Land use and site effects on the distribution of carbon in some humic soil profiles of KwaZulu-Natal, South Africa

Land use effects on the stocks of soil organic carbon (SOC) are generally based on the topsoil. Although subsoil horizons have lower concentrations, they contain a significant amount of SOC which may be more strongly protected than that in the topsoil layers. Analysis of SOC storage must therefore include the whole profile in respect of climate change mitigation. Humic soils in South Africa have high organic C in the A horizon, while the amount of C stocks through the whole profile depth is unknown. This study was conducted at six sites in KwaZulu-Natal Province to determine the effect of land use and site factors on C stocks, texture, pH and extractable aluminium (Al) and iron (Fe) concentrations and their vertical distribution to 100 cm in soils with thick (>45 cm) and thin (<45 cm) humic A horizons. The land use at some sites had been changed from grassland to maize and cultivated pasture and at others from forest to sugarcane farming. Cultivation with field crops reduced the organic C, mainly in the upper 20 cm (from 110 to 22 g C kg(−1)), with limited effect in deeper layers. The soils with thick humic A horizons and coarser texture stored more C in the deeper layers compared to those with thin humic A horizons and finer texture which had more of the C stocks in the 0–20 cm depth. Although cultivation reduced the soil C stocks in the surface layers, land use did not significantly affect the overall C stock (0–100 cm) at all sites. The high contents of extractable Fe (up to 340 mg kg(−1)) and Al (up to 3700 mg kg(−1)) stabilised the soil C and were more important than the effects of either land use or other site factors.