The global nitrogen cycle governs the availability of this fundamental element for all life on Earth. Nitrogen must cycle between various chemical forms to remain accessible to organisms, and nitrification and denitrification are the primary microbial mechanisms controlling this flow. Nitrification converts ammonia into the plant-usable form of nitrate, while denitrification returns nitrogen from nitrate back into the atmosphere as an inert gas. These opposing forces maintain the nitrogen balance in ecosystems.
Nitrification: The Step-by-Step Aerobic Conversion
Nitrification is a biological oxidation process that converts reduced nitrogen compounds, primarily ammonium (\(\text{NH}_4^+\)), into nitrate (\(\text{NO}_3^-\)) in two distinct steps. The entire conversion is strictly aerobic, requiring the presence of free molecular oxygen to proceed. This process is carried out by specialized groups of autotrophic microorganisms that use the energy released from these chemical reactions for their growth.
The first step, called nitritation, involves the oxidation of ammonium to nitrite (\(\text{NO}_2^-\)). This reaction is performed by ammonia-oxidizing bacteria (AOB), such as Nitrosomonas, or by ammonia-oxidizing archaea (AOA). This initial step is typically the slower, rate-limiting phase of the overall nitrification process.
The second step, known as nitratation, involves the rapid oxidation of the intermediate nitrite into the final product, nitrate. This oxidation is carried out by nitrite-oxidizing bacteria (NOB), such as Nitrobacter and Nitrospira. Nitrate is the form of nitrogen most easily taken up by plants. Some recently discovered organisms, known as comammox bacteria, are capable of performing the complete conversion from ammonia to nitrate in a single cell.
Denitrification: The Anaerobic Return to Gas
Denitrification is a respiratory process performed primarily by facultative anaerobic bacteria when oxygen is scarce or absent. These bacteria use oxidized nitrogen compounds, specifically nitrate, as a substitute for oxygen to accept electrons during the breakdown of organic matter for energy.
The process is a sequential reduction of nitrate (\(\text{NO}_3^-\)) back into inert dinitrogen gas (\(\text{N}_2\)), involving several intermediate gaseous compounds. The chain begins with nitrate being reduced to nitrite (\(\text{NO}_2^-\)), then to nitric oxide (\(\text{NO}\)), followed by nitrous oxide (\(\text{N}_2\text{O}\)), and finally, to dinitrogen gas.
Bacteria that carry out this process, such as Pseudomonas and Paracoccus, are common in soils and sediments. Denitrification often occurs in waterlogged soils, deep sub-soils, and aquatic sediments where microbial respiration consumes oxygen faster than it can be replaced. A byproduct of this chain is nitrous oxide (\(\text{N}_2\text{O}\)), a potent greenhouse gas that can be released if the complete reduction to \(\text{N}_2\) is not achieved.
Environmental Factors Governing Reaction Rates
The rates of nitrification and denitrification are sensitive to abiotic factors, which determine which process dominates a given locale. Oxygen availability is the most significant factor due to the different metabolic requirements. Nitrification is aerobic, requiring dissolved oxygen concentrations above \(2.0 \text{ mg/L}\) for optimal activity. Conversely, denitrification is anaerobic, and its enzymes are inhibited when oxygen concentrations are above \(0.2 \text{ mg/L}\). This contrast means a shift in oxygen level can switch the dominant nitrogen pathway.
Temperature influences the reaction rates of both processes because they are driven by microbial enzymes. Optimal temperatures for nitrification are between \(25\) and \(35^\circ \text{C}\), and similar ranges are seen for denitrification. Activity slows drastically at cold temperatures, falling sharply below \(10^\circ \text{C}\).
The acidity or alkalinity of the environment, measured by \(\text{pH}\), also exerts strong control. Nitrification is sensitive to low \(\text{pH}\), with optimal activity found in the neutral to slightly alkaline range, around \(\text{pH } 7.0\) to \(8.5\). The process can cease entirely below \(\text{pH } 5.0\). Denitrification is less affected by \(\text{pH}\), but its optimal range is also around neutral.
The availability of organic carbon is a limiting factor for most denitrifying bacteria. Denitrifiers are heterotrophs, meaning they must consume organic carbon compounds as an energy source to fuel the reduction of nitrate. A high carbon load promotes denitrification by leading to rapid oxygen depletion.
Ecological and Practical Significance
These two microbial processes are fundamental to maintaining the global nitrogen budget, preventing nitrogen from becoming permanently locked away or accumulating in toxic forms. Nitrification transforms ammonium into nitrate, a form readily assimilated by most plants and microorganisms, thereby supporting primary productivity. Denitrification completes the cycle by removing excess nitrate from soil and water systems.
In agriculture, nitrification is valued because it converts fertilizer ammonia into the mobile nitrate form that crops absorb easily. However, this mobility leads to the negative impact of denitrification, which causes economic loss by converting valuable nitrate fertilizer back into atmospheric gas. This nitrogen loss contributes to reduced crop yield and requires increased fertilizer application.
On a practical level, both processes are engineered within wastewater treatment plants to remove nitrogenous pollutants before discharge. Nitrification is used in aerated basins to convert ammonia into nitrate. This nitrate is then moved to an anoxic zone where denitrification reduces it to harmless \(\text{N}_2\) gas. This approach protects natural water bodies from eutrophication. The release of \(\text{N}_2\text{O}\) from incomplete denitrification remains a concern, as this gas is roughly \(300\) times more potent than carbon dioxide and contributes to climate change.