DISSERTATION RESEARCH: Mycorrhizal mediated competition between producers and decomposers drives ecosystem carbon storage

森林和森林土壤对地球的碳循环有重大影响。 因此，了解是什么控制着森林碳储存，对于预测气候变化对森林的影响以及森林对气候变化的影响都很重要。树木通过光合作用将大气中的二氧化碳转化为木材和树叶等生物质而生长;当树木最终死亡时，它们的生物质可以储存在土壤中，也可以通过分解过程被微生物消耗。碳在分解过程中的命运取决于什么样的微生物群体占主导地位：自由生活的土壤分解者或栖息在根部的特殊（外生菌根或EM）真菌。自由生活的微生物分解将营养素转化为植物可用的形式，否则这些形式将被锁定在死亡的植物组织中，但同时也将二氧化碳呼吸回大气中。相比之下，共生EM真菌从植物中获得碳，因此它们在分解中的作用只是释放营养物质而不是碳。如果EM真菌可以有效地竞争营养素的自由生活分解者，那么更多的碳应该留在生态系统中，而不是流失到大气中。这一假设得到了先前研究的支持，表明与EM真菌相关的生态系统在全球范围内储存了更多的碳。该研究项目支持实验来量化这些专门的真菌在森林生态系统碳储存中的重要性。这项研究的目标是更好地了解专门真菌的作用，并生成将其活动纳入生态系统和地球碳循环模型所需的信息。该项目将支持一名研究生的研究，还将为代表人数不足的少数群体提供获得重要研究经验的机会。 在陆地生态系统中，初级生产者和分解者的活动都可能受到氮（N）的限制，从而为营养级之间的竞争奠定了基础。许多植物与外生菌根真菌共生。 这些EM真菌可以产生N降解酶，使植物直接获得土壤有机N，而不需要通过自由生活的分解者转化为可获得的形式。通过“短路”传统的N循环途径，与EM真菌共生的植物可能会诱导或加剧分解者的N限制，从而增加土壤和生态系统的C储存。通过菌根真菌初级生产者和分解者之间的竞争将对生态系统生态学产生重要影响，因为它可能是土壤有机质稳定性和生态系统C储存的主要驱动因素，而独立于其他已知的驱动因素，如气候，土壤矿物学和有机质化学。该项目将评估EM介导的植物分解者竞争的重要性，并在地方和全球范围内。在当地尺度上，土壤过程将研究沿着梯度EM真菌丰度以及在对比EM和丛枝菌根（AM）森林类型。在全球范围内，EM生态系统被发现比AM生态系统在土壤中多储存70%的C，这表明微生物结构在决定碳储存方面的重要性。下一代测序和qPCR将用于更好地量化这些不同官能团在当地规模上的相对丰度。如果实证检验证明是强大的估计当地的丰度，这种方法将扩大到全球范围内使用已发表的序列数据来描述EM真菌的相对丰度和土壤地球化学性质之间的关系。

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.