Soil properties and processes driving the leaching of nitrate in the forested catchments of the eastern escarpment of South Africa

Abstract

Elevated levels of nitrate in streamwaters draining forested catchments in the eastern escarpment area of South Africa are thought to occur as a result of land use change from natural high altitude grasslands to plantation forestry. In this investigation, soil samples taken from similar pedosystems in adjacent (about 50 m from each other) forest and grassland ecosystems were tested for a range of properties in order to determine differences, which might indicate the reasons for elevated leaching of nitrate from forest soils in these areas. Soils were collected monthly in the summer season and analysed for pH, total nitrogen, total phosphorus, organic carbon and net nitrogen mineralisation rates. From the soils collected with depth (10 cm intervals), a saturated paste extract was prepared from which the major anions, cations, electrical conductivity, pH and the nitrate sorption were analysed. Three parent rock types were studied namely dolomite, mixed dolomite and quartzite and quartzite. Soil pH reduction of as much as 0.51 units was found, with the median pH of 4.30 and 4.07 for grassland and forest soils, respectively. There was a high level of soil organic matter in soils of dolomitic origin, followed by those of mixed dolomitic and quartzitic origin and finally by those of quartzitic origin. The general trend with respect to the changes in the total nitrogen, total phosphorus and the C:N ratio ( P < 0.01) followed the trends shown by the soil organic carbon. The net rates of nitrogen mineralisation pointed significantly towards more rapid nitrogen mineralisation and nitrification rates in forest soils (average 0.9 μg N g soil −1 day −1) than those under the adjacent grassland sites (average 0.3 μg N g soil −1 day −1). This was more noticeable in the more clayey, dolomitic-derived soil at the beginning of the study period in September 2000. The concentration of water-soluble nitrate in the topsoil horizons in grassland soils was low (less than 0.2 mmol l −1) compared to that of the forest soils (up to 1.1 mmol l −1). The clearest nitrate sorption trends were provided from the graph representing the lowest level of nitrate addition (3.4 mmol/kg) in grassland soils. The results of this study suggest that afforestation has changed the capacity of the soils to retain nitrate. Nitrate leaching into streamwaters may impact on other land use activities in the lower reaches of the catchment.