The Nitrogen Cycle in Soil.
The Nitrogen Cycle in Soil.

Just as water moves through the environment, so does nitrogen, in various forms. The nitrogen cycle is a representation of the various forms of N and how they relate to one another through many complex interactions.

Nitrogen Availability to Plants

For nitrogen, non-­leguminous plants — such as lawn and turf grasses, corn and most fruit and vegetable crops — must rely on either bacteria that live in the soil to "fix" the nitrogen into a usable form, or nitrogen from decomposing organic matter or fertilizers. The ammonium and nitrate ions are the most common forms taken in through plant roots. Ammonium is converted to the nitrite and nitrate forms rather quickly by nitrifying bacteria, which add oxygen to the ammonium ion and convert it to nitrate.

Nitrogen Loss from Availability to Plants

Nitrogen becomes unavailable to plants primarily in three ways.

  • Most nitrogen is lost through denitrification, which is a problem in wet or compact soils. Since these soils contain little oxygen, denitrifying bacteria remove the oxygen from nitrite and nitrate ions for their own use, releasing N2 or N2Oback to the atmosphere.
  • Nitrogen loss occurs by nitrate leaching, which is a concern with the nitrate ion. Leaching occurs when the water­-soluble nitrate ion moves through the soil as water percolates downward beyond the reach of plant roots.
  • Surface volatilization (conversion to the gaseous phase) occurs when ammonia, usually in the form of urea, volatilizes and is lost to the atmosphere. Surface volatilization is usually a problem in areas with high temperatures, and with soils that have a high pH value. Soils that have been compacted by field operations and other human activities also are a problem because it may not be possible to properly mix the urea with the compacted soil.

Nitrogen Cycle-Hydrologic Cycle: Interactions

Denitrification

Nitrifying organisms can only function when free oxygen is present. In saturated soils, free oxygen is very low, suppressing the growth of the nitrifying organisms, often causing nitrogen deficiencies in excessively wet soils. This condition is enhanced by denitrifying bacteria since they thrive in an oxygen-free environment, like a saturated soil, and therefore consume nitrate at a rapid rate. Excessive rainfall promotes nitrogen loss not only by promoting nitrate leaching from the plant root zone, but also by creating wet soil conditions that favor denitrification.

Surface Volatilization

Surface volatilization (vaporization of urea to ammonia gas) may occur when urea is applied on crop residues and is not in good contact with soil particles. To limit volatilization of the urea, producers incorporate it into the soil by tillage to bring the urea into contact with the soil. Limited rainfall also helps with proper incorporation of the urea in the upper portion of the soil profile. When water and urea combine, the result is the ammonium ion, which has a positive charge and attaches to negatively charged soil particles. Both tillage and rainfall can help make nitrogen available for plant use. Unfortunately, the interaction between tillage and excessive rainfall increases the potential for soil erosion. After tillage, the soil is more susceptible to being carried away by water during heavy rainfall.

Nitrogen Movement through Soil

The nitrate ion is the most water­-soluble form of nitrogen as well as the form least attracted to soil particles. Therefore, its interaction with the hydrologic cycle is very important since it moves where water moves. Precipitation, evaporation and transpiration may affect the movement of nitrate in the near-surface soil profile. Rainfall that infiltrates the soil surface may cause nitrate ions to move down through the soil profile by percolation. The more rain that infiltrates, the further down in the profile nitrate ions move. Nitrate movement below the plant root zone is called nitrate leaching.

To learn more about the nitrogen and hydrologic cycle, click here