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Atmospheric nitrogen exists as N2 gas with a strong triple bond between nitrogen atoms, making it relatively unreactive and unavailable to most living organisms. Nitrogen-fixing bacteria in soil convert this gaseous nitrogen into ammonia, a biologically useful form that plants and animals can utilize for protein and nucleotide synthesis.

Plants take up ammonia from soil and combine it with glutamate to produce glutamine through a metabolic process. Glutamine is then further converted into other amino acids and nucleotides, which form the building blocks for plant proteins and genetic material. This process is part of the broader overview of metabolism.

The urea cycle is a series of reactions that converts excess ammonia, which is highly toxic to cells, into non-toxic urea. Since animals cannot store large pools of amino acids, they regularly break down ingested amino acids and use the urea cycle to safely eliminate excess nitrogen through urine excretion.

Animals excrete urea through urine into the soil, where bacteria with urease enzyme activity degrade the urea back into ammonia. This ammonia can then be taken up again by plants, completing a key recycling step in the nitrogen cycle and enabling continuous nutrient availability.

Nitrification is the progressive oxidation of ammonium ions by soil bacteria into nitrite and then nitrate. Bacteria belonging to genera Nitrosomas, Nitrobacter, and Nitrospira perform this process, converting ammonium into more oxidized nitrogen forms that are readily available for plant uptake and utilization.

Denitrifying bacteria, certain archaea, and some fungi convert nitrates back into gaseous nitrogen, returning it to the atmosphere and completing the nitrogen cycle. This process transforms soil nitrogen back into atmospheric N2 gas, allowing the cycle to begin anew with nitrogen fixation.

When organisms die, soil microbes degrade their protein content to produce amino acids. Nitrifying microbes then convert these amino acids into nitrite and nitrate, which can be reused by living plants or further processed by denitrifying bacteria to return nitrogen to the atmosphere.