“The Incredible Shrinking Bison” [1] discussed how the increase of CO2 emissions has contributed to the rise of the Great Plains’ average temperature, and how the resulting warming trend has affected the bison. The bison, on the other hand, as a key species to the survival of the plains, can also have an indirect, mitigating effect on the CO2 levels in the atmosphere.

The predominate conversation around atmospheric CO2 has centered on the elimination of CO2 production.  There is, however, another conversation underway—one involving the removal of CO2 from the atmosphere. The process of capturing and storing atmospheric carbon dioxide is known as carbon sequestration.  It is one method of reducing the amount of carbon dioxide in the atmosphere, and consists of two types: geologic and biologic.  Geologic carbon sequestration involves the storing of carbon dioxide in underground geologic formations.  The CO2 is usually pressurized until it becomes a liquid, and then it is injected into porous rock formations in geologic basins [2]. 

Biologic carbon sequestration refers to storage of atmospheric carbon in vegetation, soils, woody products, and aquatic environments. For example, by encouraging the growth of plants—particularly larger plants like trees—advocates of biologic sequestration hope to remove CO2 from the atmosphere.  Within biologic carbon sequestration there are several means by which CO2 is removed from the atmosphere.  These include peatland, wetland, forestry, agriculture, carbon farming, deep soil, and ocean-related. But of all the terrestrial (as opposed to aquatic) methods, the forests receive the lion’s share of the world’s attention.  Forgotten are the grasslands which also harbor much of the wetlands. For North America the grasslands of the Great Plains and prairies, which occupy approximately one-third of the continent, are critical to carbon sequestration.  And key to the grasslands’ vitality is the North American Bison [3].

Grasslands quickly process carbon from the atmosphere and store this carbon in the root structures, which extend 8 to 15 feet into the ground, which can store 22.5 million tons of carbon. These roots can hold the carbon for decades, and process 1.7 million tons of carbon per acre to the soil annually.  This storage accumulates over time and moves carbon from the atmosphere to the ground continuously creating massive carbon deposits over the course of centuries. Prairies have the ability to store as much carbon below the ground as forests can store above the ground. When carbon is stored below ground it remains locked there and unable to enter the atmosphere.  Compared to forests, grasslands are more reliable.  In times of drought and forest fires, the carbon stored in the wood and leaves returns to the atmosphere.  During a grass fire, however, carbon is not released since it is stored in the roots underground [4].

School of Environmental Sustainability-Colorado State University [5]

Though the Great Plains and prairies occupy a vast swath of the North American continent, this does not translate into a great CO2 scrubber.  The conversion of this ecosystem into cropland has significantly reduced the ability of this region to sequester carbon [6]. Compared to native or natural vegetation, cropland soils are depleted in soil organic carbon (SOC).  When soil is converted from its native state the SOC content in the soil is reduced by approximately 30 to 40% [7].  Further, the crops replacing the native grasses are annuals with comparatively shallow root structures which are less effective in storing carbon and holding soil.  With less carbon stored and moved to soil, and increased possibility of soil loss, the effectiveness of the plains and prairies in atmospheric CO2 removal is significantly decreased.  

Short of returning the croplands back to the natural state of the region, there are agricultural methods aimed at sequestering atmospheric carbon into the soil and in crop roots, wood and leaves.  These methods are collectively referred to as carbon farming.  Besides removing CO2 from the atmosphere, increasing the soil’s carbon content—whether by reverting to the natural condition, or by carbon farming—aids plant growth, increases soil organic matter which improves agricultural yield, improves soil water retention capacity and reduces fertilizer use which is a source of the greenhouse gas nitrous oxide (N2O) [8].

Carbon farming or recovering the native perennials, however, is not the complete answer.  The ecosystems of the plains and prairies were dependent on the large herds of bison moving over the grasslands.  The grazing, trampling and recovery patterns associated with the bison were key in building soil, maintaining biological diversity and deepening plant roots, which are crucial elements in permanent carbon sequestration [9].  The bison not only provided nutrients for plant life, but tilled the soil with their hooves, working up and trampling dung into the soil, enabling plant-life to take hold, flourish and consequently become a significant carbon sink.

School of Environmental Sustainability-Colorado State University [10]

Though sequestration, as used here, is a technical term, the concept is quite familiar. When we hear the word “sequester,” the common association is with juries as in jury trials.  When a jury is sequestered, it is removed and kept apart from contact with the public.  The purpose is to ensure undue influence on, or tampering with, the deliberations of the jury, and ensure a just verdict.  As the jurors file out of the courtroom at the end of the defense’s and prosecution’s final presentations, if the judge has ordered they be sequestered, we see the tangible form of a removal to protect the integrity of the trial by jury justice system considered critical to our legal well-being.  The notion of using an act of removal in the protection of our well-being is only part of the meaning of sequestration.  What is being removed and where it is being kept are equally important.  Originally, “to sequester” meant “…to put in the hands of a trustee…” [11]. In regard to carbon sequestration the trustee is the earth itself, or more specifically, in the context of the bison and the grasslands of North America, it is the Great Plains ecosystem.  When we think of ecosystems, we tend to think of the land, the flora and the fauna.  Often missing in our consideration is the air above.  The bison—a  keystone species in regard to the flora and fauna and the land—is a crucial element of the trustee,  instrumental in the process of CO2 removal and the mitigation of the warming trend plaguing the Great Plains.

End Notes:

[1] Schuette, Keith. “The Incredible Shrinking Bison.” November 17, 2020.Bison Witness.

[2] “What is Carbon Sequestration?” What is carbon sequestration? ( Retrieved 4/24/21

[3] Schuette. “Dung Cake and Feces Pie: Yum!” April 26, 2019. Bison Witness.

[4] Davidson, William, “The Great Plains: America’s Carbon Vault” (2016). Op-Eds from ENSC230 Energy and the Environment: Economics and Policies. 73.

[5] Lavelle, Jocelyn. Soil carbon sequestration to combat climate change—a real solution or just hype? – Sustainability ( Colorado State University—School of Environmental Sustainability. Retrieved 4/30/21.

[6] 42% of the Great Plains has been converted to cropland, leaving 53% intact.  The remaining 5% holds water or has been converted to human use.  Understanding Grassland Loss in the Northern Great Plains. 2018. World Wildlife Organization.

[7] Poeplau, Christopher; Don, Axel (February 1, 2015). “Carbon sequestration in agricultural soils via cultivation of cover crops – A meta-analysis”. Agriculture, Ecosystems & Environment. 200 (Supplement C): 33–41. doi:10.1016/j.agee.2014.10.024.

[8] “Carbon Farming | Carbon Cycle Institute”. Also, “Carbon Farming: Hope for a Hot Planet – Modern Farmer”. Modern Farmer. 2016-03-25.  And Velasquez-Manoff, Moises (2018-04-18). “Can Dirt Save the Earth?. The New York Times. Retrieved 4/30/21.

[9] Wright, Pam. Bison: The Latest in Carbon Capture Tech.12/24/2017/by Regeneration International. Retrieved 4/30/21.

[10] Lavelle.

[11] Webster’s Unabridged Dictionary of the English Language. 2001. Random House.