Terrestrial Sequestration of Carbon Dioxide
Terrestrial sequestration involves the collection and storage of carbon dioxide by plants and the storage of carbon in soil.
Natural Terrestrial Sequestration
Terrestrial sequestration involves the capture and storage of carbon dioxide by plants and the storage of carbon in soil. During photosynthesis, carbon from atmospheric carbon dioxide is transformed into components necessary for plants to live and grow. As part of this process, the carbon present in the atmosphere as carbon dioxide becomes part of the plant: a leaf, stem, root, etc. Long-lived plants like trees might keep the carbon sequestered for a long period of time. Once the tree dies, or as limbs, leaves, seeds, or blossoms drop from the tree, the plant material decomposes and the carbon is released. The figure to the right depicts natural terrestrial sequestration.
As part of the natural process, some of the carbon is released into the atmosphere, but some carbon is captured within the soil and increases the soil's organic matter content. Soil organic matter consists of the living mass of microorganisms in soil and the decomposed residues like humus. This soil organic carbon component has been calculated to be about twice the amount of carbon present in the atmosphere, and about 2.5 times the amount of carbon present in the plants living on the soil. As part of the carbon cycle, the carbon present in the decomposing plant material and present within the soil is retained in the soil, or is consumed by soil organisms. Soil organisms respire carbon dioxide into the soil, which then diffuses into the atmosphere.
Preservation or Promotion of Terrestrial Sequestration
While terrestrial sequestration of carbon is a natural process that is always occurring, human actions to preserve the natural carbon sequestration capacity of land and to promote increased terrestrial sequestration are important tools to combat global warming.
Preserving Terrestrial Sequestration Capacity
Terrestrial sequestration can be thought of as the preservation of the carbon sequestration capacity associated with the land management practices of an existing plot of land. For example, terrestrial sequestration may be occurring on established timber or tallgrass prairie acreage. Because carbon is released when land is converted from natural vegetation to cropland, by preventing the switch to cropland, a land manager would preserve the terrestrial carbon sequestration capacity of the land.
Promoting Terrestrial Sequestration
Terrestrial sequestration can also be thought of as the promotion of carbon sequestration on land assets. Examples of ways to promote terrestrial carbon sequestration include: forest management, reforestation, and afforestation. Forest management involves the management of existing forests to maximize growth. Biomass fuel can sometimes result from timber thinning to promote growth, activities taken to reduce fire risk, and the removal of diseased timber. Reforestation involves planting trees on lands that have already been harvested for timber. Afforestation involves planting trees on lands that have been used for other purposes like cropland, pasture, strip-mines, etc. Both reforestation and afforestation involve establishing the appropriate environment for tree seedlings and seeds to thrive. In general, site preparation would be less burdensome for lands previously harvested for timber or used for crop production or pasture. Reclaiming mine lands to the point that trees can be planted can be more difficult because the soil resources and the topography are typically altered during the mining process. As a result, the reclamation of mine lands can involve a lot of site preparation before afforestation can occur and a newly planted forest can begin to actively sequester carbon. The figure on the right depicts a managed poplar grove that is sequestering carbon dioxide.
Land management practices such as no-till agriculture could also be considered as a way to promote terrestrial carbon sequestration. No-till agriculture on agroecosystems such as corn production would limit the release of carbon and other greenhouse gas precursors like nitrogen that are present in soil organic matter. No-till agriculture might also provide for the gradual increase of soil organic carbon, and thus, an increase in terrestrial carbon sequestration.
Over the lifetimes of some long-lived trees, terrestrial sequestration can be significant. For example, over the 70-year life span of a 410-acre grove of trees planted for a terrestrial sequestration project, over 70,000 tons of carbon dioxide will be sequestered. Noteworthy is the fact that some terrestrial sequestration projects could involve using wetlands, because wetlands have high levels of productivity. The naturally low decomposition rate in wetlands means that organic matter tends to accumulate quickly, thus sequestering carbon.
Terrestrial sequestration, while relatively easy to implement, has volume limitations. It might take over 200,000 acres of plants to offset the emissions from one average power plant. However, terrestrial sequestration is serving as a near-term bridging technology that is being used to sequester carbon dioxide while more resource-intensive and technically complex geologic sequestration projects are brought on line. The table below summarizes the range of carbon dioxide sequestration rates for four biomes, as reported in the literature.