The Nitrogen Cycle (N Cycle)

Nitrogen is crucial for all organisms because it is necessary for proteins including DNA, the carrier of genetic information. Nitrogen is the atmosphere’s most abundant element, with chemically unreactive gas (N₂), making up 78% of the volume of the troposphere. Nitrogen is one of the most important and most complex global cycles.

N cycle is the sequence of chemical and biological changes undergone by N as it moves from the atmosphere into water, soil, and living organisms, and the eventual release of this N back into the atmospheric N (Fig. 2).

                                          Fig. 2: The Nitrogen Cycle

The nitrogen cycle consists of five major steps, in which nitrogen cycles between the abiotic environment and organisms: nitrogen fixation, nitrification, assimilation, ammonification, and denitrification. Bacteria are exclusively involved in all of these steps except assimilation.

Two types of nitrogen fixation: non-symbiotic and symbiotic. In nonsymbiotic fixation, specialized bacteria (Azotobacter & Clostridium) convert gaseous nitrogen (N₂) to ammonia or ammonium (NH₄) that can be used by plants. In symbiotic fixation, Rhizobium bacteria (living in small nodules on plant roots of legumes such as beans or peas) fix nitrogen from the atmosphere and give to their host plant and the bacteria receive carbohydrate from the plant. 

Nitrification is a two-step process. It is the conversion of ammonium to nitrate. Most of the ammonium in soil (left after taken up by plants)  is converted by special nitrifying aerobic bacteria (Nitrosomonas) to nitrite ions, NO₂^-, and then (Nitrobacter) to nitrate ions, NO₃^-,which are easily taken up by plants as nutrient.

In assimilation, plant roots absorb nitrate, or ammonium, and incorporate the nitrogen of these molecules into plant proteins and nucleic acids. When animals consume plant tissues, they also assimilate nitrogen by taking in plant nitrogen compounds (amino acids) and converting them to animal compounds (proteins).

The conversion of biological nitrogen compounds into ammonia and ammonium ions is known as ammonification. The ammonia produced by ammonification enters the nitrogen cycle and is once again available for the process of nitrification and assimilation.

Denitrification: The reduction of nitrate (NO₃^-) to gaseous nitrogen (N₂) by denitrifying bacteria (mostly anaerobic) and returning nitrogen to the atmosphere through a series of  reactions. The products of denitrification are then again available for the cycle to begin.