysis of the soil fauna and the author's description of the niche the fauna have in the rhizosphere are sufficient to indicate that soil fauna do regulate pathogen populations to some degree. Investigations since 1975 have given some detail to the actual mechanisms involved and highlight some interesting relationships. These are summarized in table 1.