GROWTH OF BIRCH AND PINE-SEEDLINGS IN RELATION TO GRAZING BY SOIL FAUNA ON ECTOMYCORRHIZAL FUNGI

It is hypothesized that mutualistic relationships can be persistent due to a third ''external'' species constraining the otherwise unlimited growth of the mutualists. By consuming mycorrhizal fungi, soil fauna may exert such a control over the development of the plant-fungus symbiosis. In this study I investigated the influence of soil fauna on the mutualistic association of ectomycorrhizal (EM) fungi and seedlings of birch and pine in transparent experimental microcosms simulating coniferous forest soil. The soil was defaunated, reinoculated with soil microbes, and seedlings of silver birch (Betula pendula) or Scots pine (Pinus sylvestris) infected with 3-4 taxa of EM fungi were separately planted in the microcosms. Half of the microcosms were thereafter reinoculated with diverse soil fauna. Microcosms were incubated in a climate chamber with varying illumination and temperature regimes for two annual cycles of the plant. After 57 wk the biomass of EM fungi was significantly reduced in the refaunated microcosms, the amount of fungi on birch and pine roots being 50 and 20%, respectively, of that in the non-refaunated controls. Similarly, the total microbial biomass in the bulk soil was significantly lower in the refaunated systems. The shoot production of both tree species was significantly enhanced in the presence of fauna: by the end of the experiment 1.5 (birch) and 1.7 (pine) times more aboveground biomass was produced in the refaunated microcosms than in the controls. Concentrations of N and P in the foliage of both tree species were also higher in the refaunated microcosms. These data indicate that the obvious reduction in EM fungi due to soil fauna is not harmful for the plant-fungus symbiosis provided that the community composition of the belowground food web is complex enough to ensure efficient mobilization of nutrients in the mycorrhizosphere.