Nitrogen cycle

Key points

Introduction to the nitrogen cycle

Nitrogen sources in soil:

• Industrial fixation - converts nitrogen gas to ammonia for fertilizer production.
• Biological fixation - N-fixing rhizobia bacteria convert nitrogen gas into plant-usable ammonia. In cropping systems, the symbiotic relationship between the bacteria and legumes can contribute a significant amount of N annually.
• Plant residues - contain complex organic forms of nitrogen. Residues must decay before the nitrogen will be available to other plants.
• Animal manures - contain both plant-available (ammonium and nitrate) and organic nitrogen. The N contribution and availability depends on several factors, including livestock type, storage type and application method.
• Atmospheric fixation - can contribute 5 - 10 lb/a/yr from electrical discharges (lightning) and industrial activities.

Nitrogen pools

There are four major pools of nitrogen:

• Atmospheric nitrogen (N₂) - largest pool of nitrogen; not plant-available
• Soil organic matter (SOM) - largest pool of nitrogen in the soil; not directly plant-available
• Nitrate (NO₃^-) - the largest pool of plant-available, inorganic nitrogen in the soil
• Ammonium (NH₄⁺) - an inorganic pool of nitrogen in the soil

Nitrogen sources and transformations

There are several transformations that are important in crop production:

• Nitrification - Nitrifying bacteria convert ammonium to nitrate, a form that is plant-available, but susceptible to leaching.
• Denitrification - Denitrifying bacteria convert nitrate to nitrogen gas, a form that is no longer plant-available.
• Volatilization - The urease enzyme converts surface-applied urea to ammonia gas (NH₃) and carbon dioxide.
• Mineralization - Soil microbes convert organic nitrogen to inorganic, plant-available forms.
• Immobilization - Soil microbes take up nitrate and ammonium forms for their own use, making it unavailable to plants.