carbon & nitrogen cycle

8/7/2019 Carbon & nitrogen cycle

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Carbon Cycle

Carbon Cycle, the series of natural processes by which carbon in the air is made

available to living things, is used by them, and is then returned to the air. Such

food-making organisms as plants and algae need carbon to form carbohydrates,

which are essential for growth. They absorb carbon dioxide from the air; through

photosynthesis, the carbon dioxide is combined with water to form

carbohydrates. Other living things, such as animals, need carbohydrates for

energy, but, unlike plants, cannot manufacture their own. Herbivores obtain

carbohydrates by eating green plants and metabolize (chemically break down) the

carbohydrates into useful substances. Carnivores, in turn, obtain these useful

substances by eating herbivores. Carbon dioxide is released back into the

atmosphere when the animals breathe. Small amounts of carbon dioxide are

released into the air by the decomposition of dead organisms by the action of

certain bacteria and fungi. The burning of fossil fuels, such as coal, also releases

carbon dioxide into the atmosphere.

Diagram of the carbon cycle. The black numbers indicate how much carbon is

stored in various reservoirs, in billions of tons ("GtC" stands for GigaTons of

Carbon and figures are circa 2004). The dark blue numbers indicate how much

carbon moves between reservoirs each year. The sediments, as defined in this

diagram, do not include the ~70 million GtC of carbonate rock and kerogen.



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 organismssuch 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.

Nitrogen Fixationis 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.

Nitrification

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.

Denitrification

is the reverse of the combined processes of nitrogen fixation and nitrification. It isthe 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.

Decay Processes

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.

carbon & nitrogen cycle

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