| Summary: | Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2011 Atmospheric nitrous oxide N2O concentrations have been rising steadily for the past century as a result of human activities. In particular, human perturbation of the nitrogen cycle has increased the N2O production rates of the two major sources of this greenhouse gas, soil and the ocean. Nitrification, and particularly ammonia oxidation, is one of the major processes that produces N2O in the ocean. In this thesis, a series of stable isotopic methods have been used to characterize the biogeochemical controls on N2O production by marine nitrification as well as the natural abundance stable isotopic signatures of N2O produced by marine nitrifiers. This thesis shows that in addition to chemical controls on N2O production rates such as oxygen (O2) and nitrite (NO-2) concentrations, there are also biological controls such as nitrifier cell abundances and coastal phytoplankton blooms that may influence N2O production by ammonia oxidizers as well. Ammonia oxidizers can produce N2O through two separate biochemical mechanisms that have unique isotopic signatures. Using culture- based measurements of these signatures, we conclude that one of these pathways, nitrifier- denitrification, may be a significant source of N2O produced in the South Atlantic Ocean and possibly the global ocean. Funding for this work was provided by NSF/OCE 05-26277, the Andrew W. Mellon Founda- tion Awards for Innovative Research, the Cecil H. and Ida M. Green Technology Innovation Awards, and the W. M. Keck Foundation.
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