The nitrogen cycle connects atmospheric nitrogen, inorganic nitrogen compounds and organic nitrogen in living and dead biomass.

The main biological transfers are:

1. Nitrogen fixation converts atmospheric $N_2$ into ammonia/ammonium.
2. Assimilation incorporates inorganic nitrogen into organic molecules.
3. Consumption transfers organic nitrogen through food webs.
4. Decomposition and mineralization return organic nitrogen to inorganic ammonium-containing pools.
5. Nitrification oxidizes ammonium toward nitrite and nitrate.
6. Denitrification reduces nitrate and related oxidized forms back toward $N_2$.

These transformations create a cycle:

atmospheric N₂
   ↓ fixation
NH₃ / NH₄⁺
   ↓ assimilation
organic N in biomass
   ↓ decomposition/mineralization
NH₄⁺
   ↓ nitrification
NO₂⁻ → NO₃⁻
   ↓ denitrification
atmospheric N₂

Nitrogen also moves physically between reservoirs. Leaching carries dissolved nitrogen compounds downward with draining water, runoff moves them across land, and atmospheric deposition transfers reactive nitrogen from the atmosphere to surfaces.

The central ecological insight is that nitrogen availability depends not only on how much nitrogen exists, but on which chemical form it occupies and which organisms can transform that form. Microbial metabolism therefore controls major ecosystem-scale nitrogen fluxes.