Natural cycles such as the water cycle, carbon cycle and nitrogen cycle help in maintaining the balance in nature. If water, carbon and nitrogen are not recycled, all living things will die once these substances are used up.
The water cycle
The carbon cycle
The carbon cycle is the process in which carbon atoms are recycled over and over again on Earth. Carbon recycling takes place within Earth’s biosphere and between living things and the nonliving environment. Since a continual supply of carbon is essential for all living organisms, the carbon cycle is the name given to the different processes that move carbon from one to another. The complete cycle is made up of “sources” that put carbon back into the environment and “sinks” that absorb and store carbon.
If a diagram were drawn showing the different processes that move carbon from one form to another, its main processes would be photosynthesis, respiration, decomposition, natural weathering of rocks, and the combustion of fossil fuels.
Carbon dioxide is a greenhouse gas and traps heat in the atmosphere. Without it and other greenhouse gases, Earth would be a frozen world. But humans have burned so much fuel that there is about 30% more carbon dioxide in the air today than there was about 150 years ago, and Earth is becoming a warmer place. In fact, ice cores show us that there is now more carbon dioxide in the atmosphere than there has been in the last 420,000 years.
The nitrogen cycle
Nitrogen Cycle, the series of natural processes by which certain nitrogen-containing substances from air and soil are made useful to living things, are used by them, and are returned to the air and soil. All living things must have nitrogen to build proteins. Because of the chemical nature of nitrogen gas, however, they cannot obtain that element directly from the air. Instead, food-making organisms such as plants obtain it from the soil by absorbing nitrates (various nitrogen compounds containing oxygen) and ammonium compounds (various nitrogen compounds containing hydrogen). The nitrogen cycle is essential to plants in unfertilized soils because in such soils the nitrogen compounds are not available to the plants in any other way.
Animals, and other living things that do not make their food, depend on the nitrogen cycle indirectly. Most animals, for example, eat plants or eat plant-eating animals.
The nitrogen cycle consists of four natural processes: nitrogen fixation, nitrification, denitrification, and decay.
is the process in which nitrogen gas from the air is continuously made into nitrogen compounds. These compounds (primarily nitrates and ammonium compounds) are made by nitrogen-fixing microorganisms in the soil and by lightning.
is the process in which ammonia in the soil is converted to nitrates. Nitrification is performed by nitrifying bacteria. Plants absorb the nitrates and use them to make proteins.
is the reverse of the combined processes of nitrogen fixation and nitrification. It is the process by which nitrogen compounds, through the action of certain bacteria, give up nitrogen gas that then becomes part of the atmosphere. The amount of gas released by this process is relatively small.
are those by which the organic nitrogen compounds of dead organisms and waste material are returned to the soil. These compounds are chiefly proteins and urea. The many bacteria and fungi causing decay convert them to ammonia and ammonium compounds in the soil.
Thus, through the nitrogen cycle, food-making organisms obtain the necessary nitrogen through nitrogen fixation and (to a greater extent) through nitrification. At the same time, nitrogen compounds are returned to the soil through decay and nitrogen is returned to the air through denitrification.
In soils in which many plants are raised and few are left to decay (as in farm soils), the nitrogen cycle does not supply enough nitrogen to support plant growth. In these soils natural or artificial fertilizers, containing nitrates or ammonium compounds, are needed.