Denitrification is a microbial process in which nitrate is converted into dinitrogen gas, which then returns to the atmosphere. This transformation is a significant part of the global nitrogen cycle, facilitating the removal of nitrogen from terrestrial and aquatic systems. The process is carried out by a wide variety of microorganisms as a form of respiration.
The Biochemical Pathway of Denitrification
Denitrification is a form of respiration that unfolds as a stepwise reduction of nitrogen compounds, with nitrate (NO₃⁻) as the initial substance. This pathway is carried out by facultative anaerobic bacteria, such as species from the genera Paracoccus and Pseudomonas.
This biochemical conversion involves a sequence of intermediate gaseous nitrogen oxides before reaching the final product. The first step is the reduction of nitrate (NO₃⁻) to nitrite (NO₂⁻), a reaction catalyzed by the enzyme nitrate reductase. Following this, nitrite is converted to nitric oxide (NO) by nitrite reductase.
The pathway continues with the reduction of nitric oxide to nitrous oxide (N₂O) via nitric oxide reductase. In the final step, nitrous oxide reductase catalyzes the conversion of nitrous oxide into dinitrogen gas (N₂). The entire sequence provides the microbe with energy for its life processes.
Environmental Conditions for Denitrification
This biochemical sequence depends on specific environmental triggers. The primary requirement for denitrification is the absence or very low concentration of oxygen. In these anaerobic or anoxic environments, microbes unable to use oxygen for respiration turn to an alternative like nitrate to support their metabolism.
The availability of a sufficient supply of organic carbon is another factor. Denitrifying bacteria are heterotrophs, meaning they require organic compounds to provide the energy and electrons to drive nitrate reduction. The ratio of available carbon to nitrogen influences the process’s efficiency, and insufficient organic matter can lead to incomplete denitrification.
The rate of denitrification is also governed by temperature and pH. The optimal range for most denitrifying bacteria is between 25°C and 37°C, and lower temperatures can slow microbial activity. Denitrification proceeds most efficiently within a neutral pH range of 7.0 to 8.0, as extreme levels can inhibit the necessary enzymes. Environments that meet these criteria include waterlogged soils, sediments of lakes and rivers, and engineered water treatment systems.
Ecological Roles and Consequences
Denitrification has both advantageous and adverse effects on ecosystems. A primary benefit is its role in mitigating eutrophication in aquatic environments. Eutrophication, the enrichment of water with nutrients, often leads to excessive algal growth. When these algae die and decompose, they consume large amounts of dissolved oxygen, creating hypoxic “dead zones” where aquatic organisms cannot survive.
Denitrification removes excess nitrate from water by converting it into nitrogen gas that escapes to the atmosphere. This limits the nutrient source for algal blooms, preserving water quality and protecting aquatic life. It acts as a natural purification system against pollution from agricultural runoff and wastewater discharge.
Despite its benefits, denitrification has negative consequences in agriculture. Nitrogen is a component of fertilizers applied to enhance crop growth. In waterlogged, anaerobic soils, denitrifying bacteria convert this nitrate into nitrogen gas. This reduces crop productivity, leading to economic losses and the need for more fertilizer.
A complication arises when denitrification is incomplete. If conditions are not optimal, the pathway can stall, releasing nitrous oxide (N₂O) instead of dinitrogen gas. Nitrous oxide is a greenhouse gas with a global warming potential 300 times that of carbon dioxide. Agricultural soils are a major source of these emissions, contributing to climate change.
Human Applications and Management
Humans harness denitrification for environmental benefit while also developing strategies to limit its negative impacts. A prominent application is in wastewater treatment plants, where it is an engineered process to remove nitrogen. After nitrification converts ammonia to nitrate, the wastewater is moved to an anoxic tank. In this oxygen-free environment, bacteria convert the nitrates into nitrogen gas before discharge.
To enhance this process, a supplemental carbon source like methanol is sometimes added to the anoxic zone to fuel the microbes. A more nature-based approach uses constructed wetlands designed to replicate denitrification conditions. These systems use wetland vegetation, soils, and microbes to remove nitrates from stormwater or agricultural drainage.
In agriculture, the objective is the opposite: to inhibit or manage denitrification to prevent fertilizer loss. Management practices focus on altering the soil environment to make it less favorable for denitrifying microbes. This includes improving soil drainage to prevent waterlogging and anaerobic conditions. Timing fertilizer application to match the crop’s nutrient needs can also minimize the amount of nitrate in the soil available for loss.