Ammonia NH3

Figure 13.17 Nitrogen oxidation state extremes.

Figure 13.17 Nitrogen oxidation state extremes.

Nitrogen Oxidation States

Nitrogen Oxidation States

Figure 13.18 Biochemical nitrogen transformations.

a limiting nutrient in many environments, especially for photoautotrophs. Combined nitrogen, whether organic or inorganic, is referred to as fixed-N. In addition to microbial activity, nitrogen is naturally fixed in small amounts through photochemical reactions and electrical discharges (lightning) in the atmosphere. Beginning in the early twentieth century, large amounts of nitrogen have also been fixed through fertilizer production, and to a lesser extent, through high-temperature combustion (automobiles, electric power generation, and industrial processes) and explosives manufacturing. These human activities now account for almost 4 x 107 metric tons per year, more than 20% of all fixation. As of yet, this large increase in nitrogen fixation on a global scale does not seem to have "overloaded" other parts of the nitrogen cycle.

In addition to the essential role of microorganisms in nitrogen fixation, there are only a limited number of prokaryotes and a few fungi that can oxidize nitrogen. And although many prokaryotic and eukaryotic microorganisms and plants can utilize inorganic nitrogen in the form of ammonium and/or nitrate, many other microorganisms and all animals can obtain their nitrogen only from organic forms.

When referring to the concentrations of nitrogen-containing compounds, it is common to report them on the basis of the amount of N present rather than the amount of the compound. This makes it easier to keep track of the nitrogen as it is converted from one form to another. Thus, if 1.0 mg/L of NH3-N is completely converted to nitrate, it yields 1.0 mg/L of NO— —N. If this same conversion was reported as the concentration of the species themselves, it would instead be

since for ammonia, and for nitrate,

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