Carbon Sequestration in Grassland Ecosystems - A Review of Scientific Literature

Dr. Kay L Kottas and Alicia Admiraal; Prairie Legacy Inc. (March 2020). Carbon sequestration in grassland ecosystems. With appended details by James E. Ducey and Dr. David M. Sutherland.

Introduction

The Sands Hills occur primarily within Nebraska though also in a south-central extent of South Dakota – and comprise more than 19,000 square miles of vast native grassland tracts. The region is internationally recognized for its expanses of mixed-grass prairie since there are both cool-season and warm-season species. More than 95 species of grasses are known to occur (Dr. David M. Sutherland, 1984; Vegetative key to grasses of the Sand Hills region of Nebraska) and most thrive. The grasses are essential range forage for livestock but are also a key vegetative feature stabilizing the sandy soils and a key aspect to other essential environment values. This includes carbon sequestration. [Introduction by James E. Ducey, 15 May 2020]

Review of Scientific Literature

The amount of carbon in the soil worldwide (1,600 billion metric tons) is double that in vegetation (560 billion metric tons) or in the atmosphere (750 billion metric tons) (Rice et al. 1998). Carbon cycling through plants occurs when plants bring in atmospheric carbon for use in photosynthesis. The carbon is stored in plant tissues both above and below ground. Eventually that carbon is released back into the atmosphere as plant tissue dies and is decomposed by microbial activity. The same result happens more quickly if the vegetation is burned. Much of the below ground carbon remains there, even as it decomposes. Grasslands store most of their carbon below ground. The extensive root systems of these plants sequester nearly as much carbon as forested lands, but far more efficiently (Kerlin 2018). When a tree decomposes or a forest burns, most of the stored carbon is released to the atmosphere. When the above ground biomass of a grassland decomposes or is burned, the below ground biomass and carbon remains sequestered. Approximately 60 to 80 percent of biomass in a tallgrass system occurs below ground (Wedin and Tilman 1990).

The ability of prairie systems to sequester carbon is affected by soil texture, weather patterns, temperature and the amount of ground cover (Jones and Donnelly 2004, Mengistu and Mekuriaw 2014), and by several other factors including precipitation, season, management such as grazing, and functional group of the dominant grasses. Nitrogen input can also affect plant growth and therefore how well carbon is sequestered. Interactions between these factors greatly affect response of grassland in a particular location. We know that cropped land can reduce methane (CH4) oxidation by 90% of what it is in native grassland (Mosier et al. 1997). Methane is broken down (oxidized) by bacteria into carbon dioxide (CO2). Because methane is 25 times more potent than carbon dioxide as a greenhouse gas, that breakdown in grasslands is important. In cultivated soils among the tallgrass prairie, carbon cycling has been reduced by 50%. Even if cropland is returned to native perennial vegetation recovery of CH4 oxidation takes many decades (Mosier et al. 1997). Rosenzweig et al. (2016) estimate that in restored cultivated fields, the amount of time for soil carbon sequestration to return to the level of undisturbed native prairie can be centuries (more than 350 years).

In the Nebraska sandhills mixed grass prairie is similarly affected. Mixed grass communities provide ground cover in the sandhills across the dunes within the swell and swale topography. The swales have more above ground production and deeper A horizon soils and tend to hold more soil carbon than the swells (Hartmann 2013). In a study done by Hartmann (2013) removal of above ground vegetation, reduced CO2 flux rates by as much as 60%, but mean soil CO2 flux did not return to control levels even after ten years. In managing grasslands for carbon sequestration, limiting soil disturbance is of primary importance. Soil disturbance disrupts soil structure, results in increased soil temperature and aeration, allowing more decomposition and potential release or reduced uptake of carbon (Janowiak et al. 2013; Conant 2010). Disturbances to sandhills vegetation can therefore have serious long-term effects to carbon cycling and sequestration. Disturbances to soil, particularly in sandy soils, takes many decades of careful management to return soil carbon exchange to its natural state. Increased above ground biomass also increases potential for carbon storage. By managing plant growth and the cover on prairies and on agricultural land, we can help increase above ground biomass, help maintain or increase the presence of below ground mycorrhiza and enhance carbon sequestration.

Spangler (2011) presents some of the varied responses of grassland to grazing, citing studies that show both positive and negative grazing effects. Soil chemistry and associated microbial communities can change in response to grazing. Effects from trampling that would result from heavy grazing can remove aboveground biomass and change the physical characteristics of the soil. This in turn can limit water infiltration and root growth. Wilson et al. (2009) found a highly significant correlation between the abundance of arbuscular mycorrhiza hyphae in the soil and the ability of the soil to store carbon and nitrogen, exposing serious consequences to the soil ecosystem when disturbances cause a loss of arbuscular mycorrhiza. Ultimately, positive influence on soil organic carbon and nitrogen content seems to be realized under light grazing pressure, in contrast with heavy grazing or ungrazed treatments (Spangler 2011).

Precipitation also interacts with soil texture in regard to the impact of certain management practices, such as grazing. Grasslands on sandy, coarse-textured soils have increased soil carbon if rainfall increases under a grazing regime; grasslands on clay soils exhibit weak to strong decreases in soil carbon under these conditions (McSherry and Ritchie 2013). Conversely, with lower rainfall, clay soils show the largest increase in soil carbon compared to sandy soils (McSherry and Ritchie 2013). Soil carbon stocks already held in the soil tend to be higher in grasslands that have higher rainfall, but stocks decrease as temperature increases because evapotranspiration also increases (Janowiak et al. 2017).

By season, the average period of CO2 uptake was mid-April through late August, with tallgrass prairies in the southern Great Plains capable of accumulating more carbon; at other times of the year, the grasslands in the northern Great Plains and in short grass and mixed grass prairies, especially in the western part of the region, release carbon (Zhang et al. 2011).

The functional group of dominant grasses (C4, warm-season, or C3, cool season) was found to influence soil carbon uptake in several studies. C4 grasses, or mixtures of C4 and C3 grasses, were more resilient in their ability to accumulate soil carbon under increasing grazing pressure (McSherry and Ritchie 2013). Another 12-year study monitored net soil carbon and nitrogen and found that accumulation was positively affected by total root biomass, which increased with the presence of C4 grasses and legumes (Fornara and Tilman 2008). The inclusion of legumes was thought to be important because the ability of legumes to accumulate nitrogen helps to build soil organic matter and store more carbon. Further, this study showed that greater plant species diversity corresponded to greater uptake of carbon when compared to a monoculture (Fornara and Tilman 2008).

Another long-term study examined the effect of nitrogen inputs to mineral soils (Fornara and Tilman 2012). The addition of nitrogen contributed to increased carbon sequestration compared to unfertilized grassland, likely by increasing root mass, particularly of C3 plants. Aboveground biomass for both C4 and C3 grasses also increased during the 27-year study, but plant species richness decreased and dominance shifted to C3 grasses (Fornara and Tilman 2012). In 2017, Hungate et al. demonstrated the economic value of increased species diversity in carbon sequestration, furthering the economic argument for conservation of grassland biodiversity. For instance, an increase of 4 species over U.S. Conservation Reserve Program lands would provide 2.3 billion dollars in carbon storage value. Conversely, the loss of these species in remnant prairies would produce a similar economic devaluation of prairie land for carbon sequestration.

Summary

The loss of either cover, diversity, or both on grassland ecosystems translates to a large loss in carbon cycling, a huge opportunity loss in the below ground sequestration of carbon, and in the ability of grasslands to breakdown. Recovery from those losses of carbon sequestration to pre-disturbance rates can be an extremely long process, on the order of centuries. The gain or loss of carbon sequestration is variably dependent on soil type, precipitation, species dominance, species diversity, climate and management. While we have no influence on soil type and weather, we can influence species diversity, dominance and management. Expressed in dollars, the economic impact of such losses can be understood. The prevention of those losses, so much as is under our control amount to replacing diverse plantings on disturbed soil, protecting diversity where we can, increasing diversity where necessary, preventing disturbances and practicing careful management with responsible grazing.

Literature Cited

Conant, R.T., ed. 2010. Challenges and opportunities for carbon sequestration in grassland systems: A technical report on grassland management and climate change mitigation. Integrated Crop Management Vol. 9. Food and Agriculture Organization of the United Nations (FAO), Plant Production and Protection Division.

Fornara, D.A., and D. Tilman. 2008. Plant functional composition influences rates of soil carbon and nitrogen accumulation. Journal of Ecology 96:2 pp 314-322. https://doi.org/10.1111/j.1365-2745.2007.01345.x Fornara, D.A., and D. Tilman. 2012. Soil carbon sequestration in prairie grasslands increased by chronic nitrogen addition. Ecology 93(9):2030-2036. https://doi.org/10.1890/12-0292.1

Hartmann, A. A., R. L. Barnard, S. Marhan, and P. A. Niklaus. 2013. Effects of drought and N-fertilization on N cycling in two grassland soils. Oecologia 171:705–17.

Hungate, Bruce A., Edward B. Barbier, Amy W. Ando, Samuel P. Marks, Peter B. Reich, Natasja van Gestel, David Tilman, Johannes M. H. Knops, David U. Hooper, Bradley J. Butterfield, Bradley J. Cardinale. 2017. The economic value of grassland species for carbon storage. Science Advances. Vol. 3, No. 4, 05 April 2017.

Janowiak, M., T. Ontl, and C. Swanston. 2017. Chapter 4: Carbon and land management. Pages 21-35 in Considering forest and grassland carbon in land management. USDA United States Forest Service. General Technical Report WO-95. June.

Jones and Donnelly. 2004. Carbon sequestration in temperate grassland ecosystems and the influence of management, climate and elevated CO2. New Phytologist. Volume164, Issue3. December 2004 Pages 423-439.

Kerlin, K. 2018, UCDavis. https://climatechange.ucdavis.edu/news/grasslands-more-reliable-carbon-sink-than-trees/ Mengistu and Mekuriaw. 2014. Challenges and opportunities for carbon sequestration in grassland system, a review. Ent J Engineering Environmental Resources 1 (1) 1-12.

Mosier, A. R., W. J. Parton, D. W. Valentine, D. S. Ojima, D. S. Schimel, and O. Heinemeyer. 1997. CH4 and N2O fluxes in the Colorado shortgrass steppe. 2. Long-term impact of land use change. Global Biogeochem Cycles 11:29–42.

McSherry, M.E., and M.E Ritchie. 2013. Effects of grazing on grassland soil carbon: a global review. Global Change Biology. 19(5): 1347–1357. Rice, C. W., T. C. Todd, J. L. Blair, T. Seastedt, R. A. Ramundo, and G. W. T. Wilson. 1998. Belowground biology and processes. Pp. 244–264. In A. K. Knapp, J. M. Briggs, D. C. Hartnett, and S. L. Collins (eds.). Konza Prairie Long-term Ecology Research. Oxford Press, New York, New York. Rice, C. W., A. B. Omay, C. J. Dell, M. A. Williams, and Y. Espinoza. 1999. Soil organic matter in grasslands: Response to climate and land management. Global change and terrestrial ecosystems. P. 44 In Focus 3 Conference on Food and Forestry: Global Change and Global Challenges. University of Reading, Reading, U.K., September 20–23, 1999.

Rosenzweig, Steven T., Michael A. Carson, Sara G. Baer, John M. Blair, 2016. Changes in soil properties, microbial biomass, and fluxes of C and N in soil following post-agricultural grassland restoration. Applied Soil Ecology. Volume 100, April 2016, Pages 186-194.

Spangler, L. 2011. Rangeland sequestration potential assessment. Final Report. U.S. Department of Energy National Energy Technology Laboratory. DOE Award Number: DE-FC26-05NT42587.

Wedin, D. A., and D. Tilman. 1990. Species Effects on Nitrogen Cycling: A Test with Perennial Grasses. Oecologia 84:433–441. Wilson, G. W., Rice, C. W., Rillig, M. C., Springer, A. & Hartnett, D. C. 2009 Soil aggregation and carbon sequestration are tightly correlated with the abundance of arbuscular mycorrhizal fungi: results from long-term field experiments. Ecol. Lett. 12, 452–461.

Zhang, L., B. K. Wylie, L. Ji, T. G. Gilmanov, L. L. Tieszen, and D. M. Howard (2011), Upscaling carbon fluxes over the Great Plains grasslands: Sinks and sources, J. Geophys. Res., 116, G00J03, doi:10.1029/2010JG001504.

Addendum

These details indicate some of the main grasses of the sandhills and is courtesy of Dr. Sutherland, emeritus professor of botany at the University of Nebraska at Omaha.

“The main overstory grasses are: Calamovilfa longifolia, sand reedgrass; Andropogon gerardii subsp. hallii, sand bluestem; Stipa comata (Hesperostipa comata), needle and thread (Sporobolus cryptandrus); sand dropseed; Eragrostis trichodes (sand lovegrass); and, Bouteloua curtipendula (sideoats grama)

“Main understory grasses: “Bouteloua gracilis (blue grama); Bouteloa hirsuta (hairy grama) in rougher spots. There is also at least one very common sedge in the understory: Carex heliophila (sunsedge).

“Important grasses for stabilizing blowouts would be: Muhlenbergia pungens (sand muhly); and, Redfeldia texuosa (blowout grass).”

Other prominent warm-season grasses, according to details in historic published articles, are:

Big Bluestem: “In the sandhills it is the most important of the native hay grasses, growing best on subirrigated meadows where the water table is 1.5 to 3 feet from the surface.” “...may be considered the king of our native hay and pasture grasses.”

Indian Grass: “...rather common in the sandhills area, both in the meadows and as rather isolated colonies in the uplands. … one of the most important of the native hay and pasture grasses.”

Switchgrass: “a high producer of good quality hay, abundant pasturage and an excellent erosion control grass, it is considered among the more valuable native grasses.”