Heterotrophic Nitrification and Nitrifier Denitrification

This chapter on describes the physiology and biochemical pathways of heterotrophic nitrification and nitrifier denitrification, a description of the genetic and organism diversity involved, and a brief description of techniques to discern one process from another. A final perspective is offered on how anthropogenic input of nitrogen affects microbial transformations of inorganic N with particular emphasis on emissions of gaseous N-oxides to the atmosphere. Ammonia-oxidizing bacteria (AOB) can produce nitrous oxide by two different pathways, hydroxylamine oxidation or nitrifier denitrification. The technical breakthrough to discriminate nitrous oxide production from nitrification, nitrifier denitrification, and denitrification was the detection of individual nitrous oxide isotopomers using isotope ratio mass spectroscopy. The δ15N of nitrous oxide produced from hydroxylamine oxidation was significantly more positive than that from nitrifier denitrification or denitrification. This study found that the site preference of 15N in nitrous oxide was significantly different during nitrifier denitrification by AOB versus denitrification by two species of Pseudomonas. The chapter touches on largely understudied, but highly significant, processes of inorganic nitrogen metabolism that impact the global nitrogen cycle. Many of the studies cited in this chapter suggest that these processes are strongly influenced by the availability of carbon, nitrogen, and oxygen in the environment. It describes microbial populations and processes that make nitrous oxide in response to increased fertilizer use, nitrogen deposition, and hypoxia.