Core Concepts
Nitrification is a main process within the nitrogen cycle that converts nitrogen into forms organisms can use. The nitrogen cycle is a biogeochemical process that cycles nitrogen between the atmosphere, soil, and living organisms. This article explores the importance of nitrification, and what drives nitrification in the environment.
Introduction
Nitrogen is a key element for all living organisms. All living organisms must synthesize proteins and replicate DNA and they require nitrogen to carry out both. In plants specifically, nitrogen is needed for photosynthesis. Within the nitrogen cycle, there are processes that convert nitrogen to forms that can be used by living organisms. Many of these processes are driven by redox reactions. Nitrification is one of the nitrogen cycle’s important processes. It is an essential process for soil health and fertility, food production, water treatment, and aquatic ecosystems. Throughout this article, we will dive deeper into the chemical reactions behind nitrification and discuss the factors that shape nitrification. To conclude, we will talk about the relevant applications of nitrification in our daily lives, and the implications this process has for the environment.
The Biochemistry of Nitrification
Nitrification can be broken down into two main oxidation steps. The two steps are aerobic in nature meaning they happen in the presence of oxygen. The first step is ammonia oxidation. Ammonia oxidation is the conversion of ammonia (NH3) to nitrite (NO2–) by microorganisms capable of oxidizing ammonia. Ammonia (NH3) can exist in an ionized form and unionized form. The ionized form is referred to as ammonium (NH4+) while the unionized form remains NH3. The pKa of ammonia is around 9.25. At any pH below the pKa (9.25), most ammonia will exist in its protonated form, ammonium. Since environmental pH ranges from 6-8, ammonia is most commonly found in its protonated form. The second step of the nitrification process is nitrite oxidation. Nitrite oxidation is the conversion of nitrite (NO2–) to nitrate (NO3–) by microorganisms capable of oxidizing nitrite. A general, simplified equation for nitrification starting from ammonium is given below:
Certain enzymes serve as a catalyst for nitrification, some examples are nitrate reductase, nitrite reductase, nitric oxide reductase and nitrous oxide reductase. They are crucial for facilitating nitrification and speeding up the reaction. A catalyst is a substance that increases the rate of a chemical reaction without being destroyed or consumed in the process.
Environmental Factors Influencing Nitrification
The surrounding environment significantly impacts nitrification. Because nitrification is an aerobic process, oxygen availability is a huge factor in nitrification rates. There needs to be adequate oxygen for both steps of nitrification. Both of the steps are broken down below:
Step 1: Ammonia Oxidation
Step 2: Nitrate Oxidation
Oxygen serves as the electron acceptor in nitrification and it gains electrons from the electron donors (either ammonia or nitrite). Without it, microorganisms are not able to get the energy they need for the nitrification process.
Additional environmental factors can also greatly impact nitrification. Temperature and pH affect the rate at which nitrification can occur. The optimal pH for nitrification is around 7-8. In highly acidic or alkaline conditions, nitrification can be inhibited. The pH of the surrounding environment can affect nitrification because it influences the protonation of the enzymes and substrates involved in the reaction. At a low pH, For temperature, soil microbes prefer a range of 20-30 degrees Celsius (68-86 degrees Fahrenheit). Extreme temperatures greatly impact the performance of the enzyme catalysts because they can become denatured. Microbial activity and growth can slow down at temperatures outside of the optimal range. Other factors such as the availability of ammonia or the presence of inhibitory substances can also impact the rate of nitrification. If there is not sufficient ammonia present (ionized or unionized forms), then nitrification will slow down. Additionally, if there are substances that affect microbial communities either through suppressing activity or slowing growth, then nitrification can also slow down. Some of these inhibitory substances are heavy metals and pesticides.
Nitrification in the Environment and Agriculture
In the environment, soil bacteria are responsible for nitrification. In the first step of nitrification (ammonia oxidation), the most common genus of soil bacteria is Nitrosomonas. In the second step of nitrification (nitrite oxidation), Nitrobacter is the most common genus of soil bacteria. Soil nitrification is essential to produce plant available forms. However, nitrification can lead to nitrate leaching depending on the rate of nitrification and the amount of nitrogen in the system. Nitrification rate is influenced by soil texture, the composition of the soil’s microbial community, and the surrounding land use. An example of nitrification in a real world scenario is given in the diagram below. It begins with nitrogen fertilizer application. The conversion of this fertilizer to nitrate is broken down into the separate oxidation steps.
Nitrification in the soil profile can enhance plant growth. This is especially beneficial for crops in agricultural systems. However, increased nitrification rates can also lead to nitrate loss through leaching and denitrification if there is an excess of nitrate in the system. It is important to note that nitrification doesn’t just happen in the soil profile. It also takes place in aquatic ecosystems in the sediments throughout the water column.
Nitrification in Wastewater Treatment and Industry
Nitrification plays a crucial role in wastewater treatment. It is leveraged to convert toxic ammonia to less toxic nitrate. It is also coupled with denitrification for complete nitrogen removal in wastewater and sewage sludge. Not only is nitrification used in wastewater and drinking water treatment, but it is also used in aquaculture. In aquaculture, nitrification is used to maintain water quality and fish health. However, it must be managed well to avoid algal blooms which can negatively impact the aquaculture ecosystem health.
Conclusion
Nitrification is a foundational process in the nitrogen cycle. It has cascading implications for soil health, agriculture, water treatment, and water quality in general. This aerobic conversion of ammonia to nitrate is facilitated by microorganisms and influenced by the conditions of the surrounding environment. Environmental conditions such as temperature, pH, oxygen and ammonia availability all impact the rate of nitrification. Nitrification is essential for producing bioavailable (available for plant uptake) forms of nitrogen, but there are potential impacts for water quality depending on the rate of nitrification. Understanding the process of nitrification is important for managing nitrogen in the environment and mitigating any potential negative impacts.