Getting to the Root of it All: Could Replanting Prairie Help Slow Climate Change?

By Sarah Hill

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A diverse array of native plants adorn a fragment of Palouse prairie in eastern Washington. Photo by James Riser

Estimated to lock away 2.3 times more carbon in their soils than there is carbon in all of the earth’s atmosphere, grasslands and prairies are some of the most valuable and threatened ecosystems in the world. Could restoring these degraded ecosystems hold the key to tackling climate change?

Prairies and the Carbon Cycle
Much of the conversation about climate change revolves around carbon – the “C” in CO2, also called carbon dioxide. Extensive burning of fossil fuels has taken carbon that was trapped deep in the earth as oil and coal, and turned it into CO2, which is causing the earth’s climate to change.

What can we do to restore the balance and slow climate change? How about enhancing existing systems that suck carbon out of the air and trap it underground! Thanks to prairie plants, photosynthesis, and inter-species cooperation, we’ve got a good starting place. During photosynthesis plants take in CO2 from the air, and breathe out oxygen. Plants use that carbon to build their stems, leaves, and roots, and make sugar that they use for food. Many plants in the prairies have incredibly deep root systems. Often what you see on the surface pales in comparison to what is underneath!

Plants also feed sugar to microbes and fungi living in the soil through their roots. A special type of fungus use the carbon to make a sticky “glue”. Soil carbon and minerals like clay get stuck together by the glue and form globs of soil called aggregates, which take a very long time to breakdown. The unique combination of massive plant roots and fungal glue makes prairies carbon sponges. Plowing prairies changes these properties. Soil aggregates are broken up and the carbon is eaten by soil microbes who release CO2 into back the atmosphere. Most crops have shallow root systems and only live for one season. Once harvested their roots decompose and release carbon back into the air. Extensive cultivation has been shown to decrease soil carbon by 20-67%.

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How plants effect the carbon cycle. Image from http://www.cecsb.org.

Carbon Recovery
If plowing prairies and planting crops reduces soil carbon, what happens when plowing stops? Can building prairies build soil carbon? This was the question at the center of a study released earlier this year by Yang et al. The research team conducted a long term experiment at a tallgrass prairie restoration in Minnesota tracking soil carbon changes over time. Yang and his colleagues also examined whether the number of different species planted affected carbon storage, and if there were certain combinations of plants that were particularly effective.

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The Cedar Creek Ecosystem Science Reserve in Minnesota, where the 2019 study took place. Photo credit: CCE-LTER

Their results were striking. They clearly showed that the more species that were planted, the more carbon was stored in the soil, and that carbon levels increased dramatically over time. After 22 years soil carbon was about 90% greater than after 13 years of restoration. When comparing the results to an abandoned field that was left to regenerate without human assistance the carbon storage at year 13 was very similar. However during the second half of the study, years 13-22, carbon storage was up to 200% greater in human-planted sites than in sites left to recover without an intervention.

These results suggest that the slow growing nature of many prairie plants might mean they need more time to reach their carbon trapping potential. Additionally, some species that were planted in the restoration areas were not present in the natural areas. Planting a high diversity of plants supercharged the restoration’s carbon storing ability.

They also found certain combinations of plants were more effective at storing carbon; a specific type of grass, called a “C4” grass, and lupine, a flowering plant. The cause for this dream team? Their unique traits. The C4 grasses had the most extensive above and below ground growth out of all the plants used, and it decomposes the slowest. Lupines add extra nitrogen to the soil, and nitrogen which is a key nutrient for plant growth. The C4 grass used the extra nitrogen from the lupines to increase its growth, which increased the rate that it could take carbon from the air. Since it didn’t decompose as quickly as other species, more carbon accumulated in the soil.

Carbon Storage in the Palouse Prairie
If prairie restoration increased carbon storage in midwestern prairies, could it do the same for the Palouse prairie? 99% of the Palouse prairie in Eastern Washington and Western Idaho has been plowed up and turned into cropland, making it one of the most endangered ecosystems in the United States. In recent years palouse restoration has gained traction, including here at Eastern Washington University.

Few studies have been done on the carbon storage capacity of the Palouse, but a study published in 2013 by Sánchez-de León and Johnson-Maynard could shine some light on its potential. They examined differences in soil carbon and soil properties in two different grasslands in the Palouse. They compared three intact native prairie remnants to adjacent agricultural land that had been planted with non-native grasses for 20-25 years.

The intact prairie had 53% more soil carbon than non-native grass sites. There were also more of the long lasting soil aggregates in native prairie, however there was a large percentage of the small aggregates in the non-native grass sites. Since soil aggregates get destroyed by tilling, the presence of the soil clusters in the non-native grass soil suggests that some significant carbon recovery had occurred.

What Could This Mean For the Future of The Palouse?
The non-native grass sites were only planted with three species, two of which have shallow root systems. The study by Yang et al. demonstrated that carbon storage significantly increased with more species, that plants with deep dense roots store more carbon, and that combinations of plants can increase the carbon storage rate. The above study suggests that carbon stocks in the Palouse can recover small aggregates within 20 years of a low-diversity non-native planting…imagine the carbon storing potential that could in a diverse, native Palouse prairie restoration!

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The Palouse prairie is home to a wide variety of native grasses and flowering plants. Plants like the yellow flowered arrow-leaf balsamroot have root systems documented to do down at least 9 feet into the ground. Photo credit: James Riser

What could this mean for the future of the earth’s grasslands and prairies?
Grasslands and prairies are some of the most threatened and most valuable ecosystems on earth. As the effects of climate change intensify, it is entirely possible that they will be looked at as a remedy – an international initiative has already been created to increase global soil carbon by .4% per year. Restoring prairies would be a boon to all the creatures that call them home, while delivering climate balancing services like taking in carbon, holding down soil, and absorbing water.


References

Community Environmental Council. 2019. Carbon Farming & Regenerative Agriculture.  www.cecsb.org/rethink-food/carbon-farming/. Accessed electronically Dec 3, 2019.

Davidson, E. and Ackerman, I. 1993. Changes in soil carbon inventories following cultivation of previously untilled soils. Biogeochemistry 20: 161–193.

Eastern Washington University. 2019. Palouse Prairie Restoration. https://inside.ewu.edu/palouserestoration. Accessed electronically Dec 3, 2019.

Harlan, B. 2015. Digging Deep Reveals the Intricate World of Roots. National Geographic Magazine. http://www.nationalgeographic.com/photography/proof/2015/10/15/digging-deep-reveals-the-intricate-world-of-roots/. Accessed electronically Dec 3, 0219.

IUCN. 2019. Grasslands. https://www.iucn.org/commissions/world-commission-protected-areas/our-work/grasslands. Accessed electronically 28 Nov. 2019.

Lal, R. 2001. World cropland soils as a source or sink for atmospheric carbon. Advances in Agronomy 71:145–191.

Lal, R. 2004. Soil carbon sequestration impacts on global climate change and food security. Science 304:1623–1627.

Noss, R., F. Laroe III, and J. Scott. 1995. Endangered ecosystems of the United States: a preliminary assessment of loss and degradation. Biological Report 28, U.S. National Biological Service, Washington, D.C

Sánchez-de León, Y., and J. Johnson-Maynard. 2013. Ecosystem Carbon Storage and Cycling in Restored and Native Grasslands of the Palouse Region. Soil Science Society of America Journal 77:929-940.

Wei, X., Shao, M., Gale, W. and Li, L. 2015. Global pattern of soil carbon losses due to the conversion of forests to agricultural land. Scientific Reports 4: 4062.

Yang, Y., Tilman, D., Furey, G. 2019. Soil carbon sequestration accelerated by restoration of grassland biodiversity. Nature Communications 10: 718.

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