Cross-rangeland comparisons on soil carbon dynamics in the pedoderm of semi-arid and arid South African commercial farms

The increase of carbon (C) in the pedoderm (0-50 mm soil layer) improves ecosystem functioning and stability. This study evaluated quantitative and qualitative C responses to grazing intensity in the pedoderm of grassland and savanna ecosystems. These ecosystems differ in terms of soil properties, climate, vegetation quality and livestock density. Three commercial farms were chosen from each ecosystem. Each farm exhibited two grazing intensities (heavy and light grazing) by livestock which induced poor and good forage quality, respectively. Vegetation quality changes were observed along a gradient of grazing pressure, with respect to the distance from the water point. Soil samples of the pedoderm were taken in each of the two zones to analyse the effects of grazing on soil quality parameters. Soil samples were analyzed for soil organic carbon (SOC), soil inorganic carbon (SIC), labile and humic C fractions. The SOC structure was determined with C-13 NMR spectroscopy. Indices of the extent of SOC decomposition were calculated. Grazing pressure per se did not influence soil C fractions. However, clayey grassland soils in the cool-moist climate exhibited more resilience to C losses than sandy savanna soils under hot-dry climatic conditions. Thus, comparisons between the two ecosystems showed that on average, all C fractions were 35-97% higher in the clayey moist-cool grassland ecosystem compared to the sandy dry-hot savanna ecosystem, regardless of the grazing pressure. A decline in O-alkyl C (7%) with concomitant increase in recalcitrant alkyl C (6%) in the grassland ecosystem, indicated that constant animal trampling induced SOC decomposition, and loss of labile C fractions in the heavily grazed zones. Molecular composition of SOC in the savanna ecosystem was more influenced by plant functional types, thus C allocation patterns, than decomposition.

Automated summary

Increasing carbon content in the top 50 mm of soil can improve ecosystem functioning and stability. This study found that grassland soils in a cool-moist climate were more resilient to carbon losses than sandy savanna soils under hot-dry climatic conditions, regardless of grazing pressure.