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The Biogeochemistry of Marine Nitrous Oxide

Caitlin Frame, Ph.D., 2011
Karen Cascioti, Advisor

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. Here, 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 (NO2-) concentrations, there are also biological variables such as nitrifier cell abundances and coastal phytoplankton blooms that may also influence N2O production by ammonia oxidizers. 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.