The role of soil fertility in the function of mycorrhizal associations

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Background - Arbuscular mycorrhizal (AM) fungi and ectomycorrhizal (ECM) fungi play key roles in plant nutrient acquisition but their contributions to the stabilization of soil organic carbon (SOC) could differ substantially. ECM fungi are thought to promote the stabilization and storage of SOC, whereas there is evidence that AM fungi can stimulate the mineralization of SOC and its release as CO2. The propensity of mycorrhizal fungi to store carbon is likely to be influenced by soil fertility. The plant-mycorrhizal mutualism is based on the provision of carbon by plants to the fungal network in return for enhanced nutrient uptake. There is strong evidence that mycorrhizal function is nutrient-limited and recent research suggests that nutrient availability can alter the relationship between plants and AM fungi from mutualistic to semi-parasitic. This project aims to: 1) Perform the first direct comparison of AM and ECM fungal contributions to soil carbon storage and release; and 2) Evaluate the role of nutrient availability in structuring the exchange of resources between plants and mycorrhizal fungi. Programme of research - The project will involve a series of greenhouse experiments using tree seedlings that associate with both AM and ECM fungi (including aspen and poplar). Pots with a common soil type and one of the two tree species will be inoculated with AM, ECM, both fungi, or a sterilized combination of both fungi. The availability of different nutrients will be manipulated experimentally by adding appropriate fertilisers.The storage or release of soil carbon will be assessed by regular measurements of soil CO2 (soil respiration) and 13C pulse-labelling to track the flow of carbon through plants and mycorrhizal fungi into the soil. Changes in mycorrhizal function will be determined from measurements of tree growth, foliar nutrients, mycorrhizal infection rates, and mycorrhizal community composition. Selected treatments will be chosen for molecular characterisation of fungal phenotypes and functional genes.The results of these experiments will be used to inform and develop additional experiments with other tree species and/or field surveys along natural fertility gradients to test the wider applicability of the research.