The dark brown to black color we often associate with healthy soil is due to the presence of highly decomposed organic matter, or ‘humus’. (Not to be confused with hummus—the delicious dish made with chickpeas, tahini, olive oil, and garlic.) Soils rich in humus tend to have lower density, higher water availability, greater fertility, better tilth, and more biological diversity. Organic residues from decaying plants (and animals) are slowly transformed from identifiable leaves and roots to complex organic molecules through the process of humification. And because humus is relatively stable once it is formed, managing your soil to increasing the humus content does more than just improve agriculture; it can also promote carbon sequestration to help mitigate climate change.
The amount of carbon in soil is both an intriguing and critical question worth exploring. Soil scientist with the USDA-NRCS, state and local governments, and land-grant universities have spent over 100 years creating an inventory of the soils of the United States. Part of this cooperative soil survey program has been to understand the story of carbon stocks, which means the total amount of carbon in soils.
Soils affect and are affected by climate conditions, with local and global implications. Soils are the product of the climate under which they form, such that the properties of the soil can be used to interpret both present and past climate histories. More importantly, our management of forests, wetlands, and agricultural lands influences whether soils are sources or sinks for important greenhouse gases (carbon dioxide, methane, nitrous oxide, water vapor). This video from the Soil Science Society of America (SSSA) highlights just a few examples of how soils and climate systems interact and the possible consequences for human populations.
Carbon is constantly being cycled through the atmosphere, living organisms (plants, animals, bacteria, fungi), and soil. Within these terrestrial ecosystems, most of the stored carbon is in the soil. However, our decisions about land use and soil management affect whether soils capture and store carbon or whether they lose carbon and add greenhouse gases to the atmosphere. This video from the Food and Agriculture Organization of the United Nations describes the critical role of soils in our efforts to mitigate and adapt to climate change.
We recognize that soils are the largest terrestrial carbon pool; however, soil carbon is not distributed evenly around the globe. In fact, cooler and wetter soils at northern latitudes are where much of the soil organic carbon is stored, with a significant quantity locked up in the soils and permafrost of arctic regions. Some recent estimates suggest that over half of the global soil carbon pool is found in these sensitive landscapes. And as our changing climate leads to the loss of permafrost, a portion of the carbon locked up in the ice is being released—adding more greenhouse gases to the atmosphere.
While all soils can help clean and capture water, some parts of the landscape perform these and many other functions because they are saturated with water for part or all of the year. While these areas may have many names—swamp, marsh, bog, fen—these ‘wetlands’ have particular value because of the ecosystem services they provide, including floodwater storage, wildlife habitat, sediment trapping, water quality enhancement, carbon storage, and commercial products. A wetland has unique hydrology, and the frequent and prolonged saturated conditions promote unique soils and unique plant and animal communities adapted to these wet environments.
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Our soil resources are under threat, and climate change is only one of many factors causing soil and land degradation around the world. Understanding how and why soils can become degraded and, more importantly, the societal consequences of that degradation can lead us to the sustainable soil management practices that can halt and reverse the damage to our global soil resources.
When land in areas with relatively dry climates become even more moisture deficient, there is an increased risk to plants, wildlife, and soils. When this happens, either through climate change or human land management, it is known as ‘desertification’. Approximately half of the Earth’s land area and one third of the global population are in arid to semi-arid regions and are, consequently, threatened by the possibility of land and soil degradation through desertification. This post from the Soil Science Society of America’s Soils Matter blog discusses the topic of how soils are being degraded by desertification and what we can do to prevent it:
The organisms living in the soil produce carbon dioxide. As a result, the air in the soil may contain several hundred times more carbon dioxide than in the atmosphere we breathe. (Also, the relative humidity of soil air is typically much higher than the atmosphere, and is often near 100%.)
While soils are the largest terrestrial carbon pool, our choices regarding land management influence whether our soils will be able to store more (or less) carbon in the future. Agricultural land can contribute to carbon sequestration goals and help with climate change mitigation. For example, adding compost, planting cover crops/green manure crops, adopting crop rotations, reducing tillage operations, and converting to perennial forage crops or agroforestry are techniques that promote sequestering carbon in soils of croplands, grazing lands, and rangelands—offering agriculture's highest potential for climate change mitigation.
Soil degradation caused by climate change is a threat to our ability to feed the growing global population. However, there are simple actions that we can take to increasing our soil’s resiliency to climate change AND reducing the amount of greenhouse gases in the atmosphere.
How? By increasing the amount of carbon stored in the soil.
Why? Soils that are richer in carbon are better able to withstand erosion, contain more nutrients for plant growth, and retain more plant available water.
The response? The French government is launching the “4 per 1000” Initiative, which encourages the adoption of sustainable yet practical farming methods that will lead to slow but steady (0.4% per year) increases in soil organic carbon. This amount is significant because such an annual growth rate in global soil carbon would make it possible to stop the present increase in atmospheric carbon dioxide.
Learn more about the “4 per 1000” Initiative here: