Date of Award:

12-2023

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Committee Chair(s)

Michelle Baker

Committee

Michelle Baker

Committee

Nancy Huntly

Committee

Jennifer MacAdam

Committee

Jeanette Norton

Committee

Bonnie Waring

Abstract

In agricultural systems, an increasingly prevalent goal is to promote carbon sequestration into stable, mineral-associated soil organic matter, as increases in soil organic matter have been linked to increases in soil water-holding capacity and increases in nutrient availability. Because microbially-produced compounds are the foundation of soil organic matter, understanding interactions between management strategies and abiotic controls on microbial activity can prove invaluable for producers and ranchers interested in building soil organic matter and safeguarding production systems under a changing climate. The ability of microorganisms to contribute to growing soil organic matter stocks is dictated by their carbon-use efficiency, which is the proportion of carbon incorporated into their biomass relative to the total carbon they consume. In effect, higher values of microbial carbon-use efficiency reflect an increased potential for building microbially-derived soil organic matter. In an effort to better understand controls on microbial physiology and its role in soil organic matter formation, irrigation, plant functional type, and fertilization effects on carbon-use efficiency were evaluated in a water-limited system over the course of growing season. Results from our experiments indicated that moisture played a fundamental role in regulating microbial carbon-use efficiency, with low moistures decreasing carbon-use efficiency and thus decreasing the potential for soil organic matter formation. Conversely, other management strategies (e.g., plant functional type and fertilization) exerted little control on microbial carbon-use efficiency, especially when soil moisture was exceedingly low. Even with irrigation to supplement low moisture conditions during a growing season, microbial carbon-use efficiency still declined, likely resulting from increased water deficits as precipitation declined and evapotranspiration increased as the growing season progressed. Results from our experiments indicate that carbon-use efficiency may be intrinsically low in semi-arid systems compared to other systems without water limitations. By consequence, building soil organic matter stocks that can buffer the effects of climate change and improve water and nutrient retention may be difficult in semi-arid systems despite management techniques intended to promote soil organic matter formation.

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