Impact of small scale gold mining on soils of the wetland forests in east Usambara, Tanzania

Abstract

Soil pits in disturbed and natural wetland forests of East Usambara were excavated and soil profile samples collected and analysed for physical and chemical properties. Their physical and chemical characteristics were found to be sharply different from those of the surrounding hills. Differences included lack of A Horizon, water saturation, presence of very high organic matter, very low pH and hence very poor in basic cations, and low microbial activity. Nutrient recycling, especially nitrogen and phosphorus, tended to shift from mineralization to immobilization. Effects of soil disturbance on both physical and chemical characteristics of the soils were also very vivid. The soils disturbed from gold mining possessed an altered soil structure, improper development of soil horizons and removal of organic matter on the surface. Burried A horizons were found in all plots at the disturbed site. Carbon:Nitrogen ratios were significantly higher in the undisturbed forests suggesting greater capacity in carbon sequestration. Both sites existed in the same general area but while the undisturbed one possessed all histosol (organic soil) characteristics, the disturbed site soils were heterogenous and possessed characteristics associated with mineral soils (inceptisols). The present study suggests that disturbance has led into a reduction in the supply capacity of minerals such as nitrogen and phosphorus, and the low supply of such minerals will inhibit regrowth of the natural vegetation.