Date of Award

Fall 2023

Project Type

Dissertation

Program or Major

Earth and Environmental Sciences

Degree Name

Doctor of Philosophy

First Advisor

Serita Frey

Abstract

Soil organic carbon (C), the largest actively cycling reservoir of C on earth, is ultimately derived from plant photosynthesis. Once in soils, a significant proportion of this C is processed by microbial communities prior to its incorporation into soil organic matter (SOM) pools, and microbial-derived organic matter inputs are increasingly recognized as a dominant source of SOM. Conceptual theory therefore posits that microbial physiological traits (e.g., growth efficiency) are central controls on SOM formation and stabilization. However, recent observations are mixed, and the specific traits that promote the formation and retention of different SOM functional pools (e.g., total vs. stable SOM) are unresolved. Moreover, the relative quantitative importance of plant- versus microbial-derived SOM is uncertain, limiting understanding of the contexts in which microbial traits will be useful predictors of SOM accumulation. This dissertation sought to address these uncertainties by (1) evaluating relationships between microbial community composition, associated traits, and SOM formation potential (Ch. 1-2), and (2) by clarifying the quantitative evidence for plant and microbial contributions to SOM, to facilitate an understanding of the contexts in which microbial traits will act as primary controls on SOM formation and stabilization (Ch. 3). I demonstrate that microbial community composition and species identity influence the quantity, chemistry and stability of SOM that is formed by soil microorganisms (Ch. 1-2), and that distinct microbial traits are related to the formation of different SOM functional pools. However, these traits do not exhibit simple two-dimensional tradeoffs; rather, ‘multifunctional’ microbes with intermediate investment across a key grouping of synergistic traits (namely, growth efficiency, growth rate, turnover rate, and biomass protein and phenol contents) have the highest SOM formation potentials (Ch. 2). Lastly, I broaden my focus to critically examine the quantitative evidence for plant and microbial-derived SOM, demonstrating that limitations with current methodological approaches constrain accurate quantification (Ch. 3). I synthesize data showing the overlap in chemical signatures of plant- and microbial-derived compounds in soils and identify key knowledge gaps for future research on the formation of SOM from plant and microbial pathways. Together, my three chapters provide critical insight into the role of soil microorganisms and their traits in SOM formation and stabilization and offer a holistic framework for approaching future research questions in this field.

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