The problem now is that the carbon cycle is lopsided. It took hundreds of millions of years to sequester carbon deep in the earth and under the ocean floor, and humans have released much of that carbon into the atmosphere during the last century. Humans tap into the geological carbon cycle by extracting oil and coal, which are both hydrocarbons, for use in automobiles and power plants. A byproduct of this combustion is CO2 gas. Since the Industrial Revolution began, carbon dioxide levels in the atmosphere have increased measurably, mostly as a result of human use of fossil fuels. Humans have also altered the biological carbon cycle, increasing atmospheric CO2 levels, through forest clearing and land use, reducing the number of plants that absorb it from the atmosphere. Thus the levels of atmospheric carbon are increased in two ways. First, during combustion, stored carbon is released directly into the air as CO2, and second, the clearing of land takes away a key mechanism for removing carbon dioxide from the atmosphere via photosynthesis.
Since carbon dioxide is a primary greenhouse gas, many scientists argue that the increase in atmospheric CO2 from human activities has resulted in an enhanced greenhouse effect and could result in corresponding changes in global climate, including higher global temperatures. The carbon that remains in the atmosphere absorbs heat, preventing it from radiating out into space. Too much heat, and the climate will change and become less habitable. The same applies to the oceans, where increased carbon changes the chemistry of sea water, making the oceans less habitable and killing sea life.
According to a 2007 report by the United Nations Intergovernmental Panel on Climate Change, “about 50 percent of a carbon dioxide increase will be removed from the atmosphere within 30 years, and a further 30 percent will be removed within a few centuries. The remaining 20 percent may stay in the atmosphere for many thousands of years.” The Earth and its atmosphere are a closed system, where carbon is neither created nor destroyed. The total amount of carbon does not change — carbon can be shuffled from pool to pool, from atmosphere to ocean, from soil to sediment, but cannot be added or removed. It has to go somewhere on Earth: taken up by plants, or dissolved back into oceans.
Nitrogen is both the most abundant element in the atmosphere and, as a building block of proteins and nucleic acids such as DNA, a crucially important component of all biological life. The nitrogen cycle is a complex biogeochemical cycle in which nitrogen is converted from its inert atmospheric molecular form (N2) into a form that is useful in biological processes.
The nitrogen cycle contains several stages:
Nitrogen fixation - Atmospheric nitrogen occurs primarily in an inert form (N2) that is converted to an organic - or fixed - form in a process called nitrogen fixation. Most atmospheric nitrogen is 'fixed' through biological processes. First, nitrogen is deposited from the atmosphere into soils and surface waters, mainly through precipitation. Once in the soils and surface waters, nitrogen undergoes a set of changes to form ammonia (NH4+). This is done by microorganisms that fall into three broad categories: bacteria living in symbiotic relationships with certain plants, free anaerobic bacteria, and algae. Crops, such as alfalfa and beans, are often planted in order to remedy the nitrogen-depletion in soils via symbiotic nitrogen-fixing bacteria.
Nitrification - Most of the nitrogen taken up by plants is converted by bacteria from ammonia - which is highly toxic to many organisms - into nitrite (NO2-), and then into nitrate (NO3-). This process is called nitrification, and these bacteria are known as nitrifying bacteria.
Assimilation - Nitrogen compounds in various forms, such as nitrate, nitrite, ammonia, and ammonium are taken up from soils by plants which are then used in the formation of plant and animal proteins.
Ammonification - When plants and animals die, or when animals emit wastes, the nitrogen in the organic matter reenters the soil where it is broken down by other microorganisms, known as decomposers. This decomposition produces ammonia which is then available for other biological processes.
Denitrification - Nitrogen makes its way back into the atmosphere through a process called denitrification, in which nitrate (NO3-) is converted back to gaseous nitrogen (N2). Denitrification occurs primarily in wet soils where the water makes it difficult for microorganisms to get oxygen. Under these conditions, certain organisms - known as denitrifiying bacteria - will process nitrate to gain oxygen, leaving free nitrogen gas as a byproduct.