What is Carbon?

Exactly what is carbon, and why does it play such an important role in our economy and environment?

First of all, carbon is an integral part of almost everything in our daily lives -- from propane to limestone rocks to the steel in your car to dandelion plants to the nose on your face and the air you breathe. Carbon exists in many forms. Among other things, it forms the backbone of DNA, floats through the air and water as carbon dioxide, and lays quietly for millions of years as underground fossil fuels and limestone and dolomite rock deposits.

Carbon is the sixth-most abundant element in the universe, after hydrogen, helium, oxygen, neon, and nitrogen. It is the product of the fusion of three helium nuclei.

In its elemental form, carbon (C) has an atomic number of 6 and an atomic weight of 12, which means it has 6 protons and 6 neutrons in its nucleus, surrounded by 6 electrons. Carbon cannot lose or gain electrons to form free ions, but it can form strong covalent bonds with many other elements. Carbon-hydrogen and carbon-oxygen bonds are especially stable in nature.

Carbon is also unique among the elements in the almost infinite capacity of its atoms to bond to each other in long chains, or in rings. This is due to the strength of bonds between carbon atoms. Carbon crystallizes in two forms, as diamonds and as graphite. When long pure chains of carbon-carbon atoms form in three dimensions, the result is the diamond. When pure carbon chains form in two dimensions, the result is graphite, which can be peeled apart easily in layers. Other long chains or rings of carbon atoms exist in combination with hydrogen, oxygen, and nitrogen atoms, and are called organic compounds.

In the gaseous state, carbon forms covalent bonds with oxygen (carbon dioxide and carbon monoxide), hydrogen (methane), and nitrogen (hydrogen cyanide), and other molecules. Carbon dioxide levels in the atmosphere have increased by 30 percent since the advent if the industrial revolution, from about 275 parts per million in the early 1700s to more than 365 parts per million today. This level is estimated to increase to 450-600 parts per million by 2100. The primary cause of this increase is increased combustion of fossil fuels. Deforestation and other changes in vegetation management and land uses have also contributed to the increase.

Carbon dioxide is also diffused within ocean waters. Once dissolved in seawater, carbon dioxide can be converted into carbonate or bicarbonate. Certain forms of sea life biologically fix bicarbonate with calcium to produce calcium carbonate. This substance is used to produce shells and other body parts by coral, clams, oysters, and other organisms. When these organisms die, the calcium carbonate materials sink to the ocean floor, where they are eventually altered into sedimentary rocks. This is where most of the carbon on earth exists.

Carbon dioxide is not a good oxidizing agent, and it does not support the combustion of carbon-based substances. Rather, it extinguishes the combustion of these substances and is often used in fire extinguishers. Carbon monoxide, however, is combustible.

All organic compounds, such as proteins, carbohydrates, and fats, contain carbon. All biological substances are based upon compounds in which carbon is combined with other elements. All plant and animal cells consist of carbon compounds and their polymers.

Within the crust of the Earth, carbon exists in many inorganic and organic forms. Carbonate minerals, calcium carbonate and magnesium carbonate, are among the most abundant and widely distributed minerals on the surface of the Earth. These minerals take the form of limestone, dolomite, and marble, among others. In terms of weight, however, oxygen and silicon together are about 2,500 times more prevalent in the Earth's surface rocks than carbon. Coral and the shells of oysters and clams are primarily calcium carbonate. In the organic form, carbon is widely distributed in the lithosphere as coal, oil, natural gas, and all plant and animal tissue.

What Is Soil Organic Carbon?

Carbon also exists in the Earth's crust as soil organic matter. Microorganisms in the soil convert decayin plant and animal tissue into soil organic matter. Soil organic carbon often is divided into three pools: active, intermediate or slow, and recalcitrant.

The active pool typically stores carbon anywhere from a few months to a few years. This pool typically accounts for less than 5 percent of the total soil carbon. The slow pool stores carbon for decades, and accounts for 20 to 40 percent of total soil carbon. The recalcitrant pool is extremely stable, and stores carbon for hundreds or thousands of years before decomposing. About 60 to 70 percent of total soil carbon is in the recalcitrant pool.