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Ectomycorrhizae and forest biogeochemistry: The role of ectomycorrhizal communities in forests impacted by global change


Ectomycorrhizal communities may play a major role in preventing decreases in forest productivity associated with the depletion of nutrients caused by anthropogenic nitrogen and in facilitating increased productivity in response to elevated atmospheric CO2 concentrations. In my doctoral research I attempted to shed some light on the following question: As nutrient demand by forest trees is altered by human induced global change how will the functioning of ectomycorrhizal communities respond?

In chapter 1, I present the results of a study of the effects of nitrogen fertilization on ectomycorrhizal communities in an Eastern US hardwood forest. I found the ectomycorrhizal communities of the organic and mineral horizon to be quite distinct, and that high, but not moderate levels of nitrogen fertilization altered the ectomycorrhizal community composition and decreased ectomycorrhizal species richness. I also found that ectomycorrhizal colonization intensity increased in the mineral soil, and when considered in conjunction with other studies conducted in the same research forest, this may indicate that the ectomycorrrhizal community is shifting in accordance with the shifting nutrient demands of the forest. In Chapter 2, I attempted to elucidate the role of soil heterogeneity in shaping fungal community composition in the mineral soil. Depth and soil carbon content were consistently correlated with fungal community composition. The parent material from which the overlying soil is derived and soil calcium content may be important in determining fungal community composition but our sampling scheme did not allow us to isolate these chemical factors from the potential influence of geographic location on fungal community. In chapter 3, I present a literature review that endeavored to determine whether there is potential for ectomycorrhizal communities or individuals to alter their mineral weathering capabilities and nutrient provision to host plants in response to altered nutrient demand from their host plants. Knowledge of the mechanisms that control belowground carbon allocation by plants in response to nutrient demand is limited, but there is a potential for plants to respond to increased demand of phosphorous and potassium by allocating more carbon to the fungal symbionts most adept at providing these nutrients. Future studies on ectomycorrhizal weathering should explicitly test the role of host nutrient demand in stimulating fungal weathering. In chapter 4, I investigated the role of plants, ectomycorrhizae, and low molecular weight organic acids in stimulating mineral weathering, as well as the potential for elevated carbon dioxide to affect biotic weathering. We found that plants, but not their associated ectomycorrhizae, stimulated weathering. Elevated CO2 did not affect weathering rates. The lack of an effect of ectomycorrhizal colonization may have been due to low levels of mycorrhizal colonization. The biotic weathering observed in this study was driven by the uptake of nutrient cations, and not by substrate acidification or root exudation. My doctoral research suggests that ectomycorrhizae may play an important role in mineral nutrient provision.

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