Decomposition of ectomycorrhizal fine roots

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Trees allocate a large portion of their productivity to their fine feeder roots. In northern forests, the majority of these roots are colonized by ectomycorrhizal fungi - fungal symbionts that aid in nutrient and water uptake in exchange for plant sugars. Although the turnover of ectomycorrhizal fine roots represents an enormous input of carbon and nitrogen into the soil ecosystem, we have very little information regarding the decomposers involved, and therefore the fate of the resources liberated. Ectomycorrhizal fine roots are approximately 40% fungal tissue, which contains chitin and therefore significantly more nitrogen than plant tissue. However, it is not clear which organisms dominate the breakdown of fungi in forest soils. Bacterial vs fungal decomposition impacts both soil nutrient availability and carbon sequestration, as resources move more quickly through bacterial communities than through fungal communities. Bacteria are thought to be the dominant decomposers of chitin in alkaline soils, and soil fungi are considered to be more important under acidic conditions. However, bacterial decomposition of fungi may increase with higher temperatures, such that a larger proportion of fungal carbon and nitrogen may move through the more labile bacterial community in a warmer climate. Fine roots are also colonized internally by fungal endophytes. Although the ecological roles of these fungi are poorly understood, they may be involved in the decomposition of the plant component of ectomycorrhizae, and are well positioned within the root tissue. The symbiotic fungus forming a given ectomycorrhiza may also influence its decomposition rate, as some species produce antimicrobial compounds. ***The objectives of the proposed research are to: 1) determine which decomposers utilize the fungal component of ectomycorrhizae under different environmental conditions using DNA based techniques, 2) track microbial activity during the decomposition of whole ectomycorrhizae using RNA techniques (gene expression) and 3) determine the decomposition rates of ectomycorrhizae formed by different fungal species, characterizing the antimicrobial compounds produced by recalcitrant species. ***A prominent feature of the Canadian landscape is the boreal forest, an ecosystem in which all dominant tree species form ectomycorrhizae. Knowing the relative importance of bacteria, soil fungi, root endophytes and ectomycorrhizal fungi in the process of fine root decomposition is key to resolving the pathways though which the resulting fluxes of resources move through our northern forests. Information on the dynamic roles of soil microbes under varying environmental conditions, including those expected under climate change, is also critical to understanding present and future boreal forest ecosystem function. ********