Abstract
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 (N2O), a climatically important greenhouse gas. Isotopic measurements of ammonium (NH+4), nitrite (NO−2), nitrate (NO−3), and N2O 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 NO−3 and NO−2 have shown that there is NO−3 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 NO−2 oxidation yields unique information about the role of this process in NO−2 cycling in the primary and secondary NO−2 maxima. Finally, isotopic measurements of N2O 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.
Highlights
NO−3 and 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
We review the isotope effects involved with the nitrification process and the insights provided by this information, provide a prospectus for future work in this area
Exchange levels were low (5%) when NO−2 concentrations were held near 1 μm by co-cultivation with nitrite-oxidizing bacteria (NOB) (Buchwald et al, 2012). These results suggested that oxygen isotope exchange during nitrification may be quite low where ammonia and nitrite oxidation are tightly coupled, but may play a role when ammonia and nitrite oxidation become decoupled, such as in the PNM
Summary
Isotopic fractionation during microbial nitrification (SNM; Brandhorst, 1959). The SNM is generally assumed to reflect active denitrification in oxygen deficient zones (ODZs), as SNM features are only found in the absence of dissolved oxygen (Brandhorst, 1959; Cline and Richards, 1972; Codispoti and Christensen, 1985). Since oxygen atom exchange occurs with an equilibrium isotope effect (18εeq) of 11–14 (Casciotti et al, 2007; Buchwald and Casciotti, unpublished), this equilibration would tend to raise the δ18O value of NO−2 relative to the initial δ18ONO2 produced by ammonia oxidation. Nitrogen isotopic fractionation during ammonia oxidation ranges from 14 to 38 for AOB (Mariotti et al, 1981; Yoshida, 1988; Casciotti et al, 2003) and 20–22 for AOA (Santoro and Casciotti, 2011) These values represent the isotope effect oexcpearenssNedH+4uncdoenrsunmonp-tliiomnitginengercaolnlycegnotersattiooncsomofpNletHio+4n., In so the the isotope effect for ammonia oxidation may not be expressed. N2O production through nitrifier denitrification (enhanced by high cell densities, high NO−2 concentrations, and low O2 concentrations; Frame and Casciotti, 2010) has low δ15Nbulk and low SPs relative to that produced by hydroxylamine decomposition (Figure 2).
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