Relations trophiques entre la microfaune et la microflore du sol; aspects qualitatifs et quantitatifs

Abstract

Abstract Trophic relationships between the microfauna and the microflora of the soil: qualitative and quantitative aspects. Decomposition of all the organic matter produced by a terrestrial ecosystem is a complex process in which both micro-organism and soil animals take part. Microbial attacks on plant matter start early and increase with the seasons so that the leaves become coated more and more with yeasts. Then in autumn, the dead leaves on the ground are rapidly infested with fungi; most small invertebrates feeding on litter are more interested probably in fungal hyphae or spores than in ordinary plant material. A field study has been made on the recolonization by microfauna and microflora in forest soil samples first heat-dried and then incubated. Oxygen consumption measurements showed greater metabolic activity in the treated soil samples than in the controls. Natural rehydration following desiccation promoted the development of a great number of microbial germs (74430- 103/g in controlled soil samples), but of restricted number of fungal species. Meanwhile surfacedwelling and edaphic Collembolan species soon invaded the treated plots in excessive numbers: these events indicate that the desiccated soil samples were acting as bait with which the soil animals were more concerned with the quantity than the quality. Electron microscopy of the faeces from four coexisting Collembolan species revealed that animals do not compete for food resources of the same biotope (spores, fungal hyphae, algae, bacteria), thus corroborating the general ecological theory that holds that groups of related species sharing habitats develop different choices in food. The ingested organisms are not completely digested and an important part, when evacuated, are still viable. Thus we may assume that the role of Collembola is important in disseminating propagules of soil microflora. The use of fine microgravimetric techniques has enabled us to estimate the weight of droppings in two coexisting Collembola species. The amount of ingested food kept inside the gut is very important; it is about 20% of the dry body weight. Ecological implications of the various aspects of the feeding biology of soil fauna are discussed. The evidence suggests that electron microscopy is very helpful in determining the feeding habits of soil animals and their role as regulators in the decomposition of soil forest litter systems.