Chapter 8 : Nitrogen Cycle

• All life requires nitrogen-compounds, e.g., proteins and nucleic acids.
• Air, which is 79% nitrogen gas (N₂), is the major reservoir of nitrogen.
• But most organisms cannot use nitrogen in this form.
• Plants must secure their nitrogen in "fixed" form, i.e., incorporated in compounds such as:
  • nitrate ions (NO₃⁻)
  • ammonia (NH₃)
  • urea (NH₂)₂CO
•        most plants can only take up nitrogen in two solid forms: ammonium ion (NH₄⁺ ) and the ion nitrate (NO₃^- ).
• Animals secure their nitrogen (and all other) compounds from plants (or animals that have fed on plants).

Four processes participate in the cycling of nitrogen through the biosphere:

• nitrogen fixation  : - atmospheric fixation by lightning.

                                        - biological fixation by certain microbes — alone or in
                                          a symbiotic  relationship with some plants and animals
                                           Nitrogen-fixing cyanobacteria -
                                         - industrial fixation

• decay  : microorganisms of decay. They break down the molecules in excretions and dead organisms into ammonia.:
• nitrification : most of the ammonia produced by decay is converted into nitrates ;accomplished in two steps:
  • Bacteria of the genus Nitrosomonas oxidize NH₃ to nitrites (NO₂⁻).
  • Bacteria of the genus Nitrobacter oxidize the nitrites to nitrates (NO₃⁻).
  These two groups of autotrophic bacteria are called nitrifying bacteria.
•  denitrification :To complete the cycle other bacteria in the soil carry out a process known as denitrification which converts nitrates back to nitrogen gas. A side product of this reaction is the production of a gas known as nitrous oxide, N2O. Nitrous oxide, also known as "laughing gas" .