Anthropogenic perturbations to the biogeochemical cycle of silicon
Maguire, Timothy J.
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Globally, human activities are altering nutrient biogeochemical cycles. The impact of humans on silicon (Si) cycles remains largely unexplored. Understanding the cycle of Si is important because weathering of siliceous rocks is a substantial sink of atmospheric carbon. Additionally, Si is required by diatoms. Diatoms form the base of important socioeconomic food webs, responsible for ~50% of oceanic net primary production, and deliver atmospheric carbon to ocean sediments as part of the ocean’s biological pump. My dissertation aims to assess the role of anthropogenic activities in altering Si cycling across the land-ocean continuum. Chapter 2 focuses on how assimilation of biogenic silica (BSi) by trees may be impacted by projected changes in climate. Using samples collected during a multi-year, snow removal experiment, I show that increased frequency and duration of soil freezing in winter significantly decreased (-28%) BSi in sugar maple (Acer saccharum) fine roots compared to control plots. Importantly, I observed that fine roots are a previously undescribed pool of BSi within sugar maples, accounting for 29% of total sugar maple BSi while only 4% of sugar maple biomass. Chapter 3 examines the origin and fate of Si within wastewater for the City of Boston. I determined the total dissolved silica (DSi) load in wastewater influent (69,500 kmol DSi year-1), then parsed the total DSi flux between Si contributions of sewage (49%), groundwater infiltration (39%), and surface runoff inflow (12%). In Chapter 4, I study the DSi load carried by treated effluent. I determined that effluent load (67,800 kmol DSi year-1) is not statistically different from influent load, indicating that wastewater treatment does not remove DSi. In Chapter 5 I demonstrate how humans impact concentrations of DSi in urban groundwater. Groundwater DSi increases with human presence and urban areas have significantly higher concentrations of DSi compared to groundwater conditions along the Massachusetts coast. I demonstrate that historic variables defining fill techniques, fill material, and pre-fill land-use out preform geologic variables in predicting urban groundwater DSi concentrations. This dissertation highlights human alterations to biological assimilation, fate, and effects of Si in sewage, and centuries-long subsurface Si impacts that perturb the distribution and availability of a nutrient intimately tied to water quality and climate.