Nitrogen Cycle

Nitrogen is an integral component of all amino acids, which are the building blocks of all proteins, including enzymes. Nitrogen is also a component of nucleic acids (the seat of genetic inheritance), and chlorophyll. N is essential for carbohydrate use within plants. A good supply of N stimulates root growth and development, and uptake of other nutrients. N deficiency most often results in stunted growth, slow growth, and chlorosis (yellowish or pale green leaves). Because N is mobile, older leaves exhibit chlorosis and necrosis earlier than younger leaves. Under many agricultural settings, N is the limiting nutrient of high growth.

The most important source of N is the atmosphere, which contains 78% N as an inert gas (N₂). In the inert phase, N cannot be used by plants and it has to be converted to forms usable by plants through mineralization. Nitrogen is absorbed by plants from soil in the form of nitrate (NO₃^–) ions, although in acid environments such as boreal forests where nitrification is less likely to occur, ammonium (NH₄⁺) is more likely to be the dominant source of nitrogen. Amino acids and proteins can only be built from NH₄⁺, hence NO₃^– must be reduced.

The main pools and forms of N, and the processes by which they interact, are illustrated in the N cycle shown below. Please note that blue lines indicate sources of N, orange lines are indicative of processes through N is lost from the soil, and black lines indicate processes by which one form of N is transformed into another.

Sources

N Fertilizers

Four general classes of chemical fertilizers are used:

Ammoniacal fertilizers are acid-forming fertilizers that reduce soil pH. Examples of these fertilizers include: ammonium phosphates (NH₄H₂PO₄), ammonium chloride (NH₄Cl), and ammonium nitrate (NH₄NO₃).
Nitrate fertilizers are subject to losses by leaching. Examples of these fertilizers include: sodium nitrate, potassium nitrate and calcium nitrate.
Slowly available nitrogen forms. Example: Substances of low water solubility that must undergo chemical and/or microbial decomposition to release plant-available nitrogen.
Miscellaneous fertilizers. Example: Inhibition of microbial activity.

Organic N Fertilizers

Organic N fertilizers are by-products from the processing of animal or plant compounds that contain sufficient N to be of value as fertilizers (e.g., manure, compost, biosolids, green manure).

Animals

Animals consume plants and in turn add various residues (excrement and remains) to the soil that then serve as a source of soil organic matter or humus.

Humans

Humans consume plants and in turn add various residues to the soil that then serve as a source of soil organic matter or humus.

Biological N Fixation

Nitrogen fixation is the process by which organisms convert the inert N₂ gas of the atmosphere to N-containing organic compounds that become available to all forms of life through the N cycle.

Ammonium fixation

Ammonium (NH₄⁺) can be fixed (i.e., strongly adsorbed) in the interlayer spacing of clay minerals, particularly vermiculite and illite (2:1 clays). The mechanism is similar to potassium (K⁺) fixation.

Wet and dry deposition

Various nitrogen compounds are added to the soil through rain, snow, and dust. The quantity of nitrogen added to soil by wet and dry deposition varies with location and season, but the range of total N added through precipitation annually is 1 to 25 kg N/ha.

Transformations

Immobilization

Immobilization is the process of conversion of nitrogen from the inorganic (mineral) to organic form in microbial or plant tissues, thus rendering nitrogen not readily available to other organisms or plants.

Mineralization

Mineralization is the overall process of conversion of nitrogen from an organic to an inorganic form as a result of microbial decomposition. The inorganic nitrogen ions released by mineralization are readily available to higher plants and to microorganisms.

Adsorption

Ammonium (NH4+) ion is electrostatically adsorbed by negatively charged clay colloids.

Desorption

Desorption of strongly bound (or fixed) ammonium ions occurs during weathering of clay minerals. During desorption, ammonium ions that were fixed in the interlayer spacing of the clay minerals are released into the soil solution.

Ammonification

Amines and amino acids released from the decomposition (by mostly bacteria in neutral and alkaline environments and mostly fungi in acidic environments) of proteins are further decomposed by heterotrophic microorganisms during the ammonification process. The microorganisms that carry out ammonification can be either aerobic or anaerobic.

R-NH₂ + HOH ---> NH₃ (ammonia) + R-OH + energy

where NH₃ = NH₄⁺ + OH^-

Some of the ammonium (NH₄⁺) released into the soil solution will be either converted to nitrites and nitrates by the process of nitrification, absorbed by plants, used by heterotrophic microorganims to build new tissues, adsorbed to clay minerals, or released to the atmosphere as elemental nitrogen.

Nitrification

Nitrification is a two-step process involving the biological oxidation of ammonium (NH₄⁺) to nitrate (NO3-). The first step is the conversion of NH₄⁺ to NO₂^- (nitrite) and is achieved mostly by a group of autotrophic soil bacteria known as Nitrosomonas. This first step proceeds as:

2NH₄⁺ + 3O₂ ---> 2NO₂- + 2H₂O + 4H⁺

The second step is the conversion of NO₂^- (nitrite) to NO₃^- (nitrate) and is achieved mostly by a second group of autotrophic soil bacteria known as Nitrobacter. The second step proceeds as:

2NO₂^- + O₂ ---> 2NO₃^-

Nitrosomonas and Nitrobacter are usually referred to as Nitrobacteria. A few heterotrophic organisms can also be involved in the process of nitrification, such as fungi, bacteria, and actinomycetes.

Losses

Leaching

Leaching or removal of ions (e.g., nitrates) present in the soil solution is very common in temperate soils in which soil colloids predominantly carry negative charges.

Ammonia Volatilization

Ammonia volatilization is a mass transfer (movement) of nitrogen (as ammonia gas, NH₃) from the soil to the atmosphere.

Plant Uptake and Harvest

The major output of nitrogen from the soil includes harvested biomass. The amount of N lost through harvesting depends on the type of plant.

Erosion

Erosion losses are determined by water management practices. The magnitude of erosion losses is dependent upon specific conditions, and so generalizations are difficult.

Denitrification

Denitrification is a biochemical reduction of nitrate to gaseous compounds, such as nitric oxide (NO), nitrous oxide (N₂O), or N₂ gas. This process is carried out by facultative anaerobic bacteria.

Forms of N

Soil Organic Matter

Ammonium

Nitrate

Nitrite

Clay Colloids

Organo-clay complexes

Proteins are commonly found in combination with clay minerals, lignin, and other materials. These complexes show strong resistance to decomposition.