Nutrient Cycling in Boreal Forests-a Mycological Perspective Studies on phosphorus translocation within and between mycelia of saprotrophic-and mycorrhizal fungi

In order to understand the mechanisms controlling ecosystem diversity, production and responses to disturbance, improved knowledge about the movement and transformation of nutrients is essential. Most currently used models of nutrient cycling in boreal forests have been developed with agricultural ecosystems in mind. Boreal forest ecosystems are characterised by a high abundance and diversity of basidiomycetous fungi. These fungi occur rarely in agricultural soils but play a pivotal role in boreal forests as decomposers of organic matter and symbiotic associates of plants. The ecophysiology of basidiomycetous fungi has to be considered, when constructing nutrient cycling models for boreal ecosystems. Decomposer fungi and symbiotic mycorrhizal fungi have traditionally been placed in distinct functional categories and treated separately. This separation has no phylogenetic justification however, and fungi from the two groups share a similar mycelial morphology as well as the same microsites on the forest floor. This thesis describes experiments in which radioactive phosphorus was used in combination with non-destructive electronic autoradiography to study nutrient translocation in soil microcosms containing saprotrophicand ectomycorrhizal fungi. Bidirectional phosphorus translocation in fungal rhizomorphs was observed, showing that nutrients may circulate throughout basidiomycetous mycelia, enabling net translocation from sources to sinks. Studies of mycelial interactions between ectomycorrhizal fungi and saprotrophic fungi suggested that ectomycorrhizal fungi can interact antagonistically with other soil fungi. Interactions were associated with transfer of significant amounts of phosphoms between the interacting mycelia. Ectomycorrhizal fungi were able to mobilise radioactive phosphorus from labelled saprotrophic mycelium and to transfer the acquired phosphoms to their host plants. Wood decomposing fungi were similarly able to mobilise phosphoms from mycorrhizal mycelium and to translocate the acquired phosphoms to colonised wood blocks. The net direction and rate of phosphoms transfer between interacting mycelia was shown to depend on the availability of resources to the interacting fungi. To explain the observed phosphoms transfer it is hypothesised that interacting basidiomycetous fungi may obtain nutrients by killing and degrading each other’s mycelia. This highly competitive foraging behaviour, in combination with the ability to translocate resources over considerable distances, makes basidiomycetous fungi well adapted to the spatial heterogeneity and low nutrient availability of the boreal forest floor. A new model of nutrient cycling in boreal forests is proposed that allows for nutrient retention in soil fungi and intense competition for nutrients between soil organisms. Symbiotic association with ectomycorrhizal fungi that can effectively compete with other soil organisms for organic nutrient sources, enables plants to acquire nutrients without the need for large scale nutrient mineralisation.