|By: Ren Lim Tutor : Abofazyl Bayat
The Nitrogen Cycle
The Nitrogen Cycle is one of the most important nutrient cycles. Nitrogen is found in amino acids, proteins and nucleic acids all of which are important building blocks of life. Nitrogen is mostly found in the atmosphere as N2 molecules.
Despite its abundance in the atmosphere, nitrogen is often the most limiting nutrient for plant growth since most plants can only take up nitrogen in two solid forms: ammonium ion (NH4+ ) and the ion nitrate (NO3- ). Most plants obtain the nitrogen they need as inorganic nitrate from the soil solution .Ammonium is used but less by plants for uptake because in large concentrations it is extremely toxic. Whereas animals receive the required nitrogen they by the consumption of living or dead organic matter containing molecules composed partially of nitrogen.
When organic matter dies , their organic nitrogen is converted into inorganic forms and re-enter the nitrogen cycle via decomposition. And bacteria known as decomposers will then chemically modify the nitrogen found in organic matter from ammonia (NH3 ) to ammonium salts (NH4+) . This is carried out by a variety of bacteria and fungi and is called mineralization.
Besides that, ammonium salts can be absorbed onto the surfaces of clay particles in the soil and then released back to the soil to be altered later by certain bacteria to nitrite. Then this can be further converted to nitrate by another type of bacteria again.
The main source of nitrogen any terrestrial ecosystem is actually the athmosphere. In fact, a significant amount enter the soil in rainfall and through the effects of lightning. However, the majority is biochemically fixed within the soil by bacteria. Members of the bean family (legumes) can form mutualistic symbiotic relationships with nitrogen fixing organisms. In exchange for some nitrogen, the bacteria receive from the plants carbohydrates and special structures (nodules) in roots where they can exist in a moist environment. According to scientist, this accounts for the biological fixation globally adds approximately 140 million metric tons of nitrogen to ecosystems every year which solves the problem.
Nitrogen in the form of ammonium can be absorbed onto the surfaces of clay particles in the soil and then released by way of cation exchange. When released, most of the ammonium is often oxidized by a type of autotrophic bacteria into nitrite (NO2- ). Further oxidation by another type of bacteria converts the nitrite to nitrate (NO3- ). these are known as nitrification.
However, nitrate is very soluble and is easily lost from the soil system by leaching. Some of this leached nitrate flows until they reaches the oceans they are denitrified by heterotrophic bacteria. The process of denitrification involves the metabolic reduction of nitrate (NO3- ) into nitrogen (N2) or nitrous oxide (N2O) gas. These two gases then diffuse into the atmosphere.
Human activities such as tThe application of nitrogen fertilizers to crops, increased deposition of nitrogen from atmospheric sources because of fossil fuel combustion and forest burning, livestock ranching and sewage waste and septic tank leaching can also affect the cycle. All of these actually leads to nitrogen being leaked into the soil then to water sources and then flowing to through hydrological cycle and to be denitrified in the end.