THOMAS BAGAMSAH, MANFRED DENICH, PAUL L. G. VLEK
Centre for Development Research (ZEF), University of Bonn, Ecology and Natural Resources Management, Germany
The research was conducted in the northern Ghana to investigate the destructive power of bushfire in terms of nutrient losses. The losses were estimated in three sites with different vegetative structures; grass/herbs with scattered trees and shrubs (Sambu), open woodland savannah (Jimle) and woodland savannah (Dagomba-Line).
The nutrient loads contained in the fuel load before fires, and that of ash were subsequently compared to determine the fluxes of the micronutrients. The magnitudes of the fluxes were greatest in the herbaceous vegetation with scattered trees and shrubs where fuel load were the highest at 6.7 t ha-1. In these sites 41-93% of all measured nutrients in the fuel load were transferred to the atmosphere or lost from the sites during the bushfires. Fore each nutrient, the proportion transferred to the atmosphere as entrained ash was calculated by assuming that calcium was not volatilised during bushfires. If the transfer of entrained ash represents local redistribution only, then rainfall accession and deposition of these particulates should replace most of the losses of all the nutrients except nitrogen (N) and to some extent phosphorus (P). Estimates of biological fixation of N appear to be insufficient to replace the annual losses of N. Further research is required to confirm this because reliable estimates of rates of non-symbiotic fixation in this area are scanty, and possible role of non-symbiotic and other flora are unknown.
The study concludes that, under a regime of annual bushfires in which the burns are complete, the nitrogen reserves of savannahs similar to those in northern Ghana may be depleted, and this may subsequently lead to the unsustainable loss of nitrogen. To reduce the likelihood of this occurring, the contribution of fire to nutrient cycling would need to be reduced.
The current work also indicates that, the transfer of P to the atmosphere during bushfires is much greater than previously recognised. Natural replacement of P losses is likely to be slow in this environment because inputs in rainfall and weathering in rainfall and from soil mineral are small.
Keywords: Bushfires, nutrient losses and fuel load, Savannah