Soil organic matter in the carbon cycle

The Carbon Cycle

Soil organic matter plays a significant part in a number of global cycles. People have become more interested in the carbon cycle because the buildup of carbon dioxide in the atmosphere is thought to cause global warming. Carbon dioxide is also released to the atmosphere when fuels, such as gas, oil, and wood, are burned. A simple version of the natural carbon cycle, showing the role of soil organic matter, is given in figure 2.8. Carbon dioxide is removed from the atmosphere by plants and used to make all the organic molecules necessary for life. Sunlight provides plants with the energy they need to carry out this process. Plants, as well as the animals feeding on plants, release carbon dioxide back into the atmosphere as they use organic molecules for energy.


In Illinois, a hand-held chart has been developed to allow people to estimate percent of soil organic matter. Their darkest soils—almost black—indicate from 3.5 to 7% organic matter. A dark brown soil indicates 2 to 3%, and a yellowish brown soil indicates 1.5 to 2.5% organic matter. (Color may not be as clearly related to organic matter in all regions, because the amount of clay and the types of minerals also influence soil color.)

The largest amount of carbon present on the land is not in the living plants, but in soil organic matter. That is rarely mentioned in discussions of the carbon cycle. More carbon is stored in soils than in all plants, all animals, and the atmosphere combined. Soil organic matter contains an estimated four times as much carbon as living plants. In fact, carbon stored in all the world’s soils is over three times the amount in the atmosphere. As soil organic matter is depleted, it becomes a source of carbon dioxide for the atmosphere. Also, when forests are cleared and burned, a large amount of carbon dioxide is released. A secondary, often larger, flush of carbon dioxide is emitted from soil from the rapid depletion of soil organic matter following conversion of forests to agricultural practices. There is as much carbon in six inches of soil with 1% organic matter as there is in the atmosphere above a field.

If organic matter decreases from 3% to 2%, the amount of carbon dioxide in the atmosphere could double. (Of course, wind and diffusion move the carbon dioxide to other parts of the globe.)

The Nitrogen Cycle

Organic matter in the nitrogen cycle

Another important global process in which organic matter plays a major role is the nitrogen cycle. It is of direct importance in agriculture, because there is frequently not enough available nitrogen in soils for plants to grow their best. Figure 2.9 shows the nitrogen cycle and how soil organic matter enters into the cycle. Some bacteria living in soils are able to “fix” nitrogen, converting nitrogen gas to forms that other organisms, including crop plants, can use. Inorganic forms of nitrogen, like ammonium and nitrate, exist in the atmosphere naturally, although air pollution causes higher amounts than normal. Rainfall and snow deposit inorganic nitrogen forms on the soil. Inorganic nitrogen also may be added in the form of commercial nitrogen fertilizers. These fertilizers are derived from nitrogen gas in the atmosphere through an industrial fixation process.

Almost all of the nitrogen in soils exists as part of the organic matter, in forms that plants are not able to use as their main nitrogen source. Bacteria and fungi convert the organic forms of nitrogen into ammonium, and different bacteria convert ammonium into nitrate. Both nitrate and ammonium can be used by plants.

Nitrogen can be lost from a soil in a number of ways. When crops are removed from fields, nitrogen and other nutrients also are removed. The nitrate (NO3) form of nitrogen leaches readily from soils and may end up in groundwater at higher concentrations than may be safe for drinking. Organic forms of nitrate as well as nitrate and ammonium (NH4+) may be lost by runoff water and erosion. Once freed from soil organic matter, nitrogen may be converted to forms that end up back in the atmosphere. Bacteria convert nitrate to nitrogen (N2) and nitrous oxide (N2O) gases in a process called denitrification, which occurs in saturated soils. Nitrous oxide (also a “greenhouse gas”) contributes strongly to global warming. In addition, when it reaches the upper atmosphere, it decreases ozone levels that protect the earth’s surface from the harmful effects of ultraviolet (UV) radiation. So if you needed another reason not to apply excessive rates of nitrogen fertilizers or manures— in addition to the economic costs and the pollution of ground and surface waters—the possible formation of nitrous oxide should make you cautious.

The Water Cycle

Organic matter plays an important part in the local, regional, and global water cycles due to its role in promoting water infiltration into soils and storage within the soil. The water cycle is also referred to as the hydrologic cycle. Water evaporates from the soil surface and from living plant leaves as well as from the ocean and lakes. Water then returns to the earth, usually far from where it evaporated, as rain and snow. Soils high in organic matter, with excellent tilth, enhance the rapid infiltration of rainwater into the soil. This water may be available for plants to use or it may percolate deep into the subsoil and help to recharge the groundwater supply. Since groundwater is commonly used as a drinking water source for homes and for irrigation, recharging groundwater is important. When the soil’s organic matter level is depleted, it is less able to accept water, and high levels of runoff and erosion result. This means less water for plants and decreased groundwater recharge.