Soil and Climate Change
Climate Change Indicators: Greenhouse Gases
The most significant anthropogenic greenhouse gases (GHGs) are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). About 81 percent of U.S. greenhouse gas emissions are carbon dioxide, 10 percent are methane, and 7 percent are nitrous oxide. Although these gases are found naturally in the atmosphere, it is their accelerated increase in concentration due to human activities, most notably burning fossil fuels, that is of concern. Greenhouse gases capture heat and energy in the atmosphere and alter long-term climate cycles. This phenomenon is called the greenhouse effect. The Earth’s greenhouse effect is, in fact, a natural phenomenon that helps regulate the temperature of the planet. When the sun heats the Earth, some of this heat escapes back into space. The rest of the heat, also known as infrared radiation, is trapped in the atmosphere by clouds and GHGs, such as carbon dioxide. If GHGs did not exist, the planet would be approximately 60 degrees F colder than it is today.
Carbon Dioxide
Plants obtain carbon dioxide (CO2) from the atmosphere and fix and convert carbon to biomass by photosynthesis. Decaying biomass subsequently and slowly accumulates in soils as organic matter. Meanwhile, in soil, root respiration and decomposition of organic matter (i.e., microbial respiration) return some carbon to the atmosphere as carbon dioxide (or as methane under anaerobic conditions) and retain a part of organic matter in soil.
Methane
Methane (CH4) is another part of the carbon cycle associated with soils. Agriculture accounts for less than half of the annual global anthropogenic emissions of methane—the main anthropogenic source of soil-derived methane is rice when the rice fields are flooded due to anaerobic conditions. Mineral soils under forests and other natural vegetation act as the strongest methane sink, followed by grasslands, with the sink strength weakest in cultivated soils and those receiving nitrogen fertilizer; as such, as cropland has expanded, the methane sink strength of soils globally will decline.
Nitrous Oxide
Nitrous oxide occurs through activities such as fertilizer application and other agricultural practices that increase nitrogen availability in the soil. Soil nitrous oxide is formed through microbial transformations of soluble nitrogen, primarily denitrification (reduction of nitrate, NO3¯), and some during nitrification (oxidation of ammonium, NH4⁺ to nitrite NO2¯ and then NO3¯). Soil nitrous oxide emissions increase sharply when abundant readily decomposable organic carbon (manure, green manure) and high levels of soluble nitrogen fertilizers (NH4⁺ and NO3¯) occur together.
Fluorinated Gases
Hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride are synthetic, powerful greenhouse gases that are emitted from a variety of household, commercial, and industrial applications and processes. Fluorinated gases (especially hydrofluorocarbons) are sometimes used as substitutes for stratospheric ozone-depleting substances (e.g., chlorofluorocarbons, hydrochlorofluorocarbons, and halons). Fluorinated gases are typically emitted in smaller quantities than other greenhouse gases, but they are potent greenhouse gases.
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