Forests and forest soils have a major effect on the Earth's carbon cycle. Understanding what controls forest carbon storage is thus important for predicting both the effects of climate change on forests, and the effects of forests on climate change. Trees grow by converting atmospheric carbon dioxide into biomass such as wood and leaves through photosynthesis; when trees eventually die, their biomass can either be stored in soil or consumed by microbes via the process of decomposition. The fate of carbon in decomposition depends on what microbial groups dominate: free-living soil decomposers or specialized (ectomycorrhizal or EM) fungi that inhabit roots. Free-living microbial decomposition recycles nutrients into plant-available forms that would otherwise remain locked up in dead plant tissues, but also simultaneously respires carbon dioxide back to the atmosphere. In contrast, symbiotic EM fungi are supplied with carbon from the plant, so their role in decomposition is only to release nutrients and not carbon. If EM fungi can effectively out-compete the free-living decomposers for nutrients, more carbon should remain within the ecosystem than is lost to the atmosphere. This hypothesis is supported by previous research showing that ecosystems associated with EM fungi store more carbon at a global scale. This research project support experiments to quantify how important these specialized fungi are in forest ecosystem carbon storage. The goals of this research are to better understand the role of specialized fungi and generate the information needed to incorporate their activities into ecosystem and Earth carbon cycling models. The project will support the research of a graduate student, and will also provide opportunities for underrepresented minorities to gain significant research experience.
Both primary producer and decomposer activities can be limited by nitrogen (N) in terrestrial ecosystems, setting the stage for competition between trophic levels. Many plants associate with ectomycorrhizal fungi. These EM fungi can produce N-degrading enzymes, allowing plants to access soil organic N directly, without requiring conversion to accessible forms by free-living decomposers. By "short-circuiting" the traditional N cycling pathway, plants associated with EM fungi may induce or exacerbate N limitation of decomposers, and thus increase soil and ecosystem C storage. Competition between primary producers and decomposers via mycorrhizal fungi would have important implications for ecosystem ecology, as it could be a major driver of soil organic matter stability and ecosystem C storage that is independent of other known drivers such as climate, soil mineralogy and organic matter chemistry. The project will assess the importance of EM-mediated plant-decomposer competition and at both local and global scales. At local scales, soil processes will be studied along a gradient of EM fungal abundance as well as in contrasting EM and arbuscular mycorrhizal (AM) forest types. At a global scale, EM ecosystems were found to store 70% more C in soil than AM ecosystems, suggesting the importance of microbial structure in determining carbon storage. Next-generation sequencing and qPCR will be used to better quantify the relative abundances of these different functional groups at the local scale. If empirical tests prove to be robust in estimating local abundances, this approach will be expanded to global scales by using published sequence data to describe the relationship between the relative abundance of EM fungi and soil biogeochemical properties.
