Graduate Thesis Or Dissertation
 

Assessing the role of reactive metal species in soil organic matter cycling using chemical imaging

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/8s45qc43n

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  • Intimate associations with reactive metal species permanently protect soil organic matter (SOM) from microbial access and oxidation, contributing to the build-up of organic carbon (C) stocks in soils. It is increasingly recognized, however, that such associations can be reversible and that reactive metal species might even facilitate the oxidation of SOM. Both processes would increase the potential for soil C loss, prompting questions regarding the environmental conditions under which interactions of SOM with reactive mineral phases are protective, reversible or even facilitate oxidation. One of the foremost challenges in characterizing these interactions is the need to describe the dynamics of both SOM and metals at fine spatial scales without disturbing existing chemical or physical associations. This thesis thus addresses the general question whether non-invasive chemical imaging techniques can be used to distinguish protective interactions between SOM and reactive metal species from those that are susceptible to or facilitate C loss. The following research aimed to identify the nature of the interactions between SOM and two abundant redox-active metals (iron and manganese) in three soil microenvironments: fungal mats, the rhizosphere and the litter layer. The first of the three chapters describes the rapid formation of protective associations between fungal-derived SOM and iron (hydr)oxide precipitates in ectomycorrhizal mats. The second chapter demonstrates how root exudates can reverse such protective associations between SOM and reactive Fe phases, promoting microbial access to SOM and causing C loss in the rhizosphere. The third chapter establishes the functional role of reactive manganese species in the microbial oxidation of aromatic compounds in forest litter decomposition. Combined, this work not only demonstrates the ability of imaging and spectroscopy techniques to probe the nature of interactions between SOM and reactive minerals in soils. It also highlights the need to extend current conceptual models to reversible and oxidative interactions between reactive minerals and SOM and their implications for SOM cycling.
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