Andrew David THOMAS " The carbon conundrum in communal African rangelands"
The Carbon Conundrumin Communal African RangelandsAndrew ThomasInstitute of Geography and Earth Sciences, Aberystwyth University, U.K.
The Global Terrestrial Carbon StoreMost terrestrial organic carbonisn’t stored in vegetation...
The Global Terrestrial Carbon Store ...but in soilsc. 3x that in vegetationc. 2x that in atmosphere
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
Soil CO2 efflux Microbial respiration of organic C is the main way C is lost from soilsThis soil-atmosphere C flux was an estimated 98 ± 12 Pg C in 2010> 10x that from fossil fuels/cement manufacturing (9.1 Pg C)
Soil CO2 efflux Deforestation, agricultural conversion and desertification deplete SOC & accelerate CO2 emissionsResponsible for 20 % of global anthropogenic CO2 emissions in 1990s (IPCC, 2007)The main cause of net C release in Africa (Henry et al., 2009)
Soil CO2 effluxPredicted 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)
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)
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
Biological Soil Crusts Assemblages of cyanobacteria, bacteria, lichens, fungi & algae on the surface of soilsAstonishing microbial diversity - DNA analysis found 7,617 different species of bacteria over c. 2 haCrusts dominated by species of cyanobacteria
Biological Soil Crusts Cyanobacteria are important for SOC because they can photosynthesise… Dry BSC on calcrete soil 20 minutes after rainfallCarbohydrates produced by cyanobacteria dominate the SOC store (Mager & Thomas, 2011) Microbial Processes in Drylands 22/42
Biological Soil Crusts and CO2 efflux Makgadikgadi Basin Soil CO2 efflux rates increase with temperature and moisture Periodic net CO2 uptake to the soilThomas et al., 2008, 2011, 2013. Thomas & Hoon, 2010. Thomas, 2012
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-1Data 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
Biological Carbon Capture DevicesBSCs organisms occupy a range of environmental niches in a well-ordered micro biome Scytonemin layer Cyanobacteria layer/5mm visible chlorophyll Concentration of EPS Heterotrophic bacteriaCross-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
What effect does grazing have on SOC and CO2 loss? Two year experiment to quantify effect of grazing intensity on SOC and CO2 effluxUnsurprisingly...Intense grazing resulted in:Significant reduction in SOC & chlorophyll aSignificant 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)
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 aNo significant difference in soil CO2 effluxNo loss of cyanobacteria Lightly grazed soils Data compared to ungrazed control with significance level of p < 0.01Increased soil roughness creates shade, improves water retention & prolongs photosynthesis Where’s the conundrum? There seems an obvious solution…
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
The carbon conundrum 1. Use it or store it? Incentivising land uses that increases C storage has been successful in forested areasAnticipated 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?
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”
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 communityRotational grazing plans - usually based on assumption of private tenure and fenced paddocksIn communal grazing areas fencing is an anathema and given huge areas, prohibitively expensive
The carbon conundrum 4. Non-equilibrium environments Precipitation & biological productivity are inherently unpredictable in drylandsConservative 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)
Concluding PointsFindingsDryland soils have their own biological CO2 capture systemGrazing-related damage of BSCs increases CO2 emissions and reduces SOCOptimisation of BSC metabolism through grazing management can: • halve soil CO2 emissions • maximise SOC storage • promote healthy rangelands and sustainable pastoralismChallengesWarming - deplete soil moisture, reduce BSC SOC uptake & increase CO2 emissionsGrazing intensification will reduce BSC cover & SOC leading to soil deteriorationOpportunitiesBut, managed grazing – whether by cattle or wildlife – is beneficial to SOCPotential for sustainable pastoral livelihoods with optimal soil C storageHuge benefits to be gained from working with local communities to find culturallyacceptable solutions to sustainable management of rangelands and SOC