Insights on the marine microbial nitrogen cycle from isotopic approaches to nitrification

The microbial nitrogen (N) cycle involves a variety of redox processes that control the availability and speciation of N in the environment and that are involved with the production of nitrous oxide (N20), a climatically important greenhouse gas. Isotopic measurements of ammonium (NHI:Fi), nitrite (NO2), nitrate (NOT), and N20 can now be used to track the cycling of these compounds and to infer their sources and sinks, which has lead to new and exciting discoveries. For example, dual isotope measurements of NOT and NO2 have shown that there is NO3 regeneration in the ocean's euphotic zone, as well as in and around oxygen deficient zones (ODZs), indicating that nitrification may play more roles in the ocean's N cycle than generally thought. Likewise, the inverse isotope effect associated with NOT oxidation yields unique information about the role of this process in NO2 cycling in the primary and secondary NO2 maxima. Finally, isotopic measurements of N20 in the ocean are indicative of an important role for nitrification in its production. These interpretations rely on knowledge of the isotope effects for the underlying microbial processes, in particular ammonia oxidation and nitrite oxidation. Here we review the isotope effects involved with the nitrification process and the insights provided by this information, then provide a prospectus for future work in this area.