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，了解控制森林碳储存的因素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。树木的生长是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量；当树木最终死亡时，它们的生物量要么储存在土壤中，要么通过分解过程被微生物消耗。碳在分解中的命运取决于占主导地位的微生物群体：自由生活的土壤分解者还是栖息在根部的特殊(外生菌根或EM)真菌。自由生活的微生物分解将养分回收成植物可利用的形式，否则这些养分将被锁在枯死的植物组织中，但同时也会将二氧化碳呼吸回大气。相比之下，共生EM真菌从植物中获得碳供应，因此它们在分解过程中的作用只是释放养分，而不是碳。如果EM真菌能够有效地在营养方面胜过自由生活的分解者，那么生态系统中保留的碳应该比大气中流失的碳更多。这一假设得到了之前研究的支持，该研究表明，与EM真菌相关的生态系统在全球范围内储存了更多的碳。这项研究项目支持实验，以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，并产生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，从而为营养水平之间的竞争奠定了基础。许多植物都与外生菌根真菌有关。这些EM真菌可以产生氮降解酶，使植物能够直接获得土壤有机氮，而不需要自由生活的分解者将其转化为可获得的形式。通过“短路”传统的氮素循环途径，与EM真菌相关的植物可能诱导或加剧分解者的氮素限制，从而增加土壤和生态系统的碳储量。初级生产者和分解者之间通过菌根真菌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碳储存的主要驱动因素，而不受气候、土壤矿物学和有机质化学等其他已知驱动因素的影响。该项目将评估EM介导的植物分解者竞争的重要性，以及在当地和全球范围内的重要性。在地方尺度上，将沿着EM真菌丰度的梯度以及EM和丛枝菌根(AM)森林类型的对比来研究土壤过程。在全球范围内，EM生态系统在土壤中储存的碳比AM生态系统多70%，这表明微生物结构在决定碳储量方面的重要性。下一代测序和定量聚合酶链式反应将被用来更好地量化这些不同官能团在地方尺度上的相对丰度。如果经验测试证明在估计局部丰度方面是稳健的，这种方法将扩展到全球范围，使用已公布的序列数据来描述EM真菌的相对丰度与土壤生物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.