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Thomas, A., Hoon, S. R., Mairs, H., Dougill, A. J. (2012). Soil Organic Carbon and Soil Respiration in Deserts: Examples from the Kalahari. In L. Mol, & T. Sternberg (Eds.), Changing Deserts: Integrating People and their Environment. (pp. 40-60). The White Horse Press.
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Context 1
... in drier areas only around forty per cent of soil C was derived from shrubs, with most originating from grasses, but these inputs were dependent on the location of woody plant species. The distribu- tion of woody shrubs also affects the distribution of the other major input of SOC into Kalahari Sands -namely biological soil crusts ( Figure 6). Biological soil crusts form aggregates of mineral grains, cyanobacteria, lichen and fungi and are common in all drylands (Belnap and Lange 2003). ...
Context 2
... crusts in the Kalahari are comprised of cyanobacteria and are typically 3-4 mm thick. They are beneficial for a range of ecosystem functions and properties, includ- ing soil erodibility, water holding capacity and the storage of nitrogen and organic C. Well-developed crusts form stratified micro-environments at the soil surface ( Figure 6). Soil cyanobacteria exude C-rich sugar compounds in the form of extra-cellular polysaccharides (EPS) as sheaths which protect bacterial cells from desiccation during dry seasons and prolonged droughts. ...
Context 3
... cyanobacteria exude C-rich sugar compounds in the form of extra-cellular polysaccharides (EPS) as sheaths which protect bacterial cells from desiccation during dry seasons and prolonged droughts. The pale green colouration of cyanobacterial chlorophyll is also visible within the EPS-bound particle layer ( Figure 6). Sand grains attached to EPS sheaths may also be seen to be dangling from the underside of the crust fragment even after removal from the soil surface. ...
Context 4
... and Belnap (1996) have observed photosynthesis in BSCs down to a depth of 600 mm. Cyanobacterial sheaths are often bound to the largest translucent quartz grains ( Figure 6) in the crust, indicating that cyanobacteria may well be utilising the light harvesting qualities of these grains to as- sist photosynthesis at depth. ...
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Citations
... The amounts of the land area concerned for each of these years are also presented in km 2 (Figure 5b). Overall, LC transitions over the 15 years contributed to a 3.3% net gain in SOCs.The 2015 SOCs map(Figure 5a) was visually compared with the carbon map over Southern Africa(Scharleman, Hiederer, Kapos, & Ravilious, 2009) and values were within the ranges estimated byThomas, Hoon, Mairs, and Dougill (2012) for southwest Kalahari (<39 t ha −1 ), and the Makgadikgadi salt pans in northeastern Botswana (>50 t ha −1 ). ...
... This underestimation is probably due to the inclusion of woody shrubs in the grassland category for this study. The Botswana landscapes are often a mix of trees, shrubs, and grasses such as in south-west Kalahari(Thomas et al., 2012). At 7% of the total land area in 2015 based on our results, croplands seem overestimated when compared with FAO data which puts cropland at 0.5% in 2015 (Statistics Botswana, 2018). ...
Achieving land degradation neutrality (LDN) has been proposed to stem the loss of land resources globally. To date, LDN operationalization at country level has remained a challenge both from a policy and science perspective. Using an approach incorporating cloud‐based geospatial computing with machine learning, national level datasets of land cover, land productivity dynamics, soil organic carbon stocks were developed using the example of Botswana. LDN and proportion of degraded land were assessed. Between 2000 and 2015, grassland lost 17% of its original size, the highest level of loss for any land category; land productivity decline was highest in artificial surface areas (11%), whereas 36% of croplands show early signs of decline. With the use of national metrics (NM), areas under degradation were found to be 32.6% compared to 51.4% surface area when global default datasets (DD) are used. Estimates of degraded land computed with NM and DD were validated in Palapye, an agro‐pastoral region in eastern Botswana, where Composite Land Degradation Index (CLDI) field‐based data exists. Comparing land degradation (LD) in the three datasets (NM, DD, and CLDI), NM estimates were closest to the field data. The extra efforts put into developing national level data for LD assessment in this study is, thus, well‐justified. Beyond demonstrating remote sensing viability for LD assessment, the study developed procedures for generating and validating national level datasets. Using these procedures, LD monitoring will be enhanced in Botswana and elsewhere since these remote sensing datasets can be updated using freely available satellite datasets.
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Land use change in the rangelands of Botswana is affecting soil properties. Pastoralism is the only viable livelihood for many rural poor in Botswana, and privatization of communal land is reducing the area of grazing available to those without land tenure. This has elevated stocking densities in communal areas, increasing pressure on soils and vegetation. Field experiments were used to quantify the impact of grazing intensity on soil organic carbon (SOC) and CO2 efflux on biologically crusted aridisols in the SW Kalahari. Differences in soil properties under an encroaching shrub species, under trees and in open-grass sites were also determined. Soil CO2 efflux was significantly higher and SOC significantly lower on soils for 2 years after intense grazing disturbance compared to control soils. In contrast, light grazing disturbance resulted in an increase in chlorophyll a and SOC but no change in soil CO2 efflux. Total C and N concentrations and soil CO2 efflux were significantly greater in soils under shrubs and trees compared to grass sites. The microclimate was also different, with higher summer temperatures but lower winter temperatures at the grass sites compared to shrubs. Shading under vegetation canopies significantly reduced solar radiation reaching the soil surface. The findings demonstrate the importance of biological soil crusts (BSCs) to SOC and that intensive grazing, which destroys crusts through trampling and burial, adversely affects C sequestration and storage. Grazing management which results in less frequent and intense disturbance to the soil surface could benefit SOC stores by maintaining a healthy BSC cover. However, simplistic notions of reducing cattle numbers or implementing rotational grazing regimes in communal rangeland are problematic given the much wider significance of cattle ownership to pastoral communities.
The Kalahari of southern Africa is characterised by sparse vegetation interspersed with microbe-dominated biological soil crusts (BSC) which deliver a range of ecosystem services including soil stabilisation and carbon fixation. We characterised the bacterial communities of BSCs (0–1 cm depth) and the subsurface soil (1–2 cm depth) in an area typical of lightly grazed Kalahari rangelands, composed of grasses, shrubs, and trees. Our data add substantially to the limited amount of existing knowledge concerning BSC microbial community structure, by providing the first bacterial community analyses of both BSCs and subsurface soils of the Kalahari region based on a high throughput 16S ribosomal RNA gene sequencing approach. BSC bacterial communities were distinct with respect to vegetation type and soil depth, and varied in relation to soil carbon, nitrogen, and surface temperature. Cyanobacteria were predominant in the grass interspaces at the soil surface (0–1 cm) but rare in subsurface soils (1–2 cm depth) and under the shrubs andtrees. Bacteroidetes were significantly more abundant in surface soils of all areas even in the absence of a consolidated crust, whilst subsurface soils yielded more sequences affiliated to Acidobacteria, Actinobacteria, Chloroflexi, and Firmicutes. The common detection of vertical stratification, even in disturbed sites, suggests a strong potential for BSC recovery after physical disruption, however severe depletion of Cyanobacteria near trees and shrubs may limit the potential for natural BSC regeneration in heavily shrub encroached areas.
