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Browsing by Author "Wallenstein, Matt, committee member"

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    A modeling-experimental (ModEx) approach to advance understanding of global controls and microbial contributions to particulate and mineral-associated organic matter storage
    (Colorado State University. Libraries, 2024) Hansen, Paige M., author; Cotrufo, M. Francesca, advisor; Schipanski, Meagan, committee member; Wallenstein, Matt, committee member; Trivedi, Pankaj, committee member
    As soils are the largest terrestrial pool of carbon (C) and provision many ecosystem services, including nutrient cycling and maintenance of plant productivity, soil C sequestration represents a promising technology to help meet urgent needs to draw down atmospheric carbon dioxide (CO2) and prevent acceleration of climate change, as well as to help feed a rapidly growing global population. Given this, a comprehensive understanding of the mechanisms underpinning observed patterns of soil C storage is necessary to ensure a sustainable future for all. In response to this need, recent breakthroughs in our understanding of soil organic matter (SOM) dynamics have led to the development of multiple frameworks articulating how climate, soil, plant, and microbial properties interact with one another to control the formation of the two SOM constituents, particulate (POM) and mineral-associated organic matter (MAOM). Despite this, environmental controls that act on POM and MAOM storage at the global scale, as well as microbial functionality, is noticeably absent from our empirical understanding of SOM fraction formation and persistence. More advanced knowledge of these controls would enable more robust identification of where SOM is most vulnerable to loss, as well as more informed implementation of 'multi-pool' management practices aimed at enhancing C storage in both POM and MAOM. In this vein, this dissertation explores global controls on and microbial mediation of SOM dynamics at multiple scales through a combination of synthesis, modeling, and experimental (i.e., ModEx) approaches. Specifically, I first synthesized climate, soil property, and fraction C data to understand global controls on C storage in POM and MAOM. I then applied a previously developed individual-based model (Kaiser et al., 2015) to determine how emergent microbial community properties resulting from microbial social dynamics (i.e., interactions among microbes that produce enzymes at different rates) impact POM retention under varying degrees of MAOM saturation. Lastly, I investigated the relevance of hypothesized microbial copiotrophic and oligotrophic life history strategies to changes in POM and MAOM storage. Results from these projects indicate that global POM and MAOM storage is controlled by disparate suites of environmental variables, with POM being primarily controlled by variables that modulate microbial activity, and MAOM being controlled by a combination of C inputs and soil properties related to the potential to stabilize new MAOM. Additionally, flexible enzyme production in response to the availability of easily-assimilable, soluble substrates may contribute to POM retention under varying degrees of MAOM saturation and POM carbon:nitrogen ratio (C:N). However, variation in microbial function does not always result in changes in POM and MAOM storage – differences in growth rate, our proxy for copio- and oligotrophy, was unrelated to changes in POM and MAOM. Despite this, this dissertation indicates that microbial functions and environmental properties controlling microbial activity rates (i.e., controls on C outputs from the soil) mediate POM storage, but that MAOM is more reflective of C inputs to the soil. This indicates that microbial interventions to support soil C storage may want to focus on ecosystem-specific microbial manipulations that support community efficiency and modulate exo-enzyme production. In combination with other management strategies that increase soil C, these types of microbial interventions may help ensure that new soil C is retained in the soil for longer periods of time. Additionally, given that microbial activity is generally expected to increase with climate warming, these results indicate a premium need to preserve existing POM stocks.
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    Mullein it over: an examination of the impacts of common mullein (Verbascum thapsus) invasion and management
    (Colorado State University. Libraries, 2015) Leipzig-Scott, Peter, author; Hufbauer, Ruth, advisor; Blumenthal, Dana, committee member; Wallenstein, Matt, committee member
    Biological invasion is one of the most important problems facing modern ecologists, and while research has shown the detrimental effects caused by many invasive species, the impact of the majority of invaders in largely unknown. I investigated the effects of the ubiquitous exotic plant species common mullein (Verbascum thapsus) on plant communities in northern Colorado. Additionally, I studied the impact of a second common invasive species, cheatgrass (Bromus tectorum), and examined its interactions with mullein. I conducted a field experiment over two years, manipulating mullein and cheatgrass presence at two sites, either removing the aboveground biomass of one or both of the exotic plants, or leaving them intact. I measured a number of plant community and abiotic characteristics as responses to these treatments. I found that mullein invasion has few negative impacts, and that the benefits associated with management are lost within a year. Removal of mullein acts as a disturbance, creating physical and ecological openings for cheatgrass and other exotics to occupy. Percent cover of exotic species was highest when mullein was removed and cheatgrass was left intact. Since cheatgrass invasion is associated with undesired changes in the community, I suggest that resources should be used for its management rather than the management of mullein, and that mullein removal in areas with cheatgrass should be accompanied by an aggressive strategy addressing cheatgrass invasion.