Nitrogen is abundant in the atmosphere but mostly unusable to organisms. This page explains how nitrogen fixation creates plant-available nitrogen (mainly ammonia) and how plants take it up to build proteins, DNA, and other biomolecules.

Why Atmospheric Nitrogen Must Be “Fixed”

Most atmospheric nitrogen is $N_2$, a molecule with a strong triple bond that most organisms cannot break.

USGS nitrogen-cycle diagram summarizing the major transformations that move nitrogen between the atmosphere, soils/waters, and living organisms. It provides a big-picture map showing where nitrogen fixation introduces reactive nitrogen and how microbial processes convert it into forms plants can absorb and eventually return to the atmosphere. Source

Ecosystems therefore depend on processes that convert $N_2$ into reactive forms plants can assimilate.

Nitrogen Fixation (Core Idea)

Fixed nitrogen is “biologically available” because it can be incorporated into organic molecules after additional steps in soils and plants.

How Nitrogen Fixation Happens

Nitrogen fixation occurs through a few key pathways, all of which increase the supply of plant-available nitrogen in an ecosystem.

Biological Fixation (Most Important in Ecosystems)

Specialised nitrogen-fixing microbes use enzymes (notably nitrogenase) to reduce $N_2$ to ammonia.

• Symbiotic fixation

  • Microbes live in close association with plant roots (often in nodules)

Photograph of a legume root system with visible nodules—specialized structures that house nitrogen-fixing bacteria. This image reinforces the symbiosis concept: the plant provides carbohydrates/energy, while microbes convert atmospheric $N_2$ into plant-usable nitrogen compounds. Source

• Plant supplies sugars/energy; microbe supplies fixed nitrogen

• Free-living fixation

  • Some soil and aquatic microbes fix nitrogen without a host plant

• Conditions that influence biological fixation include:

  • Oxygen levels (nitrogenase is oxygen-sensitive)

  • Soil moisture and temperature

  • Availability of energy sources (organic matter) for microbes

Atmospheric Fixation (Minor but Widespread)

High-energy events (e.g., lightning) can convert $N_2$ into nitrogen oxides that dissolve in rain and enter soils. This contributes some reactive nitrogen, but typically far less than biological fixation.

Industrial Fixation (Human-Caused Input)

Although not an ecosystem process, industrial fixation produces ammonia-based fertilisers that add large amounts of reactive nitrogen to agricultural soils, directly affecting plant uptake dynamics.

Conceptual nitrogen-cycle diagram that integrates natural fixation with the Haber–Bosch industrial pathway. It highlights how fixed nitrogen moves among atmospheric, soil/water, and biological reservoirs and how human fertiliser inputs increase reactive nitrogen available for plant uptake and downstream processes (e.g., runoff). Source

“Plant-Available Nitrogen”: What Plants Actually Absorb

Plants do not use $N_2$ gas directly; they primarily absorb inorganic nitrogen through roots as:

• Ammonium ($NH_4^+$)

• Nitrate ($NO_3^-$) (often abundant after microbial conversion of ammonium)

Ammonia produced by fixation in soils quickly becomes $NH_4^+$ in water, which can either be taken up by plants or transformed by microbes into $NO_3^-$. Regardless of form, the key syllabus point is that fixation creates nitrogen that can be built into plant tissue.

Plant Uptake and Assimilation into Tissue

Root uptake is followed by biochemical assimilation inside the plant, turning inorganic nitrogen into organic molecules.

Uptake (Root-Level)

• Nitrogen enters roots dissolved in soil water

• Uptake rate depends on:

  • Root surface area and growth

  • Soil pH and aeration (influences which nitrogen forms dominate)

  • Competition with microbes for the same nitrogen pool

Assimilation (Building Plant Biomass)

Inside plants, absorbed nitrogen is used to make:

• Amino acids → proteins (enzymes, structural proteins)

• Nucleic acids (DNA/RNA)

• Chlorophyll (nitrogen-rich pigment essential for photosynthesis)

Nitrogen availability therefore strongly influences plant growth and primary production because it controls how quickly plants can build new tissues.

Ecological Significance of Fixation + Uptake

• Nitrogen fixation sets the “new” input of reactive nitrogen into many natural systems.

• Plant uptake transfers fixed nitrogen from soils into biomass, making it available to herbivores and higher trophic levels through feeding.

• When fixation is limited, plant growth can be constrained due to insufficient nitrogen for proteins and chlorophyll production.