Microbially Mediated Carbonate Formation in Desert Soils
Microbiology and Geochemistry
Tom Kieft, Department of Biology, Michael Pullin, Department of Chemistry, Peter Mozley and Bruce Harrison, Department of Earth and Environmental Sciences
Project Description: This project seeks to understand the role of microorganisms in the precipitation and dissolution of carbonates minerals in soil. Carbonate-rich soil horizons comprise more than one-third of the Earth's stored terrestrial inorganic carbon, with the bulk of this carbon occurring in arid and semi-arid lands. Factors controlling the fluxes of carbon into and away from such a large pool have important implications for the global carbon budget, especially in the light of global climate change. However, the actual mechanisms by which soil carbonates form are still poorly understood. In fact, a controversy exists as to whether desert soils are currently net sources or sinks of atmospheric CO2. Conventional wisdom for many decades has been that soil carbonate precipitation results from purely abiotic processes that are driven primarily by evaporation. While evaporation is clearly an important factor, a growing body of evidence has implicated microorganisms as mediators of carbonate precipitation. Our approach to determining the importance to microbes in carbonate mineral formation involves a combination of field observations and laboratory microcosm experimentation. Microbially mediated mineral precipitation may be (1) “direct” or "enzymatic”, in which metabolic processes alter the environment in the vicinity of cells (e.g. by consumption of hydrogen ions) or (2) “indirect” or “passive”, in which conditions found at the surfaces of cells favor mineral precipitation (e.g., by the adsorption of Ca2+ onto negatively charged cell surfaces or by nucleating mineral formation). We can differentiate between these in experiments that compare live and killed the effects of microbial cultures. The quantity and quality of soil organic carbon are also important factors, with composition of the desert soil plant community being a major determinant of these. We will determine the spatial correlations between carbonates and specific types of plant communities, and we will also conduct controlled experiments in which the quantity and quality of organic carbon are manipulated.
Students engaged in this ongoing interdisciplinary project will:
- conduct field work at the Sevilleta National Wildlife Refuge (30 km north of New Mexico Tech)
- use petrographic and geostatistical analyses to correlate carbonates with biological and chemical factors
- design and perform laboratory microcosm experiments that allow for tight control of environmental parameters
- use the tools of analytical chemistry to quantify inorganic carbon and to characterize organic carbon in soils
- use molecular biological methods to characterize communities of microorganisms
- cultivate and characterize microorganisms that have metabolic capabilities that are linked to carbonate precipitation
