Colorado State University
VOLUME 22
ISSUE 2
All organic substances contain carbon (C). The C cycle, through which carbon dioxide from the atmosphere is converted to organic forms by plant photosynthesis and then returned to the atmosphere through respiration, is the basis for life on earth. Soil organic matter (SOM) contains three times as much C as is found in vegetation, on a worldwide scale. Therefore, soil organic matter plays a critical role in the global C balance and the greenhouse effect. In fact, when SOM is measured, it's actually soil organic carbon (SOC) that is measured, and then a conversion factor is used to calculate SOM.
What is the greenhouse effect? Radiation from the sun warms the earth, and the earth radiates some of that energy back into outer space. The energy radiated away from Earth has a longer wavelength than the incoming energy. This long-wavelength radiation is absorbed by gases in the air, resulting in increased temperatures. The heat-retaining process of atmospheric gases is referred to as the "greenhouse effect" Without this greenhouse effect, the average temperature of the Earth would be considerably colder (about 86o F colder) and would be inhospitable to humans. However, many scientists have documented increased global temperatures during the past century and are concerned about continuing global warming and its impacts on the Earth and human life.
The greenhouse effect is quite noticeable in the cooling of the Earth at night. When it's clear, the energy that the Earth absorbed during the day radiates to outer space, and nights are cold. When it's cloudy, the water vapor absorbs radiation, and nights are relatively warm.
The gases that absorb the long-wavelength radiation emitted by the earth are known as "greenhouse gases". They include water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (CFCs). The concentrations of these gases have been increasing steadily since the Industrial Revolution.
All three of the major gases of general concern for global warming, carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4), are important for agriculture. The overwhelming source of CO2 is fossil fuel use, with deforestation and biomass burning an important source in the tropics. Agriculture provides a potential sink for CO2, through building up soil organic matter stocks, which incorporate CO2 taken from the atmosphere by plants. This is called "carbon sequestration".
Within agriculture, methane is emitted by ruminant digestive processes and from livestock waste systems. Flooded rice cultivation is a major source of CH4 worldwide, but of less importance in the U.S. Reducing emissions is the most important component of mitigation of methane, but well-aerated soils also act as a sink (that is, take up and oxidize CH4). Soils are a major source of N2O emissions, which tend to increase with additions of nitrogen, whether from mineral fertilizers, legumes, or manure.
An important point to consider in the overall effect of greenhouse gases is the relative difference in global warming potential (GWP), which is a measure of the "heat-trapping" ability of the gas and its longevity in the atmosphere. Relative to CO2 (assigned a GWP of 1), N2O has about 300 times the effect of CO2, and CH4 has about 20 times the effect. Thus, while the concentration and flux rates of N2O and CH4 are much lower than for CO2, their effects are significant due to the characteristics of those gases with respect to global warming.
Our focus in this newsletter is on carbon, specifically CO2 and CH4. When organic matter decays in the presence of oxygen, CO2 is formed. But when there is a shortage of oxygen, C is released as CH4. We will evaluate the potential for reducing the emission of these greenhouse gases from agriculture and for increasing the sequestration of C in agricultural land. Many agricultural practices that reduce emission or increase sequestration of C have other favorable impacts. For example, many of these practices may also increase farmers' profit, conserve soil and water, or improve ecosystem health.
