Reduced nitrogen is used for the synthesis of cellular components. All organisms can incorporate ammonia nitrogen into amino acids, purine and pyrimidine bases, and so forth, so the level of NH 3 is the most useful for cell metabolism. Most plants and bacteria can reduce NO 3 ‐ and NO2 ‐ to NH 3. Nitrate and nitrite can be formed from more reduced forms of nitrogen through bacterial metabolism. Nitrate and nitrite are also produced atmospherically from elemental nitrogen, especially by burning organic compounds or during the heating of the atmosphere by lightning. Thus, in the nitrogen cycle, nitrogen shuttles through the +5 (NO 3 ‐), +3 (NO 2 ‐), 0 (N 2), and ‐3 (NH 3) levels. See Figure 1 .