Carbon sequestration in secondary pasture soils: a chronosequence study in the South African Highveld

Soil restoration is a means of combating desertification in semi-arid and arid parts of the world. There, vast areas of the cropped soil degrade, particularly because of the loss of organic matter. One approach to reverse this loss is the conversion of cropland into permanent grassland for use as pasture. This study was designed to evaluate how fast and to what degree degraded cropland may re-sequester soil organic carbon (SOC) when converted into permanent secondary pasture. Topsoil samples (0-5, 5-10 and 10-20 cm) were taken from chronosequences of secondary pastures (1 to 31 years old) at three agro-ecosystems in the semi-arid Highveld of South Africa. Long-term croplands and primary grassland used as pastures served as the controls. In bulk soil samples (< 2 mm) and their clay (< 2 mu m), silt (2-20 mu m), fine sand (20-250 mu m) and coarse sand (250-2000 mu m) fractions, the contents of carbon (C) and nitrogen were determined. In all three agro-ecosystems, using a mono-exponential model, the SOC stocks increased exponentially until a maximum was reached 10-95 years after land conversion. This gain in SOC was clearly pronounced for the top 0-5 cm of soil, but hardly detectable at 10-20-cm depth. The sand fractions recovered organic C more rapidly but less completely than did the finer size separates. Overall, between 9.0 and 15.3 t of SOC were sequestered in the 0-20 cm of surface soil by this land conversion. Thus, the SOC recovery in the secondary pastures resulted in SOC stocks that were 29.6-93.9% greater than those in the arable land. Yet, in no agro-ecosystem, at any soil depth, nor in any soil fraction, did the measured SOC content reach that of the primary grassland. In part this can be attributed to a slightly finer texture of the primary grassland that had not lost silt through wind erosion or had never been used as arable land because of slightly elevated clay contents. Overall it appears, however, that previous losses of SOM cannot easily be rectified, suggesting that the native primary grassland soils are only partially resilient to land-use change.