The Reactions:

Some N₂ in the atmosphere is oxidized by lightning to NO and NO₂. This can be hydrated and washed down into soil by rain. This provides perhaps 5 kg N per hectare per year of nitrogen for plants and soil microbes. In contrast, cultivation of maize requires perhaps 200 kg N per hectare per year; hence we fertilize corn fields at a high rate. Organisms need to fix nitrogen into organic molecules in the NH₃ form. As a demonstration of the efficiency of biological nitrogen fixation, it is estimated that legumes can provide N fixation rates of 100 to 400 kg N per hectare per year. These data on Nitrogen were taken from Chapeter 11 of H.-W. Heldt (1997) Plant Biochemistry and Molecular Biology, Oxford University Press, Oxford.

Links to other WWW sites about the Nitrogen Cycle that you may find useful:

• nitrogen cycle
• http://muextension.missouri.edu/explore/envqual/wq0252.htm [local]

Formation of ammonia

>Haber Process
N₂ + 3 H₂ -----> [450° and 200 Atm] -----> 2 NH₃ ; ΔG°' = -30 kJ/mol N₂

Organisms have a series of enzymes which produce a global cycle of nitrogen.

>Nitrogenase (very sensitive to the presence of O₂)

N₂ + 6 e^- + 12 ATP + 12 H₂O --> 2 NH₄⁺ + 12 ADP + 4 H⁺
Occurs in free living bacteria (Clostridia, Azotobaccter) or in symbiotic nodules on roots of legumes (Rhizobia)

>Nitrate reductase
NO₃^- + NADPH + H⁺ --> NO₂^- + NADP⁺ + H₂O

>Nitrite reductase
NO₂^- + 3 NADPH + 3 H⁺ --> NH₃ + 3 NADP⁺ + H₂O + OH^-
The reactions catalyzed by nitrogenase, nitrate reductase and nitrite reductase all involve a reduction and thus consume biological energy.

Nitrification

(oxidation energy [ΔG] used to reduce CO₂ in soil bacteria [Nitrobacter])

NH₃ + 3/2 O₂ --> NO₂^- + H₂O + H⁺
NO₂^- + 1/2 O₂ --> NO₃^-

Denitrification

(NO₃^- is a terminal e^- acceptor in some anaerobes [Psuedomonas])
2 NO₃^- + 12 H⁺ + 10 e^- --> N₂ + 6 H₂0

Since nitrification and denitrification are oxidation processes, they release energy and are used by these soil organisms to support their growth.

Ammonia Assimilation

Glutamate dehydrogenase
alpha-ketoglutarate + NH₃ + NADH + H⁺ glutamic acid + NAD⁺ + H₂0

Glutamine synthethase .
glutamic acid + NH₃ + ATP glutamine + ADP + Pi

Transaminase
alpha-keto acid + alpha-amino acid alpha-amino acid + alpha-keto acid