Most know that trees can temporarily store carbon dioxide from the atmosphere, but fewer realise that soils contain the largest store of carbon on the Earth’s land surface.
Research by one of C3W’s Institutional Directors into the saltpans of Botswana has demonstrated the incredible capacity and unique way in which their soils and sediments store carbon. Two expeditions in July 2010 and April 2011 saw teams, led by Aberystwyth University’s Dr Andrew Thomas, travel to the remote Makgadikgadi Basin. There they set about measuring both the amount of carbon stored in the soils and sediments, as well as the flux (changes in and out) of CO2 from the ground to the atmosphere.
Speaking of the need for the work, Dr Thomas said; “We’ve known virtually nothing about how much carbon is stored in environments like this, what form it is in, or what the annual changes are.”
“This is critical because soils and sediment store three times more carbon than all the vegetation on Earth. “
“Small changes to size of the store or in the amount it releases into the atmosphere would have a dramatic impact on atmospheric chemistry, the amount of energy absorbed from the sun, and ultimately future warming.”
The Makgadikgadi pans formed from the huge lake that once covered an area larger than Switzerland, but dried up several thousand years ago. The area is now a safari destination for those seeking encounters with the abundant wildlife.
It was a chance meeting between Dr Thomas and eco-safari operator Ralph Bousfield that led to this project. Having heard about Andrew’s work in the Kalahari he invited him to visit his concession area to see if it was suitable for experimental work.
Remarking about his first view of the saltpans Andrew said; “I was totally amazed by the place from the first visit and was determined to get funding to come back.”
In the following year Andrew and his team secured a Leverhulme Trust award to investigate the unique geochemistry and carbon cycle of the area. They found the pan sediment to be highly alkaline and saline but not devoid of life, as bacteria and fungi were thriving underneath the salt-crusted surface. These micro-organisms add significant quantities of organic carbon to the underlying sediments.
The sedimentary environment also encourages the formation of inorganic forms of carbon. Occasional floods bring calcium and magnesium salts on the pan where they are rapidly turned to carbonates, during which CO2 is used up in the process. So whilst the pans discharge some CO2 in the dry season in the wet season the pans are potentially a very important carbon sink.
The project resulted in new information on the size of the carbon store on the Pans and in the surrounding grassland and wooded islands. It appears the store is about ten times greater that the surrounding Kalahari Sands, something that is not accounted for in current global change models.
Andrew D. Thomas, Andrew J. Dougill, David R. Elliott and Helen Mairs, 2014.
Seasonal differences in soil CO2 efflux and carbon storage in Ntwetwe Pan, Makgadikgadi Basin, Botswana. Geoderma, 219-220, 72-81.