Nitrogen Cycle

Fixing an Unreactive Gas

Atmospheric nitrogen exists as N2 — two nitrogen atoms held together by an extremely strong triple covalent bond — which makes it very unreactive and unusable by most living things in that form. Converting N2 gas into ammonia, a form plants and other organisms can actually use, is called nitrogen fixation.

The Three Nitrogen Pools

The nitrogen cycle can be pictured as nitrogen moving between three connected pools: atmospheric N2, the soil 'N' pool, and biomass (see Figure 12.3).

• Nitrogen enters the soil pool from the atmosphere through three routes: biological nitrogen fixation (certain bacteria converting N2 to ammonia), industrial nitrogen fixation (man-made fertiliser production), and electrical nitrogen fixation — in nature, lightning and ultraviolet radiation supply enough energy to convert atmospheric nitrogen into nitrogen oxides (NO, NO2, N2O); industrial combustion, forest fires and vehicle exhaust add further nitrogen oxides from human activity.
• Within the soil pool, ammonia is converted step by step toward nitrate. When dead plants and animals decompose, their organic nitrogen is released as ammonia — a process called ammonification. Some of this ammonia escapes back to the atmosphere, but most of it is oxidised by soil bacteria in a two-step process called nitrification: first ammonia is oxidised to nitrite by the bacteria Nitrosomonas and/or Nitrococcus, 2NH₃ + 3O₂ → 2NO₂⁻ + 2H⁺ + 2H₂O and then the nitrite is oxidised further to nitrate by the bacterium Nitrobacter, 2NO₂⁻ + O₂ → 2NO₃⁻ Both of these nitrifying bacteria are chemoautotrophs.
• Plants absorb this soil nitrate through their roots and transport it to their leaves, where it is reduced back to ammonia and ultimately becomes the amine group of amino acids — moving nitrogen from the soil pool into plant biomass by uptake. …