The diagram illustrates the nitrogen cycle, a continuous process that converts atmospheric nitrogen into forms usable by living organisms and returns it to the atmosphere.
Nitrogen primarily enters the cycle from the atmosphere, where it constitutes the vast majority of the gas. Two key fixation processes incorporate it into the soil: lightning converts gaseous nitrogen directly into nitrates, while certain plants (such as legumes) fix it into ammonium compounds with the help of symbiotic bacteria.
Once in the soil, ammonium is oxidised by nitrifying bacteria, first into nitrites and then into nitrates – the primary form absorbed by plant roots. Plants assimilate nitrates to synthesise proteins, which are then consumed by animals, transferring nitrogen up the food chain.
Upon the death of plants and animals, or through animal excretion, organic nitrogen in proteins undergoes decomposition by decomposer microbes. This releases ammonium back into the soil, restarting the nitrification process. Alternatively, under anaerobic conditions, denitrifying bacteria convert nitrates back into gaseous nitrogen, releasing it into the atmosphere and completing the cycle.
In summary, the cycle involves fixation (by lightning and plants), assimilation (by plants into proteins), decomposition (returning ammonium), nitrification (ammonium → nitrite → nitrate), and denitrification (nitrate → gaseous nitrogen). While fixation and nitrification add usable nitrogen to ecosystems, denitrification represents a loss, balancing the input and output to maintain atmospheric reserves.