Skip to content
NatureHQ

Ecologymechanism

Nitrogen fixation

No plant fixes nitrogen. Legumes build an organ to house bacteria that do, and pay in sugar.

Breaking the bond in atmospheric nitrogen so life can use it — done only by certain microbes, at considerable energetic cost. Legumes do not fix nitrogen; they house bacteria that do, and pay them in sugar.

Nitrogen gas is two atoms held by a triple bond, and that bond is why an atmosphere full of nitrogen is not a supply of nitrogen. Breaking it requires the enzyme nitrogenase, which exists only in certain bacteria and archaea, and which is expensive to run — a large energetic cost per molecule fixed. That cost is the reason fixation has not simply spread until nitrogen stopped limiting anything. The best-known arrangement is a partnership rather than a plant capability. A legume root releases flavonoids into the soil; compatible rhizobia respond with a specific signalling molecule; the plant’s receptors recognise it, and a developmental programme begins that builds an entirely new organ around the incoming bacteria — while locally suppressing the immune response that would otherwise treat them as the infection they structurally resemble. The elegance is that much of the signalling machinery is shared with the far older partnership between plants and mycorrhizal fungi: a new relationship assembled on an existing pathway. Inside the nodule there is a problem to solve that the popular account never mentions. Nitrogenase is destroyed by oxygen, and the bacteria doing the fixing are respiring aerobically and therefore need oxygen. The nodule solves this with an oxygen-binding protein — leghaemoglobin, chemically related to the haemoglobin in blood and responsible for the pink colour inside a cut nodule — which delivers oxygen to the bacteria while keeping the free concentration extremely low. Then the accounting. Fixed nitrogen goes into the plant, not into the ground. Soil gains nitrogen when nodules and roots turn over, when litter falls and decomposes, and when the plant dies — so how much a soil actually gains depends on what happens to the biomass afterwards. "Legumes add nitrogen to soil" describes a real effect while skipping the mechanism, the cost and the condition.

Early coverage · 32% complete · reviewed 2026-09-04

What this page covers

Fixation is performed only by certain bacteria and archaea — free-living, symbiotic in nodules, and cyanobacteria in water and in lichens. No plant or animal does it alone.

Often confused with: Plants fixing nitrogen, which none of them do — bacteria do it inside them; Nitrogen going directly into the soil, when it goes into the plant first

Quick facts

Only microbes do it
Certain bacteria and archaea, via nitrogenase — no plant or animal alone
A negotiated infection
Chemical advertisement, specific answer, then an organ is built
The oxygen problem
Nitrogenase is destroyed by oxygen the bacteria need to respire
Into the plant, not the ground
Soil gains later, through turnover and decomposition

Who can actually do it

A short list, and no plants on it.

Fixation puts nitrogen into the plant, not directly into the ground. Whether the soil gains depends on what happens to that plant afterwards — and the plant pays for the whole arrangement in carbon.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Symbiotic nitrogen fixation in root nodules delivers fixed nitrogen to the host plant in exchange for photosynthate. Net soil nitrogen enrichment depends on subsequent transfer pathways — root and nodule turnover, litter return, decomposition — and is reduced where biomass is removed from the system.

Who this applies to
Legume–rhizobium symbioses; other fixation routes differ.
Studied in
Fabaceae, Bacteria
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

The mechanism and the carbon cost are both well established; the size of net soil enrichment varies with management and system.

How far it can be extended

The pathway is consistent across nodulating legumes.

Caveats

  • Some nitrogen does reach soil during the plant’s life through root and nodule turnover; the claim is that the bulk arrives via decomposition rather than direct release.
  • This is not agronomic advice — how much a soil gains depends on what is done with the crop, which is outside this site’s scope.

Still unanswered

  • How much fixed nitrogen transfers to neighbouring non-fixing plants during the growing season, which varies widely between studies.

Last reviewed 2026-09-04

The evidence (2 studies)
Where fixation happens
ArrangementWho fixesWhere
Free-livingSoil and water bacteria and archaeaIn soil, sediment and water
Root nodule symbiosisRhizobia inside a plant-built organLegume roots, and some other lineages
Cyanobacterial symbiosisCyanobacteriaIn lichens, in some ferns, in corals and sponges
Free cyanobacteriaCyanobacteriaOpen water, soil crusts, wet surfaces

The lichen row is worth noticing, because it links this page to one built two sprints ago. A lichen carrying cyanobacteria rather than green algae is fixing nitrogen as well as photosynthesising, which is part of why lichens can colonise bare rock — they arrive able to make both their carbon and their nitrogen.

What happens inside a nodule

A negotiation, an infection, and an oxygen problem.

The plant advertises chemically, the bacterium answers with a specific molecule, the plant recognises it and builds an organ to house it — while suppressing the immune response that would normally repel an infection.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Root nodule symbiosis proceeds through reciprocal signalling: host flavonoids induce rhizobial nodulation factors, recognised by host receptors, triggering infection thread formation and nodule organogenesis alongside localised suppression of plant immunity. The signalling pathway is partly shared with the older mycorrhizal symbiosis.

Who this applies to
Mechanistic detail comes chiefly from a few model legumes.Do not extend this beyond the taxa listed — the popular version over-reaches.
Studied in
Fabaceae, Bacteria
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

A well-characterised molecular pathway, though established in a small number of model species.

How far it can be extended

Model species may not represent all nodulating plants, and non-legume nodulation differs.

Caveats

  • The oxygen problem is real and separate: nitrogenase is inactivated by oxygen, and the nodule must supply respiring bacteria while keeping free oxygen very low.
  • Detail derives from model legumes; other nodulating lineages use related but distinct arrangements.

Still unanswered

  • Whether the pathway can be transferred to non-nodulating crops, which is an active and unresolved research effort.

Last reviewed 2026-09-04

The evidence (1 study)

The oxygen arrangement is the detail most worth carrying away, because it shows how much the plant is doing. Nitrogenase stops working in the presence of oxygen. The bacteria inside the nodule are respiring aerobically and need oxygen to generate the energy that fixation consumes. The nodule resolves the contradiction with leghaemoglobin, an oxygen-binding protein chemically related to the haemoglobin in blood, which ferries oxygen to the bacteria while keeping the free concentration low enough for the enzyme to survive. Cut a working nodule open and it is pink for the same reason blood is.

Related

  • Nutrient cycling

    Where fixed nitrogen goes next

  • Symbiosis

    The category, and why it does not mean friendship

  • Lichen

    Where fixation and photosynthesis arrive together

Does a legume enrich the soil?

Eventually, conditionally, and not directly.

During the plant’s life, some fixed nitrogen does reach the soil as nodules and fine roots die and are replaced. The larger transfer happens afterwards, when the plant’s tissues decompose. So the soil’s gain depends on whether the biomass stays in the system — and if the plant is removed, most of the fixed nitrogen leaves with it.

This site stops there deliberately. What follows from that is a question about land management, which is not what NatureHQ is for. The ecological statement is that fixation loads nitrogen into a plant, and that the soil gains through the ordinary route by which any plant returns nutrients: dying, and being decomposed.

Claims about this, checked

Things people have heard, and what the evidence actually supports.

The research behind this page

2 studies, newest first. Each one has a page explaining what it found and what it could not show.

2013Nature Reviews Microbiology

Speak, friend, and enter: signalling systems that promote beneficial symbiotic associations in plants

Nodulation proceeds through reciprocal chemical signalling and a controlled infection process, using a signalling pathway shared in part with the much older mycorrhizal symbiosis.

1991Biogeochemistry

Nitrogen limitation on land and in the sea: how can it occur?

Nitrogen limitation persists because fixation is energetically expensive, requires other resources including phosphorus, iron and molybdenum that are themselves often scarce, and because nitrogen is readily lost from ecosystems in gaseous and dissolved forms.

Where to go from here

Each of these follows from something on this page — a relationship in the evidence, a claim people ask about, or the next mechanism along.

How complete this page is, and what it is still missing

NatureHQ publishes its own gaps. This page is at 32% completeness against what we would call a finished subject, and was last reviewed on 2026-09-04. It carries 3 claims and answers 0 mapped search questions.

  • no research from the last few years is attached — check for newer work
  • more experiments could be explained in plain English
  • Industrial nitrogen fixation and its consequences are out of scope here.
  • Non-legume nodulating plants, such as alders with Frankia, are mentioned only in passing.
  • Nitrification and denitrification, the other ends of the cycle, have no pages of their own.