Effects of season and frequency of burning on soils and landscape functioning in a moist montane grassland

Abstract

The effects of burning on soil properties and landscape function were investigated in a long-term experiment comparing different burning strategies in a moist montane grassland. Total C, total N, total S, bulk density, plant-available nutrients, and soil acidity were determined in the top 200mm of soil, together with vegetation basal cover at the soil surface. The no-burn treatment had the lowest basal cover (14.8%). Basal cover for the burnt treatments ranged from 19.0% (five-year spring burn) to 25.4% (alternate autumn/spring, burnt every 18 months). The organic matter content of these soils was very high with total carbon ranging from 114g kg^–1 in the 0–50mm layer to 77g kg^–1 in the 150–200mm layer. Bulk density was very low, being 0.57g ml^–1 in the 0–50mm layer. There were no significant effects of burning on the quantity of soil organic matter. The C:N ratio was significantly affected throughout the top 200mm by burning treatments; in the 0–50mm layer it ranged from 14.43 in the no-burn treatment to 16.14 in the treatment burnt every 18 months. Higher C:N ratios in frequently burnt treatments suggests that grassland productivity is N-limited in these treatments. In the top 50mm, soil pH is lower in treatments burnt infrequently (5 year and no burn) than in those burnt frequently, whereas concentrations of basic exchangeable cations (K, Ca and Mg) were lower in treatments burnt infrequently (five-year and no burn) than in those burnt frequently. The higher pH and concentrations of basic cations in frequently burnt treatments was probably due to greater cycling of nutrients to the soil surface as a result of higher productivity and deposition of nutrients in ash, together with reduced leaching of cations with nitrate. Landscape Function Analysis was used to measure the functioning of the landscape in terms of scarce resources and the processes that maintain these resources. All sites were highly functional, irrespective of the burning treatment applied. The infrequently burned sites had significantly higher nutrient cycling and infiltration indices than frequently burnt sites and these indices were correlated well with soil chemical properties (acidity, acid saturation, Ca, Cu, K, Mg, P and pH). No significant differences were found between treatments for the stability index.

Keywords: basal cover, fire, landscape function analysis, soils, montane grasslands

African Journal of Range & Forage Science 2007, 24(1): 9–18