UNCCD 2nd Scientific Conference

1. The Carbon Conundrum
in Communal African Rangelands
Andrew Thomas
Institute of Geography and Earth Sciences, Aberystwyth University, U.K.

2. The Global Terrestrial Carbon Store
Most terrestrial organic carbon
isn’t stored in vegetation...

3. The Global Terrestrial Carbon Store
...but in soils
c. 3x that in vegetation
c. 2x that in atmosphere

4. Soil Organic Carbon
There are good reasons to look after SOC as it
underpins soil fertility & primary productivity…
• Improves drought resistance and reduces erodibility
• Supplies organic nutrients & improves ability to retain inorganic nutrients
• Provides energy to soil microorganisms

5. Soil CO2 efflux
Microbial respiration of organic C is
the main way C is lost from soils
This soil-atmosphere C flux was an estimated 98 ± 12 Pg C in 2010
> 10x that from fossil fuels/cement manufacturing (9.1 Pg C)

6. Soil CO2 efflux
Deforestation, agricultural conversion and desertification deplete SOC & accelerate CO2 emissions
Responsible for 20 % of global anthropogenic CO2 emissions in 1990s (IPCC, 2007)
The main cause of net C release in Africa (Henry et al., 2009)

7. Soil CO2 efflux
Predicted annual temperature changes over Africa 1980-1999 to 2080-2099 (IPCC, 2007)
Warmer soils → stimulate microbial activity → accelerate rate of SOC decomposition
Global soil-atmosphere C fluxes currently increasing by 100 million tons C yr-1
(Bond-Lamberty & Thompson, 2010)

8. Lose-lose or win-win SOC scenarios
Consequences of SOC loss wide-ranging
and central to processes of desertification:
• degrades soil quality
• reduces biomass productivity
• impacts water quality
• increases atmospheric CO2
• +ve feedback to climate change
(after Lal, 2004)

9. Soil organic carbon in the Kalahari
5 - 45 tons ha-1 to 1m - varying with rainfall, soil type, land use (Thomas et al., 2012)
SOC to 1m depthin topC ha-1
Proportion SOC tons 2cm
SW Kalahari C & N Kalahari Calcrete Soils Saline Grasslands Salt Pan
5.4 ± 2.6 39.4 ± 4.1 45.0 ± 9.3 34.7 ± 7.7 27.3 ± 5.8
10% 15% 12% 8% 20%
Surface concentration of C important for understanding dryland soils

10. Biological Soil Crusts
Assemblages of cyanobacteria, bacteria, lichens, fungi & algae on the surface of soils
Astonishing microbial diversity - DNA analysis found 7,617 different species of bacteria over c. 2 ha
Crusts dominated by species of cyanobacteria

11. Biological Soil Crusts
Cyanobacteria are important for SOC because they can photosynthesise…
Dry BSC on calcrete soil 20 minutes after rainfall
Carbohydrates produced by cyanobacteria dominate the SOC store (Mager & Thomas, 2011)
Microbial Processes in Drylands 22/42

12. Biological Soil Crusts and CO2 efflux
Makgadikgadi Basin
Soil CO2 efflux rates increase with temperature
and moisture
Periodic net CO2 uptake to the soil
Thomas et al., 2008, 2011, 2013. Thomas & Hoon, 2010. Thomas, 2012

13. Biological Carbon Capture Devices
Amount of CO2 leaving soil:
13.4 ± 3.7 mg C m-2 hr-1
Enriched CO2 from subsoil utilised by
autotrophic organisms in BSCs
c. 50% reduction in soil CO2 emissions
Proportion of CO2 remains in soil as organic C
CO2 from below surface:
27.6 ± 5.7 mg C m-2 hr-1
Data from January 2013 at sites in SW Botswana (n = 108)
Implications:
BSCs can add up to 20 mg C m-2 hr-1 to the soil – given optimal conditions
Removal of BSCs will lead to greater soil-atmosphere C fluxes & reduction in soil C

14. Biological Carbon Capture Devices
BSCs organisms occupy a range of environmental niches in a well-ordered micro biome
Scytonemin layer
Cyanobacteria layer/
5mm
visible chlorophyll
Concentration of EPS
Heterotrophic bacteria
Cross-section through a well developed cyanobacterial crust (Thomas et al., 2012)
Ability to maintain metabolic activity depends on maintenance of this structural order
BSCs therefore susceptible to disturbance by grazing animals

15. What effect does grazing have on SOC and CO2 loss?
Two year experiment to quantify effect of grazing intensity on SOC and CO2 efflux
Unsurprisingly...
Intense grazing resulted in:
Significant reduction in SOC & chlorophyll a
Significant increase in soil CO2 efflux
Data compared to ungrazed control with significance level of p < 0.01
Loss of C input from BSCs & grasses & impairment of crust C capture capability
Thomas (2012), Thomas et al., (2013)

16. What effect does grazing have on SOC and CO2 loss?
Two year experiment to quantify effect of grazing intensity on SOC and CO2 efflux
But more unexpected was that…
Light grazing resulted in:
Significant increase in SOC & chlorophyll a
No significant difference in soil CO2 efflux
No loss of cyanobacteria
Lightly grazed soils
Data compared to ungrazed control with significance level of p < 0.01
Increased soil roughness creates shade, improves water retention & prolongs photosynthesis
Where’s the conundrum?
There seems an obvious solution…

17. The carbon conundrum
Stock just below or at carrying capacity and employ rotational grazing regimes
… the SOC win-win scenario of Lal?
But, interpreting science in meaningful terms for all stakeholders in order that it has
relevance for sustainable livelihoods, policy and C management is a huge challenge…
Stringer et al., 2012; Thomas et al., 2013

18. The carbon conundrum
1. Use it or store it?
Incentivising land uses that increases C storage has been successful in forested areas
Anticipated complications in drylands:
• SOC very low. Small increases can improve soil quality, but they will not attract high value payments
• Many ecosystem services provided by soil C are obtained by using it and depleting it not storing it
• A case for an environment-based differential C pricing structure?

19. The carbon conundrum
2. Shrub encroachment
Intense grazing and atmospheric CO2 enrichment lead to thickening of shrub cover
• Increases SOC and above-ground biomass (Eldridge et al., 2010) but reduces grass production and
is widely described as a degradation process
• Care needed to ensure incentives don’t reward the “wrong type of C”

20. The carbon conundrum
3. Cultural considerations
Reducing herd sizes could result in:
• Healthier cattle
• Greater financial returns for less work
• More productive pastures
But this remains an unattractive proposition:
• Cattle ownership is an important part of cultural
identity and status within the community
Rotational grazing plans - usually based on assumption of private tenure and fenced paddocks
In communal grazing areas fencing is an anathema and given huge areas, prohibitively expensive

21. The carbon conundrum
4. Non-equilibrium environments
Precipitation & biological productivity are inherently unpredictable in drylands
Conservative grazing strategies fail to take advantage of good years and may not be appropriate
“Of course, I knew the drought
must come. Mostly, we try to keep
our cattle during a drought
knowing that if we sell them when
thin, we get little for them. The
more cattle I have when it
comes, the more chance I have
that some of them will survive…”
(in Campbell, 1990)

22. Concluding Points
Findings
Dryland soils have their own biological CO2 capture system
Grazing-related damage of BSCs increases CO2 emissions and reduces SOC
Optimisation of BSC metabolism through grazing management can:
• halve soil CO2 emissions
• maximise SOC storage
• promote healthy rangelands and sustainable pastoralism
Challenges
Warming - deplete soil moisture, reduce BSC SOC uptake & increase CO2 emissions
Grazing intensification will reduce BSC cover & SOC leading to soil deterioration
Opportunities
But, managed grazing – whether by cattle or wildlife – is beneficial to SOC
Potential for sustainable pastoral livelihoods with optimal soil C storage
Huge benefits to be gained from working with local communities to find culturally
acceptable solutions to sustainable management of rangelands and SOC