Effects of tillage on heathland soil: disturbance of a natural mountain fynbos system in the southwestern Cape, South Africa

A natural mountain heathland site in the southwestern Cape, South Africa, was cleared by fire and then tilled to simulate cultivation practices employed by commercial growers of indigenous wild flowers. The effects of tillage on components of the system were investigated, with special reference to the nutrient, energy and water regimes of the soil. No significant difference was found between nutrient levels in the soil cleared by burning, and those in soil which was tilled as well. For both situations mean values of total N (Kjeldahl analysis) were between 450 and 500 μg-1, P (Bray No. 2) less than 5 μg g-1, and exchangeable K between 0.16 and 0.18 meq per 100 g. However, results showed that tillage significantly altered the energy and water regimes of the system during the dry summer period. Short-wave albedo of the soil surface was enhanced by tillage, the maximum observed change being from 16 to 22% when soil was at its driest. As a result of this increased reflectivity, summer soil surface temperatures were significantly lower on the tilled soil. Gravimetrically measured soil water maintained significantly higher levels at a depth of 150 mm on the tilled soil during the summer (more than 10 g per 100 g) relative to undisturbed soil (less than 5 g per 100 g). Shallower soil at that time was drier than this on both treatments (both less than 5 g per 100 g), but showed the same significant trend of more water being retained in the tilled soil. A correlation analysis showed that albedo was significantly correlated with near-surface soil water, positively during the summer and negatively during the winter. The reverse was true of the relationship of albedo with soil surface temperature (i.e. cool and reflective in summer, and warmer and more absorptive in winter). The first season seedling density of a plant species occurring naturally at the site, Leucadendron xanthoconus (Proteaceae), was included in the correlation analysis, and was found to be significantly correlated with soil surface temperature (positively) and near-surface soil water (negatively) during summer, with reversal of both during winter. It is suggested that while tillage has distinct advantages for the marginal cultivation of commercially desirable species in this mediterranean-type heathland, the altered soil environment may have adverse secondary effects such as the promotion of fungal disease, and creation of an unfavourable environment for establishment of other species useful for stabilisation of the transformed system. © 1992.