The biological and mechanical role of soil fungi.

The major role assigned to soil microorganisms is that of decomposition of the litter that collects on or under the soil surface. Additional roles are: synthesis of growth factors such as vitamins and auxins, fixing of atmospheric nitrogen (restricted to bacteria and blue-green algae), production of soil-aggregating substances, and several others. The relative importance of fungi and bacteria (including actinomycetes) is not accurately known as far as their dissimilatory roles are concerned, but it is generally accepted that bacteria assimilate only a minor fraction of the dissimilation products into their bodies, whereas nearly half the decomposition products are transformed by fungi into cell materials. This seems quite plausible if one observes the amount of mushrooms formed on rotting logs, whereas bacteria have very little to show for their activities. Therefore, in terms of biomass the fungi seem to be more important than bacteria in soil; but this may differ very much from one soil to the other, depending largely on the amounts of free water present. In general, fungi can grow where there is less free water. One does not get an accurate picture of the relative importance of bacteria and fungi in the soil by plating methods. When a suspension of soil is plated on nutrient agar, a large proportion of the active microorganisms, plus the sporeformers, will produce colonies. Most soil fungi do not sporulate but live as sterile hyphae that are seldom counted as colonies, partly because of their slow growth. The most commonly counted soil fungi are the fast growers with abundant conidiospore production such as Penicillium, Aspergillus, and a few others, which, by microscopic inspection of the soil, do not seem to play an important role there, but live mainly on recently dropped litter. It has been found in studying mycelium cultures that Rhizoctonia and Basidiomycetes are more common inhabitants of soil. It is usually claimed that most microorganisms in the soil live in the rhizosphere, a narrow sheath a fraction of a millimeter in width around living roots. This may be true for bacteria and actinomycetes, but it is not correct in the case of fungi. It is easier to observe fungi around and near roots, and especially if they form ectotrophic mycorrhizas because a dense mat of hyphae will be seen around the roots. But in any humus-containing soil there is a more or less dense network of hyphae anywhere in the soil away from the roots and rootlets. This is most easily observed in sandy soils. The following observations on fungi were made mainly in two environments: (1) in the Amazonian rain forest in poor to very poor soils, in both the central and western parts of the Amazon basin, and (2) in the southwestern deserts of the United States. The Brazilian forests were studied while we were participants of the Alpha Helix expedition to the Amazon in 1967, and those in Eastern Peru were visited while we were the recipients of a travel grant from the Smithsonian