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.
