UBC Theses and Dissertations
Reactive transport modelling of soil-plant interactions Gatz-Miller, Hannah Sarah
The biogeochemical interactions between plants and soils in the critical zone (CZ) are well-known for diversity and complexity. These systems are thus difficult to characterize, despite their importance in ecology, agriculture, and role in carbon (C) and other nutrient cycles. Numerical modelling of reactive transport (RT), which includes both subsurface transport and geochemistry, has been suggested as a tool to quantitatively explore CZ systems, as RT models consider many interrelated and non-linear processes simultaneously. Here, we explore the degree of complexity (e.g. inclusion of individual processes and feedbacks) necessary to characterize soil-plant interactions in RT models through two distinct research problems at diverse physical and temporal scales of soil-plant systems, while simultaneously providing a quantitative assessment and interpretation of the controlling processes for each of the research problems. Research Problem 1 explores C sequestration through the oxalate-carbonate pathway of the Iroko tree (Milicia excelsa) in both a one-dimensional and two-dimensional context. We find the potential for the Iroko to sequester large quantities of C in the form of carbonate minerals is high, provided a continual source of calcium (Ca) from weathering of local feldspar, as well as from groundwater. While the one-dimensional study worked well for understanding C and Ca mass balances, two-dimensional analysis further illuminated processes both near to and far from the tree, particularly regarding subsurface gas concentrations and transport. In Research Problem 2, RT modeling was used to simulate and interpret a non-invasive imaging experiment of lab-grown maize (Zea mays). A series of RT models was developed, accounting for various degrees of complexity of the bulk soil, attempting to reproduce observed high-resolution soil moisture and O₂ concentration data. The simulation that best matched experimental data included spatial heterogeneity of porosity, permeability, and water retention parameters in the bulk soil, along with inclusion of heterogeneities unique to the system. Synthesis of the research problems indicates that RT modeling can be an effective tool to characterize diverse soil-plant interactions and highlights the benefit of strategic consideration of model parameterization and complexity, tailored to the research questions at hand.
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