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Item A Combined Molecular and Isotopic Study of Sulfur Bacteria in Meromictic Lakes of the Pacific Northwest(2023-12) Harris, James H., IV; Gilhooly, William P., III; Druschel, Gregory K.; Bird, Broxton W.The isotope effects that result from the activity of modern sulfur metabolizing bacteria serve as analogs to interpreting the sulfur isotope values preserved in the geologic record. This biogenic signal is vital to reconstructing the history of Earth’s ancient oceans and atmosphere. However, the isotope compositions imprinted by these bacteria were influenced by multiple factors that must be considered when using these values to make interpretations about environmental change. These factors include: (1) sulfate availability, (2) the rapid and quantitative reoxidation of sulfide (i.e., cryptic sulfur cycling), (3) the initial oxygen isotope compositions of sulfate and water, and (4) the taxonomic structure of sulfur-metabolizing bacterial communities. To address these questions, this project studied four permanently stratified, anoxic and sulfidic (euxinic), lakes in southern British Columbia, Canada, and northern Washington, USA, that have a wide range of sulfate concentrations, from 0.15 – 120 mM. This project resulted in six key findings – (1) the measurement of large Δ34SSO4-H2S values at micromolar sulfate concentrations, (2) the consistent occurrence of δ18OSO4 minima at the chemocline that may be imparted during cryptic sulfur cycling, (3) that subsequent δ18OSO4 enrichments consistently preceded sulfide accumulation and δ34SSO4 enrichment in the suboxic zone of the water column, (4) that initial epilimnion Δ18OSO4-H2O values placed constraints on the maximum extent of δ18OSO4 evolution that occurred beneath the chemocline, (5) that observable changes in the metabolic composition of sulfur bacterial communities accompanied key inflections in the sulfur and oxygen isotope profiles of sulfate and sulfide within the water column, and (6) that, despite large overall differences in community structure, Δ34SSO4-H2S and Δ18OSO4-H2O values ultimately reached similar magnitudes in each lake.Item Precipitation controls on nutrient budgets in subtropical and tropical forests and the implications under changing climate(Elsevier, 2017-05) Chang, Chung-Te; Wang, Lih-Jih; Huang, Jr-Chuan; Liu, Chiung-Pin; Wang, Chiao-Ping; Lin, Neng-Huei; Wang, Lixin; Lin, Teng-Chiu; Department of Earth Sciences, School of ScienceBiological, geological and hydrological drivers collectively control forest biogeochemical cycling. However, based on a close examination of recent literature, we argue that the role of hydrological control particularly precipitation on nutrient budgets is significantly underestimated in subtropical and tropical forests, hindering our predictions of future forest nutrient status under a changing climate in these systems. To test this hypothesis, we analyzed two decades of monthly nutrient input and output data in precipitation and streamwater from a subtropical forested watershed in Taiwan, one of the few sites that has long-term nutrient input-output data in the tropics and subtropics. The results showed that monthly input and output of all ions and budgets (output – input) of most ions were positively correlated with precipitation quantity and there was a surprisingly greater net ion export during the wet growing season, indicating strong precipitation control on the nutrient budget. The strong precipitation control is also supported by the divergence of acidic precipitation and near neutral acidity of streamwater, with the former being independent from precipitation quantity but the latter being positively related to precipitation quantity. An additional synthesis of annual precipitation quantity and nutrient budgets of 32 forests across the globe showed a strong correlation between precipitation quantity and nutrient output-input budget, indicating that strong precipitation control is ubiquitous at the global scale and is particularly important in the humid tropical and subtropical forests. Our results imply that climate change could directly affect ecosystem nutrient cycling in the tropics through changes in precipitation pattern and amount.