Influence of ectomycorrhiza on the structure of detrital food webs in pine rhizosphere

Trees infected with ectomycorrhizal (EM) fungi are known to direct several times more energy into the soil than non-ectomycorrhizal trees. Since energy-rich substrates are required by most biological processes, it is widely speculated that the flow of carbon from plants through EM mycelia into soils should have a wide range of effects in the below-ground milieu. In this study we investigated whether EM fungi of Scots pine (Pinus sylvestris) seedlings exert a short-term influence on the biomass and composition of a detrital food web consisting of up to three trophic levels. The experiment was conducted in microcosms with raw humus, perlite and sandy soil (1:1:1; vol.). The soil was defaunated, reinoculated with 10 species of soil bacteria, and 11 species of saprophytic soil fungi, and pine seedlings, either infected or non-infected with six taxa of EM fungi, were separately planted in the microcosms. Five different food webs were established: 1) saprophytic microbes alone, 2) as 1) but with two species of bacterial-feeding nematodes, 3) as 2) but two species of fungal-feeding nematodes included, 4) as 2) but with predatory nematodes, and 5) as 3) but with predatory nematodes. The microcosms were incubated in a climate chamber with varying illumination and temperature regimes for two annual cycles of the pine. After 37 weeks. ca 15 times more fungal biomass was detected on pine roots growing in the systems inoculated with EM fungi than in non-inoculated systems, and there was a significantly greater total biomass of pines in the EM systems. Despite this clearly enhanced autotroph production, neither the total nematode biomass nor the length of the food chain were affected by the EM fungi. The presence of nematodes, independent of the composition of this fauna, enhanced the production of pine biomass significantly, but only in the absence of mycorrhizal fungi. Neither the species composition nor the trophic group architecture of the fauna influenced EM production or growth of EM infected pines, except for fungal-Feeding nematodes which had a slight negative influence on the biomass of EM fungi. Our results infer that the role of EM fungi in affecting the amount of energy entering the soils is significant in the long run only, i.e. through increased litter production. We hypothesize that the short-term insensitivity of the decomposer food web to the presence of EM fungi reflects the nature of EM fungi as an efficient sink, preventing the additional production of plant-derived photosynthates from reaching the organisms of the detrital food web.