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Document Type

Open Access Thesis

Embargo Period

2-1-2020

Degree Program

Environmental Conservation

Degree Type

Master of Science (M.S.)

Year Degree Awarded

2019

Month Degree Awarded

February

Abstract

Plant roots are critical weathering agents in deep soils, yet the impact of resulting mineral transformations on the vast deep soil carbon (C) reservoir are largely unknown. Root-driven weathering of primary minerals may cause the formation of reactive secondary minerals, which protect mineral-organic associations (MOAs) for centuries or millennia. Conversely, root-driven weathering may also transform secondary minerals, potentially enhancing the bioavailability of C previously protected in MOAs. Here we examined the impact of root-driven weathering on MOAs and their capacity to store C over pedogenic time scales. I compared soil that experienced root-driven weathering, resulting in the formation of discrete rhizosphere zones in deep soil horizons (100-160 cm) of the Santa Cruz Marine Terrace chronosequence (65 ka-226 ka), with adjacent soil that experienced no root growth. Using a combination of radiocarbon, mass spectrometry, Mössbauer spectroscopy, and X-ray spectromicroscopy approaches, we characterized MOA transformations in relation to changes in C content, turnover and chemistry across four soils ranging in age (65 ka-226 ka). We found that the onset of root-driven weathering (65-90 ka) increased the amount of C associated with poorly crystalline iron (Fe) and aluminum (Al) phases, particularly highly-disordered nano-goethite. The increase in C coincided with greater overall C concentrations, longer C residence times, and a greater abundance of microbially-derived C. Continued root-driven weathering (137-226 ka) did not significantly change the amount of C associated with crystalline Fe and Al phases, but resulted in a decline in the amount of C associated with poorly crystalline metal phases. This decline in C associated to poorly crystalline phases coincided with a decrease in C concentrations and potential turnover rates, and a shift toward plant-derived C. In contrast, soil not affected by root-driven weathering showed low amounts of C bound to poorly crystalline Fe and Al phases regardless of soil age and, correspondingly, lower C concentrations and estimated residence times. My results demonstrate that root-driven formation and disruption of poorly crystalline Fe and Al phases directly controls both C accrual and loss in deep soil. Hence root impacts on soil C storage are dependent on soil weathering stage, a consideration critical for predictions of the vulnerability of deep soil C to global change.

First Advisor

Marco Keiluweit

Second Advisor

Baoshan Xing

Third Advisor

Isaac Larsen

Available for download on Saturday, February 01, 2020

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