Denitrification in the Nitrogen Cycle

What is Denitrification?

It is a microbially mediated process where nitrate is used in the form of energy for denitrifiers, i.e. soil bacteria, which in turn, they reduce it in various forms of gaseous emissions. In this process, nitrate is lifted over by plant uptake and denitrified when soil conditions become appropriate. After heavy rainfall or high irrigation, soil is compacted and it makes it vulnerable to the denitrification process. During the process, the microbes use nitrate in the place of oxygen to get the energy, grow and multiply, along their pathway they secrete various enzymes to breakdown the nitrate and various gaseous forms of Nitrogen in this stage along with reductive pathway before nitrate is fully reduced to dinitrogen. One of the gaseous forms of Nitrogen is Nitrous Oxide.

In other words, we can say denitrification allows reduction of nitrate (NO3−) to produce molecular Nitrogen (N₂). Nitrogen is utilized by microbes, plants and animals to live. The denitrification process is one of the processes (last step) of the Nitrogen cycle where Nitrogen is recycled and it can be represented as following:

Nitrite  →  Nitric Oxide  →   Nitrous oxide  →  Nitrogen gas.

Here, we can see the reduction of soil nitrate to nitrogen-containing gases such as nitric oxide, nitrous oxide and then nitrogen gas.

This process is facilitated by microbes and a series of intermediate gaseous nitrogen oxide products are released during the process. Let’s understand it a little bit more in the following points:

• Facultative anaerobes perform denitrification as the part of respiration where they reduce oxidized form of Nitrogen in response to the electron donor oxidation, for example organic matter oxidation. 

• Nitrogen electron acceptors include nitrate, nitrite, nitric oxide and nitrous oxide and these finally result in the production of dinitrogen, thus completing the nitrogen cycle. 

• Denitrifying microbes need a very low amount of oxygen (less than 10 percent) and organic Carbon for energy.

• Heterotrophic bacteria like Paracoccus denitrificans and pseudomonas, and certain autotrophic denitrifiers like Thiobacillus denitrificans perform the denitrification process. Several species of bacteria involved in denitrification undergo complete reduction of nitrate to dinitrogen and over one enzymatic pathway is involved in the reduction process.  

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Half Reactions in Denitrification

Denitrification generally proceeds through some combination of the following half reactions are involved to proceed Denitrification, with enzymes involved which are mentioned in bracket below:

• NO3− + 2 H+ + 2 e−→ NO₂− + H2O (Enzyme involved: Nitrate reductase)

• NO₂− + 2 H+ + e− → NO + H2O (Enzyme involved: Nitrite reductase)

• 2 NO + 2 H+ + 2 e− → N₂O + H2O (Enzyme involved: Nitric oxide reductase)

• N₂O + 2 H+ + 2 e− → N₂ + H2O (Enzyme involved: Nitrous oxide reductase)

A net balanced redox reaction can be represented as follows where nitrate (NO3−) gets fully reduced to dinitrogen (N2):

• 2 NO3− + 10 e− + 12 H+ → N2 + 6 H2O

Favourable Conditions for Denitrification

• Naturally, denitrification takes place in marine as well as terrestrial ecosystems. 

• It also occurs in anoxic environments, ie. place where freely or dissolved oxygen is depleted. Here, nitrate (NO3−) and Nitrite (NO2−) are used as a substitute for a terminal electron acceptor in place of oxygen which are more energetically favourable e- acceptors. Anoxic environment may include groundwater, wetlands, soils, oil reservoirs and poorly aerated corners of the oceans and seafloor sediments. 

• Another environment where it can occur is oxic zones like  intertidal zones where fluctuations of oxygen concentration in coastal sediments occur due to tidal cycles.