Biogeochemistry of Carbon and Nitrogen in Aquatic Environments

Bedrock and Terrestrial Nitrogen Cycling

Nitrogen in bedrock occurs as ammonium and nitrate minerals, and as recalcitrant organic matter inherited from sediments and retained through diagenesis and metamorphism. The forms of geologic nitrogen in bedrock and cycling between nitrogen in the lithosphere, atmosphere, hydrosphere and biosphere are the subjects of a recent paper (Holloway & Dahlgren 2002).

Diagram illustrating Nitrogen sources from bedrock.

Diagram illustrating nitrification and denitrification pathways.

Bedrock nitrogen is incorporated into soils through weathering. Nitrogen is transformed through soil microbial cycling with assimilation of nitrogen by biota and vegetation. Nitrogen in excess of biological uptake is leached to surface and ground waters. Disruption of soils through irrigated agriculture, removal of vegetation or land development can lead to elevated concentrations of nitrate in runoff. The release of nitrogen from bedrock is examined by this project in two contexts: 1) bedrock weathering, and 2) hydrothermal transport.

Bedrock Weathering

The relationship between carbon and nitrogen in rock, assimilation of these nutrients in soil, and the release of nitrogen through leaching and through denitrification was examined. The photograph to the right shows a typical outcrop of Mancos Shale, which is a prominent rock type in the Grand Valley of western Colorado. This rock type has elevated carbon and nitrogen concentrations; however, surface and ground waters have relatively low concentrations of nitrate. We believe that soil microbial processes sequester nitrogen weathering from the Mancos shale and release some of this nitrogen to the atmosphere through denitrification, thus minimizing nitrate concentrations in the absence of irrigated agriculture.

Photograph of Mancos Shale outcrop, Colorado.

Hydrothermal Transport

Photograph of Cinder Pool, Yellowstone National Park.

Sources and fate of nitrogen in thermal waters are being investigated at Yellowstone National Park in collaboration with Kirk Nordstrom (USGS, Boulder, Colo.) and J.K. Böhlke (USGS, Reston, Va.). Hot spring waters throughout the park have been analyzed for nitrogen concentration and speciation as part of a multi-year study. The biogeochemistry of nitrogen in hot spring runoff channels has been addressed using a combination of in situ chemistry and laboratory assays of microbial activity. Jim Ball, member of the Nordstrom project, is shown at Cinder Pool.

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Yellowstone National Park