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Research Projects Research OverviewDetailed description Images |
Detailed project descriptions
Across wetland ecosystems, numerous studies have documented that the rate of methane emission is tightly correlated with the productivity of the ecosystem. In these systems, methane metabolism depends on tightly interactive processes including decomposition, methane production, and consumption, and the transport of both methane and oxygen between the soil and the atmosphere. The historic limitations for measuring these rhizosphere processes have hampered the development of theory for why methane emission and ecosystem productivity are so tightly correlated. In an NSF-funded project (von Fischer PI), I test hypotheses that seek to explain this empirical relationship. Working with a soil hydrologist and modeler (Todd Walter, Cornell University) and an ecosystem modeler (Dennis Ojima, Colorado State University), I use a suite of tracers to simultaneously and uniquely measure the physical transport and microbial transformations that collectively regulate methane emissions. These tracers, including isotopically-labeled methane, a hydrologic tracer and an inert gas tracer, move through the rhizosphere of wetland ecosystems, and sampled in space and time for alteration from their initial ratios. Using coupled hydrologic and biological models, we interpret this data to understand the interactions of plants, methane producers and methane consumers.
Two factors drive variability in methane consumption rates in upland soils: the transport of methane from the atmosphere into the soil and the consumption of methane by the community of methanotrophs. This microbial activity is, in turn, determined by methanotroph population size and enzymatic properties (i.e., Michaelis-Menten kinetics). My work on the Shortgrass Steppe Long Term Ecological Research site (SGS LTER) seeks to characterize the relative importance of these controls for structuring variance in the exchange of methane between soils and the atmosphere within and among sites. With funding from the SGS LTER, I regularly assay methanotroph activity using additions of an inert tracer that are coincident with methane flux measures. By measuring dynamics of both methane and the inert tracer, I simultaneously and uniquely measure the physical transport and biological transformations that collectively regulate methane flux. My results are revealing that the response of methane flux to changes in soil moisture differed among study areas such that some showed steep declines with increasing soil water content while other sites had minimal changes. Underlying this response, I am finding that the activity of the methanotroph community is greatest in soils that show the least sensitivity to soil moisture. In collaboration with an applied mathematician (Roger Thelwell, University of Washington) and a microbial ecologist (Mary Stromberger, Colorado State University) I am developing a methane reaction-diffusion model to constrain the potential importance of methanotroph population size, spatial distribution and enzymatic properties for structuring the observed differences in methane uptake capacity. Using trace additions of isotopically labeled methane to soils, I have also shown that methane production, a strictly anaerobic process, is common in upland soils. This has been surprising, but it is consistent with culture-based microbiological work that commonly extracts strict anaerobes from oxic soils. I am highly interested in the theoretical aspects and applied importance of these anoxic microsites for the maintenance of soil microbial diversity and for the function of ecosystems. This methane production may provide an important supplemental (i.e., non-atmospheric) source of methane to the methane consuming microbes. Additional ecosystem effects are also likely given the impact of oxidation status on nitrogen and phosphorus availability.
Among the most tantalizing questions in ecology are those that relate soil microbial community structure and function. A project in my lab addresses the question: Which microbes specialize on old vs. new sources of soil organic matter? With funding from the SGS LTER, I am working with microbiologist Teri Balser (University of Wisonsin, Madison) to measure soil microbial community structure by quantifying bacterial phospholipid fatty acid (PLFA) abundances and to characterize function from the carbon isotope composition of those PLFAs. Because PLFAs differ among phylogenetic and functional groups of soil microbes, they can be used to characterize community structure. In this project, we are measuring spatial and temporal variability in the PLFAs in light of several well-established properties of the SGS LTER ecosystem:(1)C4 and C3 grasses each have characteristic carbon isotope compositions, (2) C3 and C4 grasses have characteristic phenologies such that C3 dominate in spring and C4dominate in summer, (3) grazing and landscape position generate variation in C3 vs. C4 dominance. As soil microbes grow, they build PLFAs with isotopic compositions that reflect the carbon source that they consume. From the data we are collecting, we are identifying microbial groups that specialize on labile carbon (as indicated by shift in carbon source from C3 to C4 over the growing season) as compared to microbial groups that specialize on more recalcitrant carbon pools or stronger plant-type affiliations (as indicated by more constant carbon isotopes).
The carbon isotope composition of soil organic matter has the potential to yield a wealth of information on the present and past distributions of C3 vs. C4 types, and on the nature of the decomposition process. In USGS-funded research, I am working with Larry Tieszen (USGS EROS Data Center) to interpret data on the d13C of soil organic matter collected from native prairie relicts across the Great Plains. Our work is showing that, because C3 vs C4 grass distributions are strongly controlled by growing season temperatures, the d13C of soil organic matter can be used to infer long-term average temperatures. In related work with Lee Nordt (Baylor University) we have used the modern temperature vs. d13C relationship to calculate temperature from paleosols (buried soils) that were formed at various time periods over the past 14,000 years, thus creating the first Holocene temperature record for the Great Plains.
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