Nitrogen cycle.

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.