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

Reservoirs: The main reservoir is the atmosphere, which is 80% nitrogen gas (N₂). The other reservoirs are bound in soils, aquatic sediments, dissolved in water, and in living biomass.

Key processes: Nitrogen enters ecosystems by nitrogen fixation, the conversion of N₂ to NH₄⁺ or NO₃^-. Ammonification decomposes organic nitrogen to NH₄⁺. Nitrifying bacteria convert NH₄⁺ to NO₃^-, while denitrifying bacteria use NO₃^- instead of O₂ in their metabolism and releases N₂. 1