CONDITIONAL OUTCOMES IN THE RELATIONSHIP BETWEEN PINE AND ECTOMYCORRHIZAL FUNGI IN RELATION TO BIOTIC AND ABIOTIC ENVIRONMENT

An increasing number of studies demonstrate the variable and context-d ependent nature of mutualistic interactions, the outcome of which may vary in space and time in response to abiotic and biotic factors. In t his study we tested whether the intensity of grazing by soil fauna bri ngs about different costs and benefits in the Scots pine-ectomycorrhiz al (EM) fungi symbiosis with respect to nitrogen availability for the pine. The experiment was conducted in transparent microcosms in which a soil profile mimicking that of coniferous forest soil with Pinus syl vestris seedlings was created. The seedlings were grown either in N-ri ch or N-poor humus soil. The soils were defaunated, re-inoculated with heterotrophic soil microbes, and one seedling of P. sylvestris - eith er non-infected or infected with 3 species of EM fungi - was planted i n the microcosms. Half of the microcosms were thereafter re-faunated w ith diverse and numerically rich soil fauna typical to coniferous fore st soil to represent intensive grazing pressure on EM fungi, while the other half received bacterial feeding protozoans, and microbial feedi ng nematodes only (representing strongly reduced grazing pressure). Th e microcosms were incubated in a growth chamber with varying illuminat ion and temperature regimes for two growing seasons of the pine. After 45 weeks ca 10 times more EM fungal biomass was found on pine roots g rowing in N-poor soils than in N-rich soils. The amount of EM fungi wa s significantly reduced by the complex and abundant faunal community, particularly in N-poor soils, where the amount of EM fungi was less th an 16% of that found in systems with reduced grazing pressure. The sho ot and root production were significantly higher in the N-poor systems than in the N-rich systems. Seedlings grown in the N-rich soils had a markedly lower P:N-ratio (0.07) in the needles as compared to the one s in the N-poor soils (0.14). Soil type and soil fauna had a significa nt interaction on the total pine biomass production; in N-poor soils c omplex fauna reduced the amount of pine biomass, whereas in N-rich soi ls the effect of this fauna was negligible. Despite the clear faunal i nduced changes in the biomass production of the fungal symbiont, and a lthough the effects of grazing on pine growth were distinctly related to the nitrogen status of the soil. the costs and benefits that determ ine the net effects of the plant-fungus association were not unambiguo usly related to feeding by fauna on EM fungi. We hypothesize that the divergent influence of soil fauna in relation to pine growth between t he two soil types was not associated with reduced costs by the EM fung i for the pine but due to differences in the availability of P for pla nt uptake in the soils.