Overview, Cycle & Equation - Video & Lesson Transcript

Nitrification Process

The nitrification process involves the conversion of ammonia, an organic compound, to nitrate, an inorganic compound. This occurs in two main steps. The first step is nitritation, which is the oxidation of ammonia to nitrite ({eq}NO_{2}^{-} {/eq}). The second step is nitration, which is the conversion of nitrite to nitrate ({eq}NO_{3}^{-} {/eq}).

Nitritation/Ammonia Oxidation

The first step of nitrification is nitritation. During nitritation, bacteria such as Nitrosomonas convert ammonia to nitrite. This is called ammonia oxidation. Oxidation is the loss of electrons from one atom, in this case, nitrogen, to an oxygen atom. The chemical equation for this process is as follows:

{eq}NH_{3} + O_{2} \rightarrow NO_{2}- + 3H^{+} + 2e^{-} {/eq}

There are enzymes that assist with this reaction. Enzymes are proteins that speed up specific chemical reactions. Example enzymes involved in the nitritation process include:

• Ammonium monooxygenase: Conducts the intermediate step of converting ammonium to hydroxylamine ({eq}NH_{2}OH {/eq}) during nitritation.

• Hydroxylamine oxidoreductase: Oxidizes hydroxylamine, converting it to nitrite.

Nitration/Nitrite Oxidation

The second step of nitrification is nitration. During nitration, bacteria such as Nitrobacter convert nitrite to nitrate. This is called nitrite oxidation. The chemical equation for this process is as follows:

{eq}NO_{2}^{-} + H_{2}O \rightarrow NO_{3}^{-} + 2H^{+} + 2e^{-} {/eq}

An example of an enzyme involved in the nitration process is nitrite oxidoreductase. This enzyme oxidizes nitrite during conversion to nitrate.

Nitrification in the Nitrogen Cycle

Nitrification is one of the processes that occur during the nitrogen cycle. The nitrogen cycle includes the many different chemical reactions that convert nitrogen compounds into different forms throughout the environment. Nitrogen is an important element in organic compounds such as proteins and nucleic acids (DNA and RNA).

The nitrogen cycle traces the movement of nitrogen in the environment as it is converted into different compounds.

Major steps of the nitrogen cycle include the following:

• Nitrogen fixation: Specialized bacteria, industrial processes, and lightning strikes can split atmospheric nitrogen ({eq}N_{2} {/eq}) and create nitrogen-containing compounds such as ammonia, ammonium, ammonium nitrate (used in fertilizer), and organic compounds (amino acids to build protein). This adds nitrogen to the soil.

• Nitrification: Bacteria convert ammonia into nitrites, then nitrates in a two-step process.
• Denitrification: Bacteria reduce nitrates and nitrites back into atmospheric nitrogen.
• Ammonification: Ammonia ({eq}NH_{3} {/eq}) is converted to ammonium ({eq}NH_{4}^{+} {/eq})

• Assimilation: Organisms, including plants and fungi, convert nitrates and ammonia into essential organic compounds, such as amino acids, to build protein.
• Decomposition: Decomposers, such as bacteria and fungi, break down organic compounds, like animal waste and decaying plants, into ammonia. This returns nitrogen to the ground.

Note that bacteria are present in many steps of the nitrogen cycle. Different types of bacteria are essential to the conversion of nitrogen-containing compounds into different forms and for fixing nitrogen gas from the atmosphere into the soil as ammonia.

Nitrifying Bacteria

Bacteria convert ammonia into nitrates during nitrification. Some of these nitrifying bacteria may obtain energy through the oxidation of ammonia and nitrates. Most nitrification is conducted by bacteria, but there are other organisms that can conduct nitrification too.

Organisms involved in nitritation (ammonia oxidation) convert ammonia to nitrite. Example organisms include:

• Crenarchaeota, which includes archaeal bacteria species that dwell in the soil and ocean. These organisms are more abundant than nitrifying bacteria.
• Nitrifying bacteria, including those of the genus Nitrosomonas, Nitrosospira, Nitrosococcus, and Nitrosolobus. These bacteria are soil-dwelling.

Organisms involved in nitration (nitrite oxidation) convert nitrite to nitrate. Example organisms include:

• Soil-dwelling bacteria including Nitrobacter, Nitrospina, and Nitrococcus.
• Legumes, which are a type of plant including soybean and peanut, are also able to produce nitrates. These nitrates would return to the soil when the plant decays.

Note that some bacteria of the genus Nitrospira can conduct both nitritation and nitration.

The root nodules of legumes host bacteria, which help the plant to convert nitrogen compounds.

Factors Affecting Nitrification

As with all chemical reactions, there are ideal conditions for nitrification to occur. If these conditions are not met, this can slow down the rate of nitrification in the environment. Some of the factors that affect nitrification include:

• Soil temperature
• pH (acidity/alkalinity)
• Water content
• Oxygen aeration
• Soil salinity
• Soil texture
• Source of nitrogen

All of the above factors can impact important bacteria that are responsible for nitrification. Nitrifying bacteria are more sensitive to changes in their environment than other types of bacteria, which is why it is important to maintain a balance of these factors.

Function of Nitrification

Nitrification is essential for converting organic nitrogen (ammonia) into inorganic nitrogen (nitrate). Living things depend on these nitrogen-containing compounds to synthesize important organic compounds of their own. These include amino acids and nucleic acids. Without these compounds, living things would be unable to build proteins, enzymes, DNA, and RNA. Plants use nitrates to create amino acids and proteins. Animals are unable to create amino acids but can obtain them by consuming plants. Nucleotides, which contain nitrogen, are the building blocks of DNA and RNA. These can also be created by plants and animals as long as they have a source of nitrogen.

There are also other functions for nitrification. These include:

• Agriculture: The fertilizer applied to crops often contains ammonia. The ammonia will undergo nitrification, producing nitrates for plants.
• Wastewater treatment: The ammonia present in waster products is processed to eliminate nitrogen. Wastewater treatment plants will conduct nitrification to convert the ammonia to nitrate and then denitrification to remove nitrogen by converting it to nitrogen gas. The nitrogen gas returns to the atmosphere.

Lesson Summary

Nitrification is the process in which ammonia is converted to nitrate. Nitrification is part of the nitrogen cycle, which traces the flow of nitrogen through the environment as it is converted into different compounds. Nitrogen compounds are important for plants and animals to grow, reproduce, and develop. Nitrogen first enters the ground by lightning strikes, decaying plants, and animal waste. Nitrifying bacteria in the ground will then convert the organic nitrogen compounds into inorganic forms such as nitrites and nitrates. These bacteria contain enzymes that facilitate the chemical reactions that occur during nitrification.

Nitrification consists of two main steps, including nitritation and nitration.

• Nitritation, also known as ammonia oxidation, converts ammonia into nitrite: {eq}NH_{3} + O_{2} \rightarrow NO_{2}- + 3H^{+} + 2e^{-} {/eq}

• Nitration, also known as nitrite oxidation, converts nitrite to nitrate: {eq}NO_{2}^{-} + H_{2}O \rightarrow NO_{3}^{-} + 2H^{+} + 2e^{-} {/eq}

Nitrates can be used by plants to create organic nitrogen-containing compounds. Both plants and animals need these organic compounds, such as amino acids and nucleotides, to create proteins, enzymes, and DNA. Other functions of nitrification include agriculture and wastewater treatment.