Carbon Sequestration in Grasslands

Prairie systems contain much more soil organic carbon than other ecosystems due to rooting characteristics of the vegetation that grows there. These systems have adapted to frequent fire and grazing by developing deep root systems. Grasslands and shrubland carbon stock make up 34% of all carbon in the U.S. Great Plains region.¹

Deep Root Systems

Native grassland species have extensive root systems, some growing up to 15 feet deep. In fact, most of native grassland species' biomass is found below ground. Grasslands dominated by native species store more carbon in soil than those dominated by non-native species. Native prairie species are known to have up to twice the deep root biomass of introduced species.¹ 

Deep root systems deposit carbon into deep soil layers, which is important because the rate of carbon sequestration increases with soil depth.² Deep roots of native species are more likely to contribute to very stable carbon pools¹ via  the exchange of plant sugars for nutrients with associated soil microbial life in the root zones. Biomass of soil microbes increases with plant species diversity.³ Late successional plant diversity increases carbon in the root zones as more diverse, perennial plant communities promote more diverse micro-organisms which contribute to long term, below ground carbon storage.⁴

Root Turnover

Each year as much as one-half of a native prairie plant's root system dies and regenerates. This annual root turnover contributes large amounts of organic materials to the soil system. Living roots also contribute organic material through exudation of carbohydrate-containing compounds and sloughing dead cells. Root associative fungi and bacteria exchange carbohydrates for minerals and water, over time shifts in community assembly shift towards fungal dominated biota.⁴ Root-derived soil organic carbon is well protected against oxidation and decay because it is buried deep in the soil profile. 

Disturbance - a hindrance and a help

Disturbance, depending on the type, can either reduce or enhance soil organic carbon accumulation. Disturbance to prairie soils from tillage results in losses of soil organic carbon from oxidation to the atmosphere in the form of carbon dioxide. Some prairie soils have lost more than 50% of their organic matter as a result of breaking the sod and subsequent tillage for crop production. Soil organic carbon is protected from oxidation and release to the atmosphere when disturbance from tillage is eliminated.⁴

Naturally occurring periodic fires increase the carbon storage of prairies. Fire, a natural disturbance to prairie systems, causes oxidation of organic carbon while simultaneously stimulating root growth and increasing organic matter additions to the soil. Fire results in a net gain of carbon to the soil system because enhanced root growth exceeds oxidative losses. Woody plant encroachment is controlled by periodic fire, but could actually increase carbon sequestration while altering grassland composition.¹

Managed grazing can also increase carbon storage, rotational grazing practices can stimulate root growth. This practice is most effective at building soil carbon stocks when grazing management prevents overgrazing and over compaction which can reverse the sequestration rates of grassland soils by depleting vegetation and preventing healthy vegetative growth.¹

1. USGCRP. Second State of the Carbon Cycle Report. (2018). Available at: https://carbon2018.globalchange.gov/chapter/10/ (Accessed: 21st June 2019)

 2. Gill, R.A. and I.C. Burke. Plant and Soil (2002) 241:233-2423.

3. Lange, Markus, et al. “Plant diversity increases soil microbial activity and soil carbon storage” 2015. Nature Communications. 6, 6707 (2015).

4. Yang, Y., Tilman, D., Furey, G. & Lehman, C. Soil carbon sequestration accelerated by restoration of grassland biodiversity. Nat Commun 10, 718 (2019).