Last update: February 19, 2026

The Nitrogen Cycle: From Air to Life

Nitrogen cycle illustrating fixation, nitrification, assimilation, and denitrification — The major steps of the nitrogen biogeochemical cycle: fixation by bacteria and lightning, assimilation by plants, release through decomposition, and return to the atmosphere.
Image source: astronoo.com

Pourquoi le cycle de l’azote est‑il essentiel à la vie sur Terre ?

Parce que l’azote, indispensable à la construction des protéines et de l’ADN, ne peut être utilisé directement par la plupart des êtres vivants sous sa forme gazeuse pourtant abondante dans l’atmosphère. Le cycle de l’azote transforme continuellement cet élément, grâce à l’action combinée des bactéries, des plantes, des animaux et des phénomènes naturels, en formes assimilables par le vivant. Sans ces conversions successives — fixation, nitrification, assimilation, ammonification et dénitrification — les écosystèmes s’effondreraient rapidement. Ce cycle invisible mais fondamental maintient l’équilibre biologique de la planète et conditionne la fertilité des sols, la productivité des océans et la stabilité du climat.

Why is nitrogen, though abundant, so difficult to capture?

Earth's atmosphere contains nearly 78% dinitrogen (\(N_2\)) by volume or 75% by mass. Yet, this vast potential resource for life is inaccessible to most living beings. No animal, human, or plant can transform it into nutrients.

The reason lies in the exceptional strength of the triple chemical bond between the two nitrogen atoms (N≡N). Considerable energy is required to break this bond.

Two pathways exist: lightning at 30,000 °C, or the chemical elegance of bacteria and their nitrogenase enzyme, capable of reducing \(N_2\) to \(NH_3\) (dinitrogen → ammonia). Without this prior rupture, there would be no proteins, no DNA, no muscles, no neurons, no blood: no life.

The nitrogen cycle is a cascade of transformations that converts an inert gas into assimilable nutrients, then returns it to the atmosphere.

N.B.: Antoine Lavoisier (1743-1794) proposed the term "nitrogen" (from Greek "a-zôê", without life). Paradoxically, this inert element in gaseous form proves indispensable to every living cell once fixed.

Which microorganisms for which transformation?

Comparison of major nitrogen cycle pathways and their biological actors
Process	Chemical transformation	Bacteria + Fungi	Required conditions	Ecological role
Symbiotic fixation	\(N_2 \rightarrow NH_3\)	Rhizobium, Bradyrhizobium	Symbiosis with legume roots, microaerophilia	Direct nitrogen supply to cultivated plants
Free fixation	\(N_2 \rightarrow NH_3\)	Azotobacter (aerobic), Clostridium (anaerobic)	Soils, aquatic environments, available organic carbon	Diffuse but constant contribution to natural fertility
Nitrification (step 1)	\(NH_3 \rightarrow NO_2^-\)	Nitrosomonas, Nitrosospira	Strict aerobiosis, neutral to slightly alkaline pH	Formation of nitrites, precursors of nitrates
Nitrification (step 2)	\(NO_2^- \rightarrow NO_3^-\)	Nitrobacter, Nitrospira	Aerobiosis, tolerance to a wide range of temperatures	Production of nitrates, highly assimilable form for plants
Denitrification	\(NO_3^- \rightarrow N_2\) (via \(NO_2^-\), NO, \(N_2O\))	Pseudomonas, Paracoccus, Bacillus	Anoxia (saturated soils, sediments), labile organic matter	Cycle closure, return of \(N_2\) to the atmosphere
Ammonification (mineralization)	Organic N \(\rightarrow NH_4^+\)	Saprophytic fungi (Bacillus, Streptomyces, etc.)	Aerobiosis or anaerobiosis, decomposition of necromass	Recycling of nitrogen from organic waste

N.B.: Human activity through synthetic fertilizers has doubled the flow of reactive nitrogen entering terrestrial ecosystems since 1950.

When humans disrupt the nitrogen cycle

Before the industrial era, natural fluxes of biological and atmospheric fixation maintained a stable stock of reactive nitrogen in soils and waters. The work of Justus von Liebig (1803-1873) had already foreseen the importance of nitrogen compounds for fertility. But it was the invention of the Haber-Bosch process (first decade of the 20th century) that multiplied the production of synthetic nitrogen fertilizers. Today, anthropogenic fixation exceeds natural fixation.

Excess nitrates leached by rain reach rivers and then coastal areas, causing eutrophic dead zones where algal blooms suffocate aquatic fauna. The Gulf of Mexico or the Baltic Sea illustrate this diffuse pollution. Moreover, nitrous oxide (\(N_2O\)), a byproduct of nitrification and denitrification, is a greenhouse gas 300 times more potent than carbon dioxide, contributing to global warming.

FAQ – Nitrogen Cycle

What is the nitrogen cycle?

It is the set of processes that convert atmospheric nitrogen into biologically usable forms and eventually return it to the atmosphere.

Why can’t organisms use atmospheric nitrogen directly?

Because the N₂ molecule is extremely stable and only certain bacteria can break it apart.

What role do bacteria play?

They drive the key steps: nitrogen fixation, nitrification, ammonification, and denitrification, making nitrogen available or returning it to the air.

How do plants use nitrogen?

They absorb nitrates and ammonium from the soil to build proteins, amino acids, and DNA.

How do human activities disrupt the cycle?

Fertilizers, pollution, and industrial emissions add excess reactive nitrogen, causing eutrophication, ecological imbalance, and greenhouse gas release.