Underneath our feet is an undervalued and underestimated capacity for carbon storage: soil organic carbon. Understanding the Earth’s processes and implementing proper land use management will provide a key component in mitigating climate change.
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What Is Soil Organic Carbon?
Soil organic carbon (SOC) is a component of Earth’s biosphere in the soil. More specifically, the term refers to the measurable amount of soil organic matter (SOM) in the top zero to 10 centimetres of soil. SOM is composed of microbes (bacteria & fungi), decaying plant and animal material, faecal matter, and decomposition products. SOC is the product of carbon dioxide being stored in the soil through photosynthesis, primarily through plants.
In addition to the soil providing homes to small and large organisms and helping plants grow, it is an important facet of halting global warming. Soil is crucial in mitigating changes in our climate as it holds more carbon than land vegetation and the atmosphere combined. To fully understand how soil stores carbon, we break down how each facet works.
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The Process
Increasing carbon storage in the soil from the atmosphere is dependent on photosynthesis, decomposition, and respiration in ecosystem processes. During photosynthesis, plants intake carbon dioxide from the atmosphere to convert into energy to grow. This is the predominant process in which carbon is stored in the soil as SOC. During decomposition, biomass, a byproduct of plants and animals, is broken down by soil microbes. During photosynthesis and decomposition, plant and microbes respire. Despite carbon dioxide being emitted during the respiration process, the amount is lower than that being stored.
The Composition
SOC is made up of organic matter whose composition is dependent on soil type, climate as well as land and soil management. These variables can help lead to carbon being sequestered and take a natural approach to mitigating climate change.
Soil type, the makeup of soil consisting of sand, clay, and silt, influences how much organic matter is in the soil. Clay in soil acts as a blanket for the organic matter found in it, limiting the amount of decomposition and respiration from microbes and other organisms from breaking it down. Sandy soils, on the contrary, do not protect the organic matter in the soil from being broken down by microbes and other organisms.
In addition to soil type, climate also influences SOC. Climate variables, precipitation and temperature influence both the moisture availability in the soil (precipitation dependant) and the rate at which organic matter decomposes in the soil (temperature dependant). The right amount of precipitation acts as a transport system for nutrients to be carried from the soil into the plant to help it grow. And the greater the plant growth, the greater the amount of SOM.
Same goes for temperature. Any decrease results in organic matter decomposing at a slower rate. For example, a study in Western Australia found that “under moist conditions, each 10C increase in temperatures doubles the rate of organic matter decomposition.”
One last factor influencing SOC levels is soil and land management. As the first 10 centimetres of soil represents a large proportion of SOM, proper land and soil management are imperative. These can influence the amount of organic matter in the soil by putting mitigative measures in to prevent soil erosion, adding manure and/or straw to increase the SOM content, and understanding the quantity of water crops require to prevent runoff.
SOC Influence on Climate
Agricultural land covers over one-third of the global land area. Such large proportion of global land area has the potential, with proper land and soil management, to enhance SOC which increases the capacity to sequester carbon dioxide from the atmosphere and help reduce anthropogenic influence on climate. Unfortunately, current agricultural practices are deteriorating soil health.
Although current agricultural practices are deteriorating soil health, actions can be taken to mitigate these effects, increase SOC, and enhance carbon sequestration. For example, this can be achieved through less intensive tilling, cover crops, and perennial crops.
Intensive tilling of soils decreases the amount of SOC being stored as it exposes the soil to microbial decomposition, resulting in an increased amount of carbon being released back into the atmosphere. Cover crops, such as peas and clover, are planted after the main crop harvest to help increase the uptake of carbon into the soil. Perennial crops, which are present during all seasons of the year, are able to establish deep root systems. Deeper root systems and year-round growth gives these crops an advantage to store more carbon in the soil than annual crops.
Conclusion
Understanding the potential of the natural world and implementing proper land and soil management practices will be a key component in mitigating climate change.
In particular, applying less intensive tilling of soils, planting cover crops and perennial crops, and adopting other such agricultural practices will help increase the amount of SOC, which is essential to improve soil health, reduce carbon dioxide in the atmosphere, and improve food security.
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