What Is A Nitrogen Cycle?
The nitrogen cycle is an important biogeochemical cycle process in which nitrogen is converted into multiple chemical forms. These cycles naturally occur in the atmosphere, terrestrial and marine ecosystem. The conversion of nitrogen can be carried out through both biological and physical processes.
Nitrogen is essential to all life on earth because it is a key component of proteins and nucleic acids. The largest source of nitrogen can be found in its gas form (N2) in the atmosphere. Where the majority of earth’s atmosphere is made of 78% nitrogen, 21% oxygen and 1% argon and 0.04% carbon dioxide.
In general, there are five key processes in a nitrogen cycle:
Nitrogen Fixation – The process by which nitrogen (N2) is converted into ammonia (NH3) or ammonium (NH4+) by biological fixation or straight into nitrate (NO3-) through a physical process that requires a lot of energy. This is due to the stable state of (N2), which need a great deal of energy to break the bonds joining the 2 nitrogen atoms together. Interesting, between 5 to 10 billion kg of nitrogen in the atmosphere each year is fixed by lightning strikes. Atmospheric nitrogen must be fixed into unstable states (ions) before they can be absorbed by plants.
Ammonification – Happens when a plant or animal passes away or when an animal expels waste. The initial form of nitrogen is organic. Where bacteria or fungi convert the organic nitrogen back into ammonia (NH3). Note: ammonia becomes ammonium when reacts with Bronsted acids (proton donors). Given by chemical formula: (H+) + NH3 -> NH4+. When pH is low, the equilibrium shifts to the right, hence more ammonia molecules are converted into ammonium. If the pH is high (concentration of hydrogen ions is low), the equilibrium shifts to the left where hydroxide ion (OH−) abstracts a hydrogen ion (H+) from ammonium, generating ammonia.
Nitrification – The two-step process in which NH4+ get converted in NO3- for plants to absorb. The first step is to convert ammonium (NH4+) to nitrite (N02-) with bacteria such as the Nitrosomonas species. The second is to convert N02- into N03- which is done by other bacterial species such as Nitrobacter. Note: high quantities of ammonia and nitrite are toxic to fish, whereas nitrate is much less harmful.
Assimilation – This is a process by which plants and animals incorporate nitrogen in the forms of ammonium(NH4+) and nitrate(NO3-). Plants can absorb ammonium and nitrate from the soil by their root hairs and then incorporate them into plant proteins and nucleic acids. Animals utilise nitrogen from plant tissues.
Denitrification – Completes the nitrogen cycle by converting (NO3-) into the gas form N2 by anaerobic bacteria. This process only occurs when the oxygen level is very low.