The Nitrogen Cycle

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The Nitrogen Cycle Explained:

Q: How is nitrogen gas (N2) converted to plant-available nitrogen?

Dr. Khan:

The ultimate source of all nitrogen used by plants is N2 gas, which accounts for 78% of air. In order for plants to use it, N2 must first be converted to ammonium (NH4+) through biological or chemical fixation.

Biological fixation is the major process for converting atmospheric N2 to plant-available forms. It takes place through the activities of symbiotic or non-symbiotic microbes. Symbiotic microbes reside in root nodules on legume plants like soybean, whereas the nonsymbiotic group occurs as free-living forms in the soil.

Chemical fixation is what happens when nitrogen fertilizers are made by the Haber-Bosch process, which uses large amounts of energy to convert atmospheric N2 into ammonia (NH3) for direct application (as anhydrous NH3) or for making other nitrogen fertilizers.

Q: What is the primary source of nitrogen for plants?

Dr. Khan:

Regardless of whether it originated through biological or chemical fixation, soil nitrogen occurs largely in the organic matter. This organic nitrogen becomes plant-available through a process of microbial decomposition known as mineralization, which usually serves as the major source for nitrogen uptake by corn and other cereal crops. Fertilizer inputs provide a supplemental source of nitrogen that is most apt to be beneficial during the period of peak crop nitrogen demand.

Q: What is nitrogen mineralization and what factors affect it?

Dr. Khan:

Mineralization is the microbial conversion of nitrogen in proteins and other organic forms to mineral nitrogen as ammonium (NH4+), which is then converted to nitrate (NO3) and becomes available for plant uptake. This process is favored by the absence of soil acidity (i.e. a soil pH close to neutral), warm weather, and rainfall that keeps the soil moist but not waterlogged.

Q: What is nitrogen immobilization and how does it affect availability of nitrogen to the crop?

Dr. Khan:

Immobilization is the conversion of inorganic nitrogen (NH4+ or NO3) to organic forms (such as amino acids and amino sugars) by the soil microbes, and is the reverse of the mineralization process. Microbes require both nitrogen and carbon to drive the immobilization process. The carbon comes from carbonaceous residues such as corn stalks, and will deplete the soil of mineral nitrogen needed by microbes to utilize the residues as a carbon and energy source. The consequences for the next crop can be yield reduction due to nitrogen deficiency. This reduction is sometimes called the 'carbon penalty', which can be avoided by knowing the soil’s nitrogen supplying power and, if necessary, adjusting the nitrogen fertilizer rate to compensate for the adverse effects of nitrogen immobilization.

Q: How is nitrogen lost from soil and how can these losses be reduced?

Dr. Khan:

There are two major processes that lead to nitrogen loss from the soil – leaching and denitrification of NO3. Both are driven by excessive rainfall. Leaching occurs because NO3 is highly soluble and readily carried by water moving through the soil profile. Denitrification occurs when the soil becomes saturated with water, which sharply reduces the amount of oxygen in the soil pores. Under such conditions, microbes convert NO3 to nitric oxide (NO), nitrous oxide (N2O), and finally nitrogen gas (N2).

Nitrogen can also be lost through volatilization of ammonia (NH3). This happens when anhydrous NH3 is applied to soils that are too wet for the applicator slit to seal, or when fertilizer urea or animal manure is left on the surface of dry soil or crop residues.

Nitrogen losses by leaching or denitrification can be controlled by adjusting fertilizer rates to account for soil nitrogen supplying power, and by matching nitrogen availability with periods of high nitrogen demand from the crop. To reduce volatilization losses, fertilizer urea should be incorporated into the soil whenever possible, or else applied with a urease inhibitor.