Carbon dioxide (CO2) is released to the atmosphere when humans burn oil, coal, and gasoline, and is the major cause of global warming. Soils can store carbon (C), helping counteract rising carbon dioxide, but the future of the soil C sink is uncertain. Will it be converted to soil organic C, which can stay put for thousands of years, or will soil microorganisms convert it back to CO2, returning it to the atmosphere? This is a major uncertainty about the future C sink on land. Recent work suggests a surprising response, called the priming effect, in which adding C to soil boosts the metabolism of microorganisms, causing them to produce even more CO2 than expected. Yet, this phenomenon is variable and very poorly understood. Proposed mechanisms fail to explain what conditions modulate the occurrence and magnitude of the priming response. Preliminary data suggest that the soil mineral assemblage, reflecting the chemical and geological properties of soil, interacts strongly with the soil microbial community to influence the priming effect. This research will test the idea that the priming response depends on interactions between the soil mineral assemblage and the soil microbial community. Thus, this research lies at the interface among geology, biology, and chemistry. The research will investigate priming responses in nine soils, spanning a broad range of climatic and environmental conditions. Laboratory experiments will evaluate how priming responds to variation in the mineral assemblage, and samples from the experiments will be tested for carbon cycling and microbial community characteristics. The work will use state-of-the-art techniques, including in-line isotope-ratio measurements using a cavity ring-down instrument, and new stable isotope probing techniques paired with gene microarrays capable of identifying microorganisms performing specific ecological functions. This project emphasizes integrating research and teaching, will provide interdisciplinary training for undergraduate students at institutions with strong histories of minority enrollment. Students will gain experience with the cutting-edge methods, and with a research field with strong implications for policy decisions surrounding global climate change and carbon management.

Project Report

The goal of this research was to understand how soil minerals controls the cycling of carbon in forested ecosystems of the western U.S. We employed a combination of field and laboratory experimentation to achieve this goal. A matrix of environmental gradients along the western slope of the Sierra Nevada of California that encompass vegetation communities including those dominated by ponderosa pine, white fir, and red fir, and geologic substrates including from granite, andesite, and basaltic lithology were used to capture a broad range of geochemical and mineralogical controls over soil carbon cycling. The soils collected from the field were subjected to range of physical and chemical analyses to measure the types of minerals present, the geochemical composition of the soils, the organic matter content of the soils, and the residence time of the carbon in the soils. The results from this research demonstrate the importance of minerals in controlling how carbon is physically partitioned in forest soils and the residence time of that carbon in the subsurface. Forests on andesite in particular have a tremendous capacity for soil carbon storage and sequestration due to the unique soil mineral assemblages that form on these materials. The mineral properties that facilitate carbon storage in soils include a large, charged surface area that can interact with soil organic matter to preserve and protect it from microbial decomposition on the mineral surface. Laboratory experimentation to address the research goals included isolating the unique minerals from the andesite derived soils and measuring the interaction of these minerals with the soil microbial community and different types of plant substrates. The results from the research are highly relevant to understanding how carbon moves through the earth surface system and the potential for certain sets of ecosystems and parent materials to stabilize and sequester carbon.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Deborah Aruguete
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University of Arizona
United States
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