Experimental constraints on contributions of mycorrhizal symbioses to bedrock weathering of calcium and magnesium

智力价值：尽管众所周知，生物对风化过程和大陆环境中元素的分布做出了重大贡献，但不同生物对镁和钙等关键元素循环的相对影响仍然知之甚少。菌根共生已被认为是陆地生态系统元素循环的重要组成部分，其中丛枝菌根真菌在农田、热带和一些温带地区占主导地位，外生菌根真菌在温带和北方森林中占主导地位。然而，这些共生体在风化过程中的作用以及它们的存在对关键元素循环(如镁和钙)的影响并未量化。为了定量和系统地解决这些问题，我们进行了一系列松树和枫树苗木的培养试验，这些苗木生长在不同的真菌共生体上，供应花岗岩和碳酸盐基岩。通过这项工作，我们将确定真菌群落对风化过程的影响，以及元素和同位素的生物和地球化学分馏的幅度。我们的实验将使我们能够评估：(1)丛枝(枫树)和外生菌根(松树)共生对风化(即基岩矿物溶解)的相对意义；(2)真菌生物量丰度与风化之间的联系；(3)养分供应速率对风化的影响；(4)叶化学中基岩类型记录的保真度；以及(5)重要元素和重要同位素循环的关键生物地球化学贡献。我们将通过结合使用基岩渗滤液、基岩消化液、渗滤液和生物组织(真菌、根、茎和叶)的微量元素和同位素分析来评估这些问题。我们特别感兴趣的是利用这些实验来阐明对钙和镁同位素循环的生物贡献，这两个最重要的新兴同位素系统需要在不同的地球表面和生态系统水库中进行元素收支。这些系统内的一个关键悬而未决的问题仍然是确定沿生物和非生物途径的同位素分馏。基于文献和对松树幼苗钙同位素系统学的初步研究，我们假设钙和镁在生物组织中都有生物分馏，树木组织的同位素特征反映了非生物和生物分馏的机制。广泛影响：我们将开发的综合数据集最终将提高我们对陆地水库内元素循环的理解，并提供地球科学界几个领域感兴趣的结果，包括研究水文学、生物地球化学、生态系统内的地球化学循环以及大陆对海洋地球化学循环的贡献。这项拟议的工作将构成一篇博士论文的最后几章，该论文讲述的是一位有前途的早期职业女性生物地球化学家。此外，拟议的研究继续并进一步加强了UNH小组与比利时哈塞尔特大学和美国地质调查局的科学家之间富有成效的跨学科合作。这一合作将加强北卡罗来纳大学在生物地球化学方面的研究计划和实验室发展。PIs积极与高中教师和高中生就与拟议研究有关的主题开展工作，研究成果将广泛出版，并纳入北卡罗来纳大学的地球化学课程。

Intellectual Merit: Although it is well known that organisms contribute significantly to the weathering process and to the distribution of elements within continental environments, the relative influence of different organisms on key elemental cycles such as magnesium and calcium remain poorly understood. Mycorrhizal symbioses have been recognized as important parts of elemental cycles within terrestrial ecosystems, with arbuscular mycorrhizal fungi dominant in croplands, the tropics, and some temperate regions, and ectomycorrhizal fungi dominant in temperate and boreal forests. The role of these symbioses, however, in weathering and the influence of their presence on key elemental cycles such as magnesium and calcium are not quantified. To address these questions in a quantitative and systematic way, we have carried out a series of culture experiments of pine and maple seedlings, grown with different fungal symbionts and supplied with granitic and carbonate bedrock layers. With this work, we will identify the influence of fungal communities on the weathering process and the magnitude of biological and geochemical fractionation of elements and isotopes. Our experiments will enable us to assess: (1) the relative significance of arbuscular (maple) versus ectomycorrhizal (pine) symbioses on weathering (i.e. bedrock mineral dissolution); (2) the links between the abundance of fungal biomass with weathering; (3) the influence of nutrient supply rate on weathering; (4) the fidelity of the record of bedrock type in foliar chemistry; and (5) key biogeochemical contributions to important elemental and important isotopic cycles. We will assess these questions through the combined use of trace elements and isotope analyses of bedrock leachate solutions, bedrock digestions, percolating fluids, and biological tissues (fungi, roots, stems and foliages). We are specifically interested in using these experiments to elucidate biological contributions to the calcium and magnesium isotope cycles, two of the most important emerging isotope systems requiring elemental budgets within different Earth surface and ecosystem reservoirs. One key outstanding question within these systems remains the identification of isotope fractionation along biological and abiotic pathways. Based on the literature and preliminary studies of calcium isotope systematics in pine seedlings, we hypothesize that calcium and magnesium will both be biologically fractionated within the biological tissues, and that the isotopic signatures of the tree tissues reflect mechanisms of both abiotic and biotic fractionation.Broader Impacts: The integrated dataset we will develop will ultimately improve our understanding of element cycling within terrestrial reservoirs and provide results that will be of interest to several segments of the earth sciences community, including those studying hydrology, biogeochemistry, geochemical cycling within ecosystems, and continental contributions to marine geochemical cycles. The proposed work will form the final chapters of a Ph.D. dissertation of a promising early career female biogeochemist. Additionally, the proposed study continues and further strengthens productive interdisciplinary collaborations between the UNH group and scientists from Hasselt University in Belgium and the U.S. Geological Survey. This collaboration will enhance the research program and laboratory development in biogeochemistry at UNH. The PIs actively work with high school teachers and high school students on topics related to the proposed studies, and the research results will be published widely and incorporated into the UNH geochemistry curriculum.