Nitrogen is essential for all living things because it is a key building block of proteins, nucleic acids (DNA and RNA), and many other biomolecules. Organisms use nitrogen through a series of processes that move nitrogen through ecosystems, from the atmosphere into living tissues and back again. What you asked: how organisms use nitrogen Key roles and pathways

• Assimilation by plants
  • Plants take up inorganic nitrogen forms, mainly nitrates (NO3−) and ammonium (NH4+), from the soil and incorporate them into amino acids, proteins, nucleic acids, and chlorophyll. This is the primary way nitrogen enters the food web. Animals then obtain nitrogen by eating plant material or other animals.

• Nitrogen fixation
  • Certain bacteria (including some free-living soil bacteria and specialized symbionts in legume roots) can convert atmospheric N2 into ammonia (NH3), which is then usable by plants. This is the entry point of nitrogen into most ecosystems.

• Nitrification
  • Ammonia or ammonium in soil is oxidized first to nitrite (NO2−) and then to nitrate (NO3−) by nitrifying bacteria. Nitrates are readily taken up by plants and are a major form of nitrogen available in agricultural soils.

• Ammonification (decomposition)
  • After organisms die or excrete waste, decomposers such as fungi and bacteria break down organic nitrogen compounds, releasing ammonium back into the soil. This ammonium can be reused by plants or microbes.

• Denitrification
  • In oxygen-poor environments, some bacteria convert nitrate back to gaseous N2 or nitrous oxide (N2O), returning nitrogen to the atmosphere and closing the cycle. This process reduces available nitrogen in soils and sediments.

• DNRA (nitrate to ammonium)
  • Some microbes reduce nitrate to ammonium instead of releasing nitrogen gases, helping conserve bioavailable nitrogen in the ecosystem.

How nitrogen moves through ecosystems

• Atmosphere to soil/organisms: Nitrogen fixation and mineralization events introduce usable nitrogen forms into soils; plants assimilate NO3− and NH4+ to synthesize organic molecules.

• Within food webs: Plants provide nitrogen to herbivores; those animals pass nitrogen up the chain as they are consumed, with nitrogen continually recycled via decomposition.

• Human impacts: Fertilizers, manure management, and other agricultural practices alter nitrogen availability and cycling, often increasing nitrogen runoff and emissions and affecting ecosystems.

Why nitrogen matters for life

• The nitrogen in proteins and nucleic acids is essential for growth, metabolism, and reproduction in all organisms. Without a usable form of nitrogen, organisms cannot synthesize critical biomolecules.

Common misconceptions clarified

• Plants cannot use atmospheric N2 directly; they rely on fixed forms such as NO3− and NH4+. Nitrogen fixation by microbes is needed to convert N2 to usable forms.

• Denitrification does not "create" nitrogen for plants; it returns nitrogen to the atmosphere, reducing the pool of available soil nitrogen.

If you’d like, I can tailor this to a specific organism group (plants, soil bacteria, aquatic systems) or provide a simple diagram-like step-by-step flow of the nitrogen cycle.