Carbon storage

The carbon cycle refers to the fixation of atmospheric carbon dioxide (CO2 ) through photosynthesis and the simultaneous or subsequent release of carbon dioxide through respiration. Through this process, carbon is cycled continuously through three main global reservoirs: the oceans, the atmosphere, and the terrestrial biosphere (including vegetation and soils). Over time, human activities have altered the amount of carbon that flows through and is stored in the various reservoirs.

To stop rising concentrations of CO2 in the atmosphere, countries are actively seeking ways to increase carbon storage capacity on land. The large amount of land area covered by grasslands as well as the relatively unexplored potential for grassland soils to store carbon has increased interest in the carbon cycles of these ecosystems.

The following summarizes key findings of the PAGE study regarding carbon storage in grassland ecosystems, as well as the quality and availability of data.

Conditions and trends

• Grasslands store approximately 34 percent of the global stock of carbon in terrestrial ecosystems while forests store approximately 39 percent and agroecosystems approximately 17 percent.

• Unlike tropical forests, where vegetation is the primary source of carbon storage, most of the grassland carbon stocks are in the soil.

• Cultivation and urbanization of grasslands, and other modifications of grasslands through desertification and livestock grazing can be a significant source of carbon emissions.

• Less than 16 percent of approximately 4,500 relatively large protected areas are at least 50 percent grassland; protected grasslands cover approximately 4 million square kilometers or 3 percent of the total land area, just 7.6 percent of the total grassland area.

• Biomass burning, especially from tropical savannas, contributes over 40 percent of gross global carbon dioxide emissions.

• Some exotic grassland plant species may decrease total carbon storage because they have less extensive below-ground root networks for storing organic matter than native grassland plants.

Information status and needs

• Estimates of carbon storage in terrestrial ecosystems worldwide vary widely; we need continued updating of models to refine estimates of carbon storage in grassland vegetation and soils.

• Carbon storage estimates need to reflect the influence of different vegetation and soil types and conditions and management practices.

• Soil greatly affects the storage potential of grasslands; comprehensive soil studies are needed to improve the accuracy of estimates of that potential.

Quality and availability of data

PAGE measures and indicators	Data sources and comments
Potential carbon stored in grasslands and other terrestrial ecosystems
Estimates for storage in above-and below-ground live vegetation	Above- and below-ground vegetation carbon storage estimates (Olson et al. 1983) as modified by USGS/EDC (1999).
Estimates for storage in soil	Soil carbon storage estimates based on the International Soil Reference and Information Centre (ISRIC) and World Inventory of Soil Emission Potentials (WISE) global data set of derived soil properties developed by Batjes (1996) and Batjes and Bridges (1994); FAO digital soil map of the world (FAO 1995).
Trends and modifications in storage capacity	Various studies reporting on loss of organic carbon or on a reduction in carbon storage potential based on current practices.