ABSTRACT
Nitrogen (N) is an element essential to life. It is abundant in the atmosphere, mostly as dinitrogen (N2), representing roughly four-fths by volume or three-fourths by weight of the total gas. Trace amounts of oxides of nitrogen (nitrous oxide [N2O] and nitric oxide [NO]) are also present, which play a role in the greenhouse effect and stratospheric ozone chemistry. In soil, sediment, freshwater, and ocean water, nitrogen exists in both inorganic and organic forms. Inorganic forms of N include gaseous NO and N2O, as well as dissolved ammonia (NH3), ammonium (NH4+), nitrate (NO3−), and nitrite (NO2−). Interconversions between these forms of N can be used microbially to produce energy under aerobic conditions (oxidation of ammonia to nitrite and nitrate via nitrication) and anaerobic conditions (reduction of nitrate and nitrite to gaseous compounds nitric
oxide, nitrous oxide, and dinitrogen via denitri-cation). Figure 14.1 illustrates the complexity of the microbial N cycle in the environment. While research on organic nitrogen is limited compared to inorganic nitrogen, this pool is believed to be important in both aquatic and terrestrial ecosystems (Neff et al., 2003; Aluwihare and Meador, 2008; Letscher et al., 2013). Organic nitrogen compounds include humic and fulvic acids, proteins, peptides, amino acids, purines, pyrimidines, pyridines, other amines, and amides.
