Impact of grazing intensity on seasonal variations in soil organic carbon and soil CO ₂ efflux in two semiarid grasslands in southern Botswana

Abstract

Biological soil crusts (BSCs) are an important source of organic carbon, and affect a range of ecosystem functions in arid and semiarid environments. Yet the impact of grazing disturbance on crust properties and soil CO 2 efflux remain poorly studied, particularly in African ecosystems. The effects of burial under wind-blown sand, disaggregation and removal of BSCs on seasonal variations in soil CO 2 efflux, soil organic carbon, chlorophyll a and scytonemin were investigated at two sites in the Kalahari of southern Botswana. Field experiments were employed to isolate CO 2 efflux originating from BSCs in order to estimate the C exchange within the crust. Organic carbon was not evenly distributed through the soil profile but concentrated in the BSC. Soil CO 2 efflux was higher in Kalahari Sand than in calcrete soils, but rates varied significantly with seasonal changes in moisture and temperature. BSCs at both sites were a small net sink of C to the soil. Soil CO 2 efflux was significantly higher in sand soils where the BSC was removed, and on calcrete where the BSC was buried under sand. The BSC removal and burial under sand also significantly reduced chlorophyll a, organic carbon and scytonemin. Disaggregation of the soil crust, however, led to increases in chlorophyll a and organic carbon. The data confirm the importance of BSCs for C cycling in drylands and indicate intensive grazing, which destroys BSCs through trampling and burial, will adversely affect C sequestration and storage. Managed grazing, where soil surfaces are only lightly disturbed, would help maintain a positive carbon balance in African drylands.

Enclosures in West Pokot, Kenya: Transforming land, livestock and livelihoods in drylands

“…As an estimated 70 % of the world's poorest rely on income from pastoralism and agro-pastoralism, changes in vegetation and its associated ecosystem functions through the use of enclosures will have an immediate effect on their livelihoods (Thomas 2012). Vegetation ecological dynamics in drylands are often referred to as following either equilibrium or nonequilibrium paradigms, where the former largely is governed by biotic factors, such as grazing pressure and partial rest, while the latter is governed by abiotic factors like rainfall and especially the variability of rainfall.…”

Microbial Nursery Production of High-Quality Biological Soil Crust Biomass for Restoration of Degraded Dryland Soils

“…Yet, several land uses, such as agriculture (20), livestock grazing (21)(22)(23), and recreation (24), negatively affect biocrusts. Recovery is a slow process, given that biological activity in these organosedimentary assemblages is highly limited by water and restricted to short periods of time after precipitation events (25) and has been estimated to be at least decades (3).…”

Seasonal differences in soil CO2 efflux and carbon storage in Ntwetwe Pan, Makgadikgadi Basin, Botswana

“…⁎ The negative value implies a net uptake of CO 2 to the crust. palm woodland, saline grasslands and salt pan sediments (Table 1)is considerably more than the estimated 6-10 tons organic C ha − 1 for the surrounding Kalahari Sand soils and 39 tons C ha − 1 for calcrete pans (Thomas, 2012).…”

“…Soil/sediment CO 2 efflux was determined using static respiration chambers (based on a design described in Hoon et al, 2009 with modifications outlined in Thomas, 2012). Chambers and the methodology were designed to minimise the likelihood of errors in efflux estimation associated with the use of closed chambers (Davidson et al, 2002;Pumpanen et al, 2004), particularly changes in the surface environment, pressure, and the soil-gas diffusion gradient (further details can be found in Thomas et al, 2011).…”

“…2). Organic C inputs from biological crusts to soils are known to be regionally and globally significant (Elbert et al, 2012;Thomas, 2012), but there are very few estimates of their contribution to the C store of salt pans (Chairi et al, 2010 is a rare example).…”