Nitrogen Cycle

Nitrogen Fixation

Atmospheric nitrogen (N₂) is very stable and unusable to most organisms. Certain bacteria (in soil, water, or root nodules of legumes) convert N₂ into ammonium (NH₄⁺) or ammonia (NH₃), which plants can use.

This brings nitrogen from the air into the biological cycle.

Litterfall

Leaves, stems, dead roots, and animal waste fall to the ground and decompose, returning organic nitrogen (proteins, nucleic acids, etc.) into the soil.

It recycles nitrogen from living organisms back into the soil pool.

Mineralization

Microbes decompose organic matter in litter, converting organic nitrogen compounds into inorganic ammonium (NH₄⁺).

Makes nitrogen available again in a form microbes and plants can use.

Immobilization

Soil microbes take up inorganic nitrogen (NH₄⁺ or NO₃⁻) from the soil to build their own biomass.

This temporarily "locks up" nitrogen so it's unavailable to plants until those microbes die and decompose.

Nitrification

Specialized bacteria convert ammonium (NH₄⁺) → nitrite (NO₂⁻) → nitrate (NO₃⁻).

Nitrate is the main form of nitrogen that most plants absorb, but it is also more prone to loss from soil.

Leaching

Because nitrate (NO₃⁻) is soluble in water, it can easily be washed out of the soil profile into groundwater and rivers.

This is another loss pathway that reduces soil fertility and can pollute water.

Plant Uptake

Plants absorb nitrogen primarily as nitrate (NO₃⁻) and ammonium (NH₄⁺) from the soil. They then use it to build proteins, DNA, and other essential molecules.

This is how nitrogen re-enters the food chain.

Denitrification

Under low-oxygen (anaerobic) conditions, certain bacteria convert nitrate (NO₃⁻) back into nitrogen gas (N₂) or nitrous oxide (N₂O), which return to the atmosphere.

This closes the cycle, completing the movement of nitrogen from land back to the atmosphere.

Flow of Nitrogen

Atmosphere (N₂) → Fixation → Soil organic matter (via litterfall & mineralization) → Ammonium (NH₄⁺) → Nitrification → Nitrate (NO₃⁻) → Plant uptake OR lost via leaching/volatilization → Eventually returned to atmosphere through denitrification.

Volatilization

Ammonium (NH₄⁺) can be converted into ammonia gas (NH₃) and released back into the atmosphere, especially in warm or alkaline soils.

It's a loss pathway, meaning nitrogen escapes the soil system.