Effects of land use and site on organic carbon fractions in some humic soil profiles of KwaZulu-Natal, South Africa

Soil organic matter (SOM) and its different carbon (C) fractions play a vital role in storing C and are a source of plant nutrients in natural and managed ecosystems. This study was conducted at six sites in KwaZulu-Natal Province to determine the effect of land use on the distribution of C fractions in selected humic soil profiles from different sites, using chemical and physical fractionation. Land use at some sites had been converted from natural grassland to maize and cultivated pasture and at others from native forest/wattle plantation to sugarcane farming. Humic acids (HA), fulvic acids (FA), hot water extractable C (HWEC), particulate organic matter C (POM-C) and mineral-associated organic C (MAC) were analysed to 1 m depth. Cropping lowered POM-C and HWEC, in the top 100 mm, irrespective of site differences, under pasture, grassland, forest and wattle plantation. Amongst the most labile fractions, FA predominated, resulting in a HA:FA ratio < 1 for pastures. The MAC was not affected by land use, irrespective of site differences. These findings reveal that organic carbon in humic soils can be decreased by cultivation but does not inevitably indicate the loss of C storage in the stable pool.