Carbon creates life on earth. We are made of carbon, and our homes, our means of transportation, and our civilizations are built on carbon. All humans living on Earth needs carbon, but that need causes one of the most serious problems facing us today, which is global climate change. Changes that put carbon gases into the atmosphere result in warmer temperatures on Earth.
Over time, the carbon cycle will maintain a balance that prevents all of Earth's carbon from entering the atmosphere or from being stored entirely in rocks. This balance helps keep Earth's temperature stable like a thermostat. This means that the average temperature on Earth can vary time to time.
Reference: http://earthobservatory.nasa.gov/Features/CarbonCycle/
Nitrogen Cycle
Nitrogen is important to all life. Nitrogen in the atmosphere can go through many complex biological changes and chemical changes, be combined with nonliving and living material, and returns back to the air or soil in a continuing cycle.
Plants and animal wastes decompose, adding nitrogen to the soil. Plants use the nitrogen in the soil to grow. Bacteria in the soil convert those forms of nitrogen into forms plants can use. People and animals eat the plants, then plant and animal residues return nitrogen to the soil again, completing the cycle.
Reference: http://earth.rice.edu/mtpe/bio/biosphere/topics/energy/50_nitrogencycle.html
http://www.windows2universe.org/earth/Life/nitrogen_cycle.html
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