The Nitrogen Cycle

Overview

The nitrogen cycle describes how nitrogen moves between the atmosphere, soil or substrate, microorganisms, plants, and water.

Nitrogen is unique among nutrients because: - It has a large atmospheric reservoir - It exists in multiple chemical forms - It is strongly influenced by biology - It is easily lost from the root zone

Major nitrogen pools

Atmospheric nitrogen

• Exists as N₂ gas
• Unavailable to plants directly
• Becomes available through biological fixation

Organic nitrogen

• Found in organic matter, composts, plant residues
• Not directly available
• Must be mineralised by microbes

Inorganic nitrogen

• Ammonium (NH₄⁺)
• Nitrate (NO₃⁻) These are the primary plant-available forms.

Key processes

Biological nitrogen fixation

Conversion of N₂ gas into ammonium by: - Symbiotic bacteria (e.g. legumes) - Free-living soil bacteria

Mineralisation

Microbial conversion of organic nitrogen into ammonium (NH₄⁺).

Immobilisation

Temporary microbial uptake of ammonium or nitrate during decomposition of high C:N materials, making N unavailable to plants.

Nitrification

Microbial oxidation of ammonium to nitrate: NH₄⁺ → NO₂⁻ → NO₃⁻

Favoured by: - Warm temperatures - Good aeration - Neutral pH

Denitrification

Microbial reduction of nitrate to gaseous forms (N₂, N₂O) under low oxygen conditions, resulting in nitrogen loss.

Nitrogen loss pathways

• Leaching (mainly nitrate)
• Denitrification (waterlogged conditions)
• Volatilisation (ammonia loss from urea/ammonium at high pH)
• Crop removal

Practical implications

• Nitrate is mobile and easily lost
• Ammonium is retained but biologically active
• Nitrogen availability depends on biology as much as fertiliser input
• C:N ratio strongly influences N availability
• Nitrogen management must consider timing, form, and environment

Agronomic note

Many “nitrogen deficiencies” are actually immobilisation, leaching, or uptake inhibition problems, not lack of applied nitrogen.