2.3: Organic Matter And Natural Cycles

The Carbon Cycle

Soil organic matter plays a significant role 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 the primary cause of climate destabilization.

A simple version of the natural carbon cycle that leaves out industrial sources, 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. Carbon dioxide is also released to the atmosphere when fuels, such as gas, oil, coal and wood are burned.

Color and Organic Matter

In Illinois, a handheld chart has been developed to allow people to estimate percent of soil organic matter. Their darkest soils, almost black, indicate 3.5–7% organic matter. A dark brown soil indicates 2–3%, and a yellowish brown soil indicates 1.5–2.5% 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.) Recently, mobile apps have been developed that use smartphone cameras to estimate soil organic matter content and have proven to work quite well for rough estimates.

Soils are amassing the cumulative carbon and nutrient capture from plant production, and the largest amount of carbon present on the land is not in the living plants but is instead stored in soil organic matter. It has taken a while, but that understanding is now finding its way into 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, and in fact carbon stored in all the world’s soils is two to 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 through the rapid depletion of soil organic matter following conversion of forests to agricultural practices. There is as much carbon in seven inches of a 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, and it can be absorbed by the oceans and taken up by plants downwind during photosynthesis.)

Climate Change and Soils

Climate change is already having profound effects on the planet by warming seas, melting glaciers and sea ice, thawing frozen soil (permafrost), and increasing weather extremes: more heat waves, increasing intensity of rainfall in many places and more frequent dry conditions in other locations. As we write this, the last five years (2015, 2016, 2017, 2018 and 2019) have been the warmest since record keeping began in the 1880s. The 2018 and 2019 heat waves in North America, Europe, and southeast and eastern Asia, as well as during the following Australian summer (beginning in December 2018 and then again in their 2019–2020 summer, accompanied this time by historic wildfires), have been especially severe. July 2019 was the warmest month ever recorded. Farming has already been affected in many parts of the world, with increasing night temperatures lowering grain yields as more energy that plants produce during the day is used up by greater nighttime respiration, and with regional droughts causing crop failures.Gases such as carbon dioxide (CO₂), methane (CH₃) and nitrous oxide (N₂O) trap heat in the atmosphere, resulting in a warming Earth, the so-called greenhouse effect.

Atmospheric carbon dioxide concentrations increased from around 320 parts per million (ppm) in the mid 1960s to 415 ppm as we write these words, and it is increasing at the rate of about 2 to 3 ppm per year. The historical conversion of forests and grasslands to farming was responsible for a large transfer of carbon (from accelerated soil organic matter decomposition) into the atmosphere as CO₂. This agricultural conversion is second to the burning of fossil fuels as the largest contributor to increasing atmospheric CO₂ concentrations (remember, fossil fuels are derived from carbon stored in ancient plants). As forests are burned and soils are plowed in order to grow crops (enhancing the use of organic matter by soil organisms), CO₂ is emitted into the atmosphere.

But soils managed in ways that build up organic matter can become net sinks for carbon storage and can enhance their health at the same time. Increasing soil organic matter is no silver bullet for combating climate change, but it can help to slow the increase in CO₂ for a while if done on a massive scale all over the world. A number of non-governmental organizations in the United States, along with a number of international efforts, are encouraging farmers to increase soil organic matter levels in the form of payments for sequestering carbon. (Large-scale “geoengineering” schemes have been proposed to take CO₂ out of the atmosphere or to shoot particles into the atmosphere to reflect some of the incoming radiation from the sun. The costs and potentially negative side effects of such proposals have not been established. Thus, at present, drastically reducing fossil fuel use through switching to renewable energy sources and reducing total energy use is the only sure way we know to stop or reverse climate change.)Ecologically sound management of agricultural soils using practices that promote the buildup of organic matter certainly has a part to play in combating climate change. It offers win-win outcomes because higher levels of organic matter also increase resilience of soils that are being confronted with the more intense storms and dry periods resulting from a warming planet with increasingly destabilized weather patterns. Read further about the role of soil health in climate resilience in the SARE bulletin Cultivating Climate Resilience on Farms and Ranches (www.sare.org/climate-resilience).

The Nitrogen Cycle

Gains. 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. Both nitrate and ammonium can be used by plants, but most nitrogen used by plants is taken up in the nitrate form, with a small amount as ammonium. Small quantities of some sources of amino acids and small proteins can be absorbed. Figure 2.9 shows the nitrogen cycle and how soil organic matter enters into the cycle. 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. Every percent organic matter in a surface soil (to 6 inches deep) contains approximately 1,000 pounds of nitrogen. Each year bacteria and fungi convert some portion of the organic forms of nitrogen into ammonium, and different bacteria convert ammonium into nitrate. Depending on the soil organic matter levels, a typical crop may derive 20–50% of its nitrogen from mineralized organic matter.

Animal manures can also make large contributions to the plant-available nitrogen pool in the soil. They typically have high organic nitrogen contents that are made readily available when microorganisms convert organic forms to ammonium and nitrate. Most of the crop’s nitrogen demand can be met with manure on livestock farms where large amounts of it are generated.

In addition to decomposing organic matter and manure, nitrogen is also derived from some bacteria living in soils that can “fix” nitrogen, converting nitrogen gas to forms that other organisms, including crop plants, can use. These can be modest amounts of nitrogen in typical cereal crop systems but large quantities when growing a legume. Also, inorganic forms of nitrogen, like ammonium and nitrate, exist in the atmosphere naturally and are sometimes enhanced by air pollution. Rainfall and snow deposit these inorganic nitrogen forms on the soil, but generally in modest amounts relative to the needs of a typical crop. Inorganic nitrogen also may be added in the form of commercial nitrogen fertilizers, which for most cash grain crops (except legumes like soybeans) is generally the largest nitrogen addition. These fertilizers are derived from nitrogen gas in the atmosphere through an industrial fixation process that requires quite a lot of energy.

Figure 2.9. The role of organic matter in the nitrogen cycle. Illustration by Vic Kulihin.

Losses. Nitrogen can be lost from a soil in a number of ways. Soil conditions and agricultural practices govern the extent of loss and the way in which nitrogen is lost. When crops are removed from fields, nitrogen and other nutrients also are removed. When uncomposted manure or certain forms of nitrogen fertilizer are placed on the soil surface, gaseous losses (volatilization) may occur, which may cause losses of up to 30%. The nitrate (NO₃^–) form of nitrogen leaches readily from soils and may end up in groundwater at levels unsafe for drinking or may enter surface waters where it causes low-oxygen “dead zones.” Leaching losses are greatest in sandy soils and in soils with tile drainage. 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 to nitrous oxide (N₂O) gases in a process called denitrification, which can be a significant pathway of loss from soils that are saturated. Nitrous oxide (also a potent greenhouse gas) contributes strongly to climate change, and in fact is estimated to be the largest agricultural contribution to greenhouse gas emissions (more than carbon dioxide and methane). 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 to use nitrogen fertilizers and manures efficiently—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 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 oceans 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 and increase storage of water in soil. When we look at the increasing occurrence of major flooding in parts of the world, especially in the U.S. grain belt, we point to climate change. But surely this is worsened by the gradual degradation of regional soils that are mostly used for intensive crop production.

The water that has entered the soil 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 and store water, and high levels of runoff and erosion result. This means less water for plants and decreased groundwater recharge.

Note

It is very difficult, if not impossible, to come up with a meaningful monetary value for the worth of organic matter in our soils. It positively affects so many different properties that taking them all into account and figuring out their dollar value is an enormous task. One percent organic matter in the top 6 inches of an acre of soil contains about 1,000 pounds of nitrogen. At about 45 cents per pound, this alone is worth about $450 for every percent organic matter in your soil. Adding in the value of 100 pounds each of phosphorus, sulfur and potassium, the total comes to $500 per acre for every percent of organic matter. But we also need to consider other nutrients that are present and the beneficial effects that organic matter has on reducing other inputs and increasing yields. And what are the monetary benefits of reduced flooding, water pollution and climate change? We know it truly is an invaluable resource, but it is difficult to put an exact price on it.