Tritrophic interactions in a soil community enhance decomposition rates

Microbivorous soil fauna can influence decomposition rates by regulating bi omass and composition of the microbial community. The idea that predators a t higher trophic levels regulate population densities of microbivorous faun a and thus indirectly increase microbial growth and activity has often been suggested but rarely examined in soil ecosystems. In this paper the effect s of tritrophic interactions on decomposition processes in the soil are stu died and expressed as soil respiration, hyphal lengths, cellulase and chiti nase activities. The experiments were carried out in soil microcosms in a f actorial design with three fungal species (Alternaria alternata, Fusarium o xysporum, Trichoderma viride), the fungivorous collembolan Folsomia fimetar ia and the predatory mite Hypoaspis aculeifer. The respiration rate was sig nificantly higher with three trophic levels than in those with two and lowe st in those with only fungi present. This indicates that a low level of gra zing stimulates microbial respiration more than a high level or no grazing at ail. The effect was similar for all three fungal species but most pronou nced in microcosms with the fungus A. alternata which was a preferred food source by the collembolans. Hyphal lengths were in all cases but with T, vi ride reduced in the presence of collembolans and predatory mites. T. viride had a slightly higher chitinase activity than the other fungi but increase d numbers of trophic levels did not affect the enzymatic activities of any of the fungi.