Elemental nitrogen (N2) makes up 80% of the atmosphere (by volume) and represents the dominant form of atmospheric nitrogen gas. Despite its high atmospheric abundance, N2 is generally nonreactive, due to strong triple bonding between the N atoms, making much of this N2 pool unavailable to organisms. In fact, only 2% of this N2 pool is believed to be available to organisms at any given time (Galloway, 1998). Consequently, N2 must be “fixed” into ionic forms such as NH4+ before it can be used by plants. Since N is essential for the synthesis of amino acids and proteins and because it is often in low concentrations, N is usually considered to be limiting to organisms in many ecosystems. Nitrogen has five valence electrons and can occur in a broad range of oxidation states that range from +V to -III, with NO3− and NH4+ being the most oxidized and reduced forms, respectively. Some of the most common N compounds that exist in nature, along with their boiling points, ΔH0, and ΔG0, are shown in table 10.1 (Jaffe, 2000); these thermodynamic data can be used to calculate equilibrium concentrations. Fluxes in the global N cycle have been seriously altered by anthropogenic activities (Vitousek et al., 1997; Galloway et al., 2004). For example, fluxes of many nitrogen oxides, which are largely derived from burning fossil fuels, have increased significantly in the atmosphere resulting in photochemical smog and acid precipitation (table 10.2; Jaffe, 2000). Similarly, the advent of artificial N fertilizers (e.g., the Haber–Bosch process, where N2 is fixed to NH3 by industrial processes), which were developed to compensate for the general nonavailability of N2 to most agricultural crops, has resulted in increased N loading from soils and sewage to rivers and estuaries around the world, and considerable eutrophication problems in these aquatic ecosystems. For example, biological N2 fixation accounted for a major fraction of newly fixed N before the 1800s (∼90–130 Tg N y−1) (Galloway et al., 1995).
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