Nitrogen can go through many transformations in the soil. These transformations are often grouped into a system called the nitrogen cycle, which can be presented in varying degrees of complexity. The nitrogen cycle is appropriate for understanding nutrient and fertilizer management. Because microorganisms are responsible for most of these processes, they occur very slowly, if at all, when soil temperatures are below 50° F, but their rates increase rapidly as soils become warmer.

The heart of the nitrogen cycle is the conversion of inorganic to organic nitrogen, and vice versa. As microorganisms grow, they remove H4+ and NO3- from the soil’s inorganic, available nitrogen pool, converting it to organic nitrogen in a process called immobilization. When these organisms die and are decomposed by others, excess NH4+ can be released back to the inorganic pool in a process called mineralization. Nitrogen can also be mineralized when microorganisms decompose a material containing more nitrogen than they can use at one time, materials such as legume residues or manures. Immobilization and mineralization are conducted by most microorganisms, and are most rapid when soils are warm and moist, but not saturated with water. The quantity of inorganic nitrogen available for crop useoften depends on the amount of mineralization occurring and the balance between mineralization and immobilization.

Ammonium ions (NH4+) not immobilized or taken up quickly by higher plants are usually converted rapidly to NO3- ions by a process called nitrification. This is a two-step process,during which bacteria called Nitrosomonas convert NH4+ to nitrite (NO2-), and then other bacteria, Nitrobacter, convert the NO2- to NO3-. This process requires a well-aerated soil and occurs rapidly enough that one usually finds mostly NO3- rather than NH4+ in soils during the growing season.
 

The nitrogen cycle contains several routes by which plant-available nitrogen can be lost from the soil. Nitrate-nitrogen is usually more subject to loss than is ammonium-nitrogen. Significant loss mechanisms include leaching, denitrification, volatilization and crop removal.

The nitrate form of nitrogen is so soluble that it leaches easily when excess water percolates through the soil. This can be a major loss mechanism in coarse-textured soils where water percolates freely, but is less of a problem in finer-textured, more impermeable soils, where percolation is very slow.

These latter soils tend to become saturated easily, and when microorganisms exhaust the free oxygen supply in the wet soil, some obtain it by decomposing NO3-. In this process, called denitrification, NO3- is converted to gaseous oxides of nitrogen or to N2 gas, both unavailable to plants. Denitrification can cause major losses of nitrogen when soils are warm and remain saturated for more than a few days.

Losses of NH4+ nitrogen are less common and occur mainly by volatilization. Ammonium ions are basically anhydrous ammonia (NH3) molecules with an extra hydrogen ion (H+) attached. When this extra H+ is removed from the NH4 ion by another ion such as hydroxyl (OH-), the resulting NH3 molecule can evaporate, or volatilize from the soil. This mechanism is most important in high-pH soils that contain large quantities of OH- ions.

Crop removal represents a loss because nitrogen in the harvested portions of the crop plant is removed from the field completely. The nitrogen in crop residues is recycled back into the system, and is better thought of as immobilized rather than removed. Much is eventually mineralized and may be reutilized by a crop.

 

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