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Title:A modeling framework for characterizing root exudation-driven geochemical dynamics in the Critical Zone
Author(s):Roque-Malo, Susana Lucia
Director of Research:Kumar, Praveen
Doctoral Committee Chair(s):Kumar, Praveen
Doctoral Committee Member(s):Druhan, Jennifer L; Dere, Ashlee D; Valocchi, Albert J; Wander, Michelle M
Department / Program:Civil & Environmental Eng
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Root exudation, coupled root exudation-reactive transport modeling, Critical Zone
Abstract:Land use change and intensive agricultural practices have induced significant shifts in the transport and transformation of water, carbon, and nutrients across landscapes. The long-term implications of such changes on soil health and water chemistry remain an open challenge, and the mechanisms that drive changes in solute chemistry and eventually stream water chemistry are not completely understood. Vegetation plays a central role in driving Critical Zone (CZ) hydrobiogeochemistry through root exudation, the process by which plant roots respond to their environment and release reactive carbon (C) into the soil to influence soil microbial symbionts to their advantage. Although previous studies have demonstrated that such root processes promote soil weathering, there are no existing models capable of describing the interaction of root exudation with temporally-variable processes ranging from mineral dissolution to energy and moisture fluxes in the soil column in a single framework. The goal of this thesis is to address gaps in the numerical simulation of hydrobiogeochemical dynamics in the shallow subsurface CZ through the development of a modeling framework that links root-microbe-soil-water interactions and feedbacks with above-ground natural and anthropogenic forcings and below-ground influences of soil parent material. This framework was achieved through the development of (1) the root exudation model REWT (Root Exudation in Watershed-scale Transport) which explicitly describes root exudation and associated feedbacks with the soil microbiome; and (2) the model CrunchREWT, which couples REWT with the existing reactive transport model CrunchFlow to incorporate fluid-mineral interactivity, acid/base chemistry, solute complexation, and other geochemical processes to link root-microbe feedbacks to the broader soil environment. These models are driven by a oneway coupling with the existing multi-layer canopy-root ecohydrologic model MLCan, which vertically resolves canopy- and root-system moisture and temperature gradients and fluxes, and plant uptake of moisture and nutrients for various plant species in both natural and intensively managed ecosystems. We present REWT and CrunchREWT simulations for an intensively managed site in Bondville, Illinois, USA which undergoes corn-corn-soybean rotation. REWT simulations indicate that rates of nitrification and respiration are substantially altered due to the explicit consideration of root exudation. CrunchREWT results show that root-sourced reactive C inputs can lead to the augmentation or reduction of solute concentrations in the soil by several orders of magnitude. Silicate weathering products illustrate episodic leaching patterns, and calcium simulations reveal the development of a stable weathering front consistent with observations. Aluminum concentrations are particularly responsive to geochemical transformations driven by root-sourced reactive C, and analysis of leaching concentration vs leaching flux indicates hysteresis behavior. This work demonstrates the importance of systematically incorporating root exudates into hydrobiogeochemical models and can serve to inform experimental design for shallow subsurface CZ processes. Although many insights can be gleaned from the simulations presented, several challenges impede our ability to directly compare results with observational data, including the difficulty of procuring the broad array of data necessary to parameterize and validate CrunchREWT. We envision the management-induced reactive zone (MIRZ) flux monitoring system, designed to provide the depth-resolved MIRZ biogeochemical data CrunchREWT requires. We also outline further model improvements that will allow for validation with observational data, including the representation of multi-phase gas diffusion through the soil, tile drain water fluxes and injection of gases in the soil through the tile line, exudation of organic acids, and expansion to a three-dimensional simulation framework. The work presented here lays the foundation to explore the role of vegetation in soil development and landscape co-evolution. It contributes to a more integrated representation of the interconnected physical, chemical, and biological processes that govern ecosystem functioning.
Issue Date:2021-04-19
Rights Information:Copyright 2021 Susana Roque-Malo
Date Available in IDEALS:2021-09-17
Date Deposited:2021-05

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