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Nutrient cycling

Adding dead leaves to soil can leave plants with less nitrogen, not more — for months.

How the elements life is built from move between living things, dead things and the ground. The part people miss: decomposers are competitors for those nutrients, not a delivery service.

The textbook picture has decomposers releasing nutrients from dead material so plants can take them up again, which is true over long enough periods and misleading about what happens first. A microbe decomposing a dead leaf needs nitrogen to build itself, exactly as a plant does. If the leaf is carbon-rich and nitrogen-poor — and most plant litter is — the microbes take nitrogen out of the surrounding soil to process it. For a period that can run to months, adding dead plant material to soil reduces the nitrogen available to plants rather than increasing it. That is immobilisation, and it reverses into mineralisation only once decomposition has consumed enough carbon for the remaining material to be nitrogen-rich relative to what the decomposers need. There is no universal threshold: it depends on how efficiently the particular decomposers convert carbon into biomass, which varies. Rules of thumb about carbon-to-nitrogen ratios are useful approximations rather than constants. Behind all of this sits a fact worth stating on its own. The atmosphere is roughly four-fifths nitrogen, and nitrogen limits growth across most of the world’s ecosystems. The bond holding N₂ together is expensive to break, the organisms that can break it need other scarce elements to do it, and once nitrogen is in a usable form it leaches away in water or leaves as gas. Abundance and availability are different properties, and the gap between them is why an element that surrounds everything is the thing most often in short supply. The cycles also leak, in both directions, which is why "cycle" is a slightly generous word. Nitrogen arrives by fixation and by deposition and departs by leaching and by denitrification. Phosphorus enters almost entirely from weathering rock and leaves in sediment, with no atmospheric route back — which is why phosphorus and nitrogen behave so differently over long timescales and why treating them as parallel diagrams misses the most important thing about each.

Early coverage · 43% complete · reviewed 2026-09-04

What this page covers

Every organism participates. The transformations that matter most are microbial, and several — nitrification, denitrification, fixation — are carried out by narrow groups of bacteria and archaea and by nothing else.

Often confused with: Decomposers handing nutrients back to plants, which skips a competition; A closed loop, when systems gain and lose continuously

Quick facts

Immobilisation first
Decomposers take nitrogen from soil to process carbon-rich litter
No fixed threshold
The switch depends on decomposer efficiency, not a single ratio
Abundant and unavailable
Air is four-fifths nitrogen; nitrogen limits most ecosystems
Phosphorus has no air route
It enters from rock and leaves in sediment — a one-way trip

The bit that runs backwards first

Decomposers are competitors, not couriers.

Add carbon-rich dead material to soil and nitrogen available to plants can fall, not rise. The microbes decomposing it need nitrogen to build themselves, and they take it from the soil around them.

Established

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

Decomposing litter with a high carbon-to-nitrogen ratio immobilises inorganic nitrogen from the surrounding soil into microbial biomass, reducing plant-available nitrogen until decomposition narrows the ratio sufficiently for net mineralisation to begin. The threshold depends on decomposer carbon-use efficiency rather than being a universal constant.

Who this applies to
Litter decomposition in soils generally, across biomes.
Studied in
Bacteria, Fungi, Plantae
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

A consistent pattern across a large compilation, with a clear mechanism in decomposer nitrogen requirements.

How far it can be extended

Compiled across litter decomposition datasets from many biomes.

Caveats

  • The carbon-to-nitrogen threshold is not a fixed number; rules of thumb quoting one are approximations.
  • This concerns plant-available nitrogen in the short term; over longer periods the nitrogen is released again.

Still unanswered

  • How much decomposer carbon-use efficiency varies in the field, since it sets the threshold and is hard to measure.

Last reviewed 2026-09-04

The evidence (1 study)
Two processes that run at the same time, in opposite directions
ProcessWhat happensEffect on plants
MineralisationOrganic nitrogen released as ammonium during decompositionIncreases what is available
ImmobilisationMicrobes take up inorganic nitrogen to build biomassReduces what is available
Net effectWhichever is larger, which depends on the materialCarbon-rich litter: negative at first, positive later

Diagram

Two processes, running at once, in opposite directions

Nitrogen leaving dead material, and nitrogen being taken back out of the soil.

Two processes, running at once, in opposite directionsIn dead materialNitrogen locked in moleculesIn the soilNitrogen plants can take upMineralisation — releasedImmobilisation — microbes take it upCarbon-rich litter runs the second one harder, for months.Which is why adding dead plant material to soil can leave plants with lessnitrogen, not more. The threshold depends on the decomposers, not on a fixed ratio.
The same explanation in words

Two boxes face each other: nitrogen locked in the molecules of dead material, and nitrogen in the soil in a form plants can take up. An arrow from the first to the second is labelled mineralisation, marking nitrogen being released. An arrow running back the other way is labelled immobilisation, marking microbes taking soil nitrogen up to build themselves. Notes record that carbon-rich litter drives the second harder and for months, that this is why adding dead plant material to soil can leave plants with less nitrogen rather than more, and that the switching point depends on the decomposers rather than on a fixed ratio.

Both processes happen continuously and simultaneously; what is observed is the balance. That is why the same material can appear to enrich soil in one setting and impoverish it in another, and why the honest answer to "does mulching add nitrogen" is that it depends on the material, the microbes and how long you wait.

Surrounded by nitrogen, short of nitrogen

The gap between abundance and availability.

The air is four-fifths nitrogen and nitrogen still limits growth across most ecosystems. Breaking the bond in N₂ is expensive, needs other scarce elements, and usable nitrogen leaves a system easily once made.

Established

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

Nitrogen limitation of primary production persists in many terrestrial and marine systems despite atmospheric abundance, because biological fixation is energetically costly, requires co-limiting elements including phosphorus, iron and molybdenum, and because fixed nitrogen is readily lost through leaching and gaseous pathways.

Who this applies to
Terrestrial and marine systems; the balance of causes differs between them.
Studied in
Plantae, Bacteria
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

A long-standing and well-supported synthesis, with the mechanisms independently established.

How far it can be extended

Documented across many ecosystem types.

Caveats

  • Human nitrogen inputs have substantially relieved limitation in many regions since this was written, which changes the picture regionally rather than the mechanism.
  • Some systems are limited by phosphorus or by other elements instead, and co-limitation is common.

Still unanswered

  • Why nitrogen-fixing organisms do not simply increase until limitation is relieved, which remains partly unresolved.

Last reviewed 2026-09-04

The evidence (1 study)

Three things keep the gap open. Breaking the triple bond in N₂ takes a great deal of energy, which is why the organisms that do it are comparatively few and why it is not simply done by everything. Fixation needs other elements — phosphorus, iron, molybdenum — that are frequently scarce themselves, so the ability to relieve nitrogen limitation is often limited by something else. And usable nitrogen is mobile: it leaches away in water and leaves as gas when microbes respire nitrate in the absence of oxygen.

Related

Why "cycle" is a generous word

Systems gain and lose continuously.

A cycle implies closure, and none of these is closed. Nitrogen enters an ecosystem by fixation and atmospheric deposition and leaves by leaching and by gaseous loss. Carbon enters by photosynthesis and leaves by respiration, by dissolved export in water, and by fire. Phosphorus is the extreme case: it enters almost entirely from the weathering of rock, and leaves in sediment washed to the sea, with no atmospheric return path at all.

That last asymmetry has real consequences. Over long timescales, ecosystems on old, deeply weathered surfaces run short of phosphorus in a way they cannot run short of nitrogen, because nitrogen has a route back from the air and phosphorus does not. Drawing the two as parallel circular diagrams hides the most important difference between them.

The research behind this page

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

2015Nature

The contentious nature of soil organic matter

Soil organic matter is better described as a continuum of progressively decomposing fragments than as discrete humic macromolecules.

2013Global Change Biology

The Microbial Efficiency-Matrix Stabilization (MEMS) framework integrates plant litter decomposition with soil organic matter stabilization: do labile plant inputs form stable soil organic matter?

The easily digested parts of litter — sugars and simple compounds — are converted efficiently into microbial tissue, and it is that microbial material which ends up bound to mineral surfaces and persisting.

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.

2008Science

The global stoichiometry of litter nitrogen mineralization

Decomposing litter initially immobilises nitrogen — taking it from the surrounding soil — when its carbon-to-nitrogen ratio is high, and only begins releasing nitrogen once decomposition has narrowed that ratio sufficiently.

2004Ecology Letters

Detritus, trophic dynamics and biodiversity

Detritus supports food webs whose dynamics differ from grazing webs in important respects: the resource does not respond to its consumers, inputs arrive independently of consumption, and the resulting couplings can stabilise or destabilise the wider community depending on structure.

1999The American Naturalist

Patterns in the fate of production in plant communities

The share of plant production eaten alive varies enormously between ecosystem types, from a small minority in forests to a large majority in phytoplankton systems.

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.

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

NatureHQ publishes its own gaps. This page is at 43% completeness against what we would call a finished subject, and was last reviewed on 2026-09-04. It carries 6 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
  • no popular claim about this subject has been checked yet
  • Nitrification and denitrification are named but not covered in their own right.
  • The carbon cycle at ecosystem scale is treated on the decomposition and soil carbon pages.
  • Human alteration of nutrient cycles is deliberately out of scope here.