After reading this essay you will learn about the nitrogen cycle.
Nitrogen atoms are constantly moving in a giant circle from the air, through the soil, into the bodies of plants and animals, and eventually back to the air. This whole process is called the nitrogen cycle. All living things need nitrogen to develop and grow.
Even though the earth’s atmosphere is made up of 78% nitrogen, plants and animals cannot use it in this form because the nitrogen atoms are too firmly bound together in molecules. So plants must draw their nitrogen from nitrogen compounds dissolved in the soil, and animals get their nitrogen by eating plants or by eating other animals that eat plants.
The nitrogen gets into the soil in a couple of different ways. A small quantity of the nitrogen found the soil by way of lightning. Lightning changes atmospheric nitrogen into nitrogen dioxide which is soluble in water. The nitrogen oxides dissolve in rainwater to form nitric acid which is absorbed by soil. The rest of the nitrogen in soil comes from bacteria. Bacteria are the only living things capable of getting nitrogen directly from the air. This is called “fixing”.
The process is started by certain kinds of bacteria in the soil that can extract nitrogen from the air. Then other bacteria convert the nitrogen into nitrogen compounds called nitrates, This process is called nitrification. Plants absorb the nitrates and turn them into more complex nitrogen compounds. Bacteria also help return nitrogen to the air. Bacteria in the soil decompose animal waste and the remains of dead animals and plants and produce ammonia. Nitrifying bacteria turn the ammonia into nitrates.
Other bacteria, called denitrifying bacteria, convert some of the nitrates back into nitrogen gas, which is released into the air. All these different steps form a massive cycle. The effect is that, over time, bacteria in the soil return almost the same amount of nitrogen to the air as other bacteria take from the air. This keeps the nitrogen content of the earth and its atmosphere in a perfect balance.
Unfortunately, humans are interfering with the natural balance when they overuse artificially produced nitrates as agricultural fertilizers. Before these nitrates can be converted into atmospheric nitrogen, they are often carried off from the soil by rain or irrigation. These dissolved nitrates are carried to streams and rivers and even seep down to groundwater. In some parts of the world, water for humans and animals contains such high concentrations of nitrates that it is unsafe for consumption.
These excessive amount of nitrates, when they reach rivers and lakes, cause too much algae to grow. This over-abundance of algae uses up too much of the oxygen in the water. When oxygen levels fall, other forms of life in the water die off. Organisms require nitrogen to produce amino acids.
Nitrogen makes up seventy-eight percent of the atmosphere, but most organisms cannot use this form of nitrogen, and must have the fixed form. The nitrogen cycle produces the fixed form of nitrogen these organisms need.
A special type of bacteria called nitrogen fixing bacteria take in atmospheric nitrogen and produce ammonia (NH3).
Other bacteria use this ammonia to produce nitrates and nitrites, which are nitrogen and oxygen containing compounds.
The nitrates and nitrites are used by plants to make amino acids which are then used to make plant proteins.
Plants are consumed by other organisms which use the plant amino acids to make their own.
Decomposers convert the nitrogen found in other organisms into ammonia and return it to the soil. A few of these types of bacteria return nitrogen to the atmosphere by a process called denitrification, however this amount is small.