The processes like formation and decay, growth and decomposition normally occur in nature. Various cycles are going on in nature as a result of these processes. Due to these cycles-plants, animals and their environment are linked to each other and a balance is established between them.

Carbon Cycle

Carbon is an essential element found in all living beings. Plants and animals get this element from carbon dioxide present in the atmosphere. Plants take carbon dioxide from the air and in the presence of water and sunlight, they make their food by the process of photosynthesis. Plants are consumed by other animals as food. In this way, they get compounds of carbon. Energy is produced by the digestion of these carbon compounds. Carnivores get carbon from the flesh of herbivores. Omnivores like man get carbon from both plants and animals. The carbon cycle shows that all animals absorb carbon dioxide directly from the atmosphere or the plants. Animals convert carbon into carbon dioxide during respiration or oxidation and again this carbon dioxide is released into the air. Carbon dioxide is also liberated during the decay of plants and animals and the burning of fuels which enters into the atmosphere and completes the carbon cycle. 

Nitrogen Cycle

In the nitrogen cycle, nitrogen circulates continuously through soil, water, air and living beings. All living beings need nitrogen. It is found in proteins and nucleic acid. Though 80% of air is nitrogen, most of the plants and animals cannot use it in a gaseous state. It is utilized by plants or animals in the form of several compounds like ammonia or amino acids. Many bacteria of soil like Azotobacter and clostridium convert the atmospheric nitrogen into nitrogenous compounds which can be absorbed by the plants. Many small nodules are present in the roots of leguminous plants which contain bacteria named Rhizobium leguminosarum. These bacteria convert atmospheric nitrogen into nitrates. Plants absorb nitrate salts through their roots and synthesize proteins in their cells. Nitrogen enters into the body of animals when they eat these plants. Bacteria and fungi decompose the dead bodies on earth. Nitrosomonas bacteria convert ammonia into nitrites and nitrobacteria convert nitrites into nitrates. In this way, plants absorb nitrates again. Some other bacteria like Thiobacillus denitrificans and micrococcus dinitrificans convert nitrates and ammonia of the soil into nitrogen gas. Apart from this, a fraction of nitrogen in the atmosphere changes in to nitric acid during lightning which forms nitrates with rainwater in soil that is absorbed by the plants. In this way, atmospheric nitrogen moves through many food chains and again comes into the air through a cycle. Though this process takes billions of years each atom of nitrogen comes back into the air.

Oxygen Cycle

Life is not possible without oxygen. About 20% of air is oxygen. All the living beings on earth need oxygen for respiration. Oxygen is essential for burning. You will think that the constant use of oxygen from the atmosphere will bring its end someday. Bu tin nature the oxygen cycle is constantly going on and no disturbance has occurred to the balance of this gas. This cycle like the nitrogen cycle is a continuous one. Green plants absorb carbon dioxide gas form the atmosphere of photosynthesis. In the daytime, all the trees and plants give out oxygen and take in carbon dioxide. At night, they give out carbon dioxide. This way, the quantity of oxygen in the atmosphere remains stable.

Hydrogen Cycle

Water is very important for all plants and animals. The hydrosphere gets its most abundant element, hydrogen, from water only. Water is formed by the combination of oxygen and hydrogen. During photosynthesis, hydrogen is produced by the dissociation of water which forms glucose after combining with carbon dioxide. Plants provide food for herbivores and these animals get glucose and proteins from plants only. Hydrogen forms carbohydrates which is an important source of energy for living beings. We take these carbohydrates in the form of food. These help our life cycle to continue.