Graduate Thesis Or Dissertation
 

Measuring Electron Activity to Constrain the Role of Soil Structure in the Formation of Biogeochemical Heterogeneity

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  • Soils have a critical role in global carbon (C) cycling, containing one of the largest fast-cycling carbon stocks on earth. Robust representation of soil organic matter dynamics in Earth System Models is critical for future climate prediction. Current C cycling models assume that all C cycling in non-hydric (i.e. ‘upland’) soils occurs solely via aerobic microbial metabolism. However for at least 30 years, it has been known that what may be a seemingly aerobic soil environment can in fact contain pockets of both aerobic and anaerobic metabolic processes. None of the modeling efforts to date have the capacity to estimate or include the spatial abundance of or contribution to C cycling from anaerobic microbial metabolic processes. What is missing is quantitative information detailing the division of ‘upland’ soils into aerobic or anaerobic environments on seasonal time scales. This thesis addresses the general question of how the division of an ‘upland’ soil into aerobic or anaerobic environments can be quantified and the driving mechanisms identified and parameterized. Platinum-based electrodes were used to measure changes in electron activities (reported as electromotive potential) as a method for distinguishing biogeochemically distinct soil environments. The first chapter details long-term field measurements of aerobic and anaerobic environments in three ‘upland’ Mollisols, all in close geographical proximity, but which formed a hydrologic gradient. The extent to which each of the soils was divided into biogeochemically distinct environments was measured using two-dimensional grids of Pt-electrodes. Variation in electron activity as a function of depth, horizontal position within the soil profile, and seasonal climatic drivers was recorded. The second chapter establishes a first-order mechanistic relationship between the volume, connectivity, and general shape of the soil pore system and the metabolic status of the soil (measured as electron activity) as a function of pore network architecture. X-ray computed tomography was used to parameterize three different pore network architectures; one native and two artificially generated using one of the soils from chapter one. The strength of the relationships between the resulting pore network metrics and electron activity dynamics were then established. This work demonstrates that diverse biogeochemical conditions can not only simultaneously coexist in ‘upland’ soils, but that the extent to which a soil is divided into heterogeneous environments is more a function of seasonal precipitation events than seasonal temperature changes. Furthermore, the pore network characteristics related to the formation of anaerobic environments are a function of the scale of observation as well as aggregate size.
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  • Existing Confidentiality Agreement
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  • 2018-01-23 to 2019-11-28

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