The process of denitrification is an integral part of nitrogen cycle occurring on the Earth since the time unknown. Nitrogen cycle starts with the biological nitrogen fixation and ends with denitrification.
It is a chemical process: Denitrification literally means removal of nitrate nitrogen. Factually, it is an anaerobic respiratory process which reduces oxidized nitrogen forms such as nitrate (NO3−), nitrite (NO2−), nitric oxide (NO) or nitrous oxide (N2O) into nitrogen gas (N 2). Here, various forms of oxidized nitrogen are electron acceptors while as organic matter (including Hydrogen) becomes electron donor. Electron donor would be oxidized in return of reduction of oxidized nitrogen. Thus in respiratory electron transport chain nitrate replaces oxygen as terminal electron acceptor. In nitrogen cycle, depending upon thermodynamic stability, the order of oxidation to reduction is NO3− →NO2 − →NO→ N2O→ N2. It is represented by redox equation: 2NO3− + 10e− + 12H + → N2↑+ 6H2O. The conversion from NO3− to N2 is enzymatically catalyzed stepwise reaction and mostly takes place in 2 pathways; the first is assimilatory and second is dissimilatory nitrate reduction. In assimilatory denitrification, nitrate is reduced to ammonia, which then serves as a nitrogen source for cell synthesis and is thus introduced into the cytoplasm. In dissimilatory denitrification, nitrate becomes electron and is converted to one of the gaseous byproduct, N2. Dissimilatory denitrification is the most efficient process for the complete removal of nitrate from the ecosystem. The rate of denitrification is dependent on many environmental parameters such as soil pH, soil type and humidity, presence of oxygen, drainage and temperature. Highly acidic soils have low N availability and low nitrification and hence denitrification rates; on the contrary, in the moist soils the rate of denitrification is the highest. Denitrification takes place in terrestrial, aquatic and marine ecosystems. Ocean floor, marshy wet lands, groundwater strata, oil reservoirs, stagnant lakes and sediments and paddy fields are ideal environments on the Earth where denitrification process is predominantly present.
Denitrifiers: Denitrification is carried out by denitrifying bacteria or denitrifiers which include heterotrophic facultative aerobic and strictly anaerobic bacteria and some species of extremophile genera. So anaerobic respiration of denitrifiers is denitrification wherein they prefer to respire nitrate instead of oxygen. The main denitrifiers' genera comprise Agrobacterium,Alcaligenes, Aeromonas,Bacillus, Citrobacter, Clostridium,Proteus, Paracoccus denitrificans , Pseudomonas,Thiobacillus and soil Actinomyces. Of these,Thiobacillus denitrificans is an autotrophic denitrifier; Paracoccus denitrificans and Pseudomonas can denitrify even in presence of oxygen. Some fungal species such as Fusarium, Aspergillus, Globobulimina pseudospinenscens, Straminipiles and Humicola are also peculiar denitrifiers from marine sediments and coastal areas as shown from the studies of Risgaard-Peterson et al and Sumathi et al.
In the absence of denitrification: In the absence of denitrification, nitrogen compounds may accumulate to toxic levels and prove detrimental especially to the growth of plants. Truthfully, nitrogen is the most growth limiting element present in the soil, under circumstances severely affecting crop productivity and yield. So removal of fixed soil nitrogen by denitrifiers is usually considered unfavorable but it has been proved that denitrification literally maintains homeostasis between soil and air nitrogen content. It is the unique mechanism by which nitrogen taken from the atmosphere is returned to the atmosphere to be refixed and thus completing cycling of nitrogen properly and which would be continued forever. Other important aspects of denitrification process are enlisted here for your reference:
- It is the vital step in the treatment of sewage and waste water which remove excess nitrogenous compounds.
- For the removal of excess nitrate from groundwater resources and agricultural lands where nitrate is accumulated from overuse of fertilizers.
- For the treatment of highly nitrified effluents which are released in water bodies like rivers, streams and lakes. Untreated effluents not only encourage algal blooms but also enter into drinking water ports which have been known to be one of the reasons of infantile methemoglobinemia.
- One negative facet of denitrification is that it is responsible for emission of nitrous oxide, one of the greenhouse gases and hence causative agent of greenhouse effect and global warming.
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