Biological importance: Nitrogen is a component of amino acids and nucleic acids.

Forms available to life: Producers can use inorganic ammonium (NH₄⁺) or nitrate (NO₃^-). Consumers can only use organic nitrogen.

Reservoirs: The main reservoir is the atmosphere, which is 80% nitrogen gas (N₂).

Key processes: Nitrogen enters ecosystems through nitrogen fixation by Rhizobium bacteria.

Ammonifying bacteria decompose organic nitrogen to NH₄⁺.

Nitrifying bacteria convert NH₄⁺ to NO₃^-, while denitrifying bacteria use NO₃^- in their metabolism and releases N₂.

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