There are two types of Biogeochemical Cycle: 1. The Gaseous Cycle 2. The Mineral Cycle.
Type # 1. Gaseous Cycle:
In the gaseous cycle, the main sources of nutrients are atmosphere and water bodies. In the sedimentary cycle, the main sources of nutrients are soil and rocks of the earth crust.
The gaseous cycles are:
(i) Water or Hydrological cycle,
(ii) The carbon cycle,
(iii) The oxygen cycle, and
(iv) The nitrogen cycle.
The carbon cycle is mainly the conversion of carbon dioxide to carbohydrates which are used as food for plants through the process of Photosynthesis. However, all the amount of carbohydrates is not necessary for plants.
The excess amount is released again to the atmosphere by the process of exhalation through leaves and roots during the night. The plants are eaten by herbivores or get decomposed. The herbivores again convert some of the carbohydrates into carbon dioxide which is released to atmosphere through respiration.
After the death of animals, they get decomposed and converted to carbon dioxide again and then returned to the atmosphere. Thus, the cycle is completed.
The oxygen cycle is a complex one. Oxygen is essential for survival of all the living beings including human being. It is the major byproduct of the process of photosynthesis. Oxygen occurs in numerous chemical forms and combinations. Decomposition of water molecules by sunlight during the photosynthesis produces much of the oxygen. It is released to atmosphere through transpiration and respiration processes of plants.
The nitrogen cycle includes the fixation of nitrogen and conversion to nitrate compounds. Our atmosphere is composed of around 79% of nitrogen. Normally, nitrogen is used only after it is fixed to form nitrate compounds. 90% of the fixed nitrogen is biological. It is formed by the actions of soil microorganisms, associated plant roots and also some bacteria. Nitrogen is directly used by soil bacteria and blue green algae.
Type # 2. Mineral Cycle:
The mineral cycle includes the utilization of mineral salts as critical nutrients to animal and plant life. The sources of minerals are inorganic such as phosphorous, sulphur, calcium and potassium. They are dissolved in water and soil in the form of salts. These salts are used by the animals and plants.
After death these salts are returned to the soil and water through the process of decomposition and transpiration or flow.
The mode of transfer of energy of food synthesised in plants through a series of organisms of an ecosystem with repeated eating and being eaten is known as food chain.
Each step of food chain is called a tropic level. If there are too many tigers and too few deer or other prey animals, the population of tigers will have to decline. Since, tigers do not eat grass; they simply would starve to death due to scarcity of their food supply. As their number declines, fewer deer will get preyed upon.
Consequently, the number of deer will slowly increase. Such mutual balance of population’s size can also be observed in the plants in certain aquatic habitats. Further, in the food chain, the number of organisms in one tropic level is higher than the number of organisms in the next higher tropic level. If the number at each level is plotted, it will take the shape of a pyramid. This is called Food Pyramid or Pyramid of Numbers.
The mixing or overlapping of several simple food chains constitutes Food Web. The food web is created mainly because of competition for food among various organisms.