Nitrogen cycle

Introduction to the nitrogen cycle

Nitrogen exists in many forms and easily transforms from one form to another.

Nitrogen pools

Review the four major pools of nitrogen.

Nitrogen sources and transformations

Review the major nitrogen sources for crops and factors that affect their transformations.

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.