Comparative Analysis of the Impacts of Commercial Exotic Forestry on Soil Fertility In the Southern Highlands of Tanzania: A Study of Pine, Eucalyptus and Black Wattle Plantations

Pine, eucalyptus and black wattle trees have been planted worldwide for commercial forestry and carbon sequester. These species affect soil fertility differently depending on climate and ecosystem. In the southern highlands of Tanzania, these trees replaced most of the native forests, grasslands and cropland, but their impacts on soil fertility are uncertain. The study compared Pinus patula, Eucalyptus grandis and Acacia meansii (black wattle) forest soils to surrounding natural forests, grasslands and cropland. 18 study sites were selected based on the related location and land use. Each site had two transects; each transect had three 25 m x 20 m rectangular plots for soil sampling. Soil pH, macro and micro-nutrients were measured to determine the soil fertility. The results show that pine, eucalyptus and wattle trees had significantly lower soil organic carbon than cropland and natural forests (p<0.05). The pine and wattle forests had lower pH than the surrounding cropland and natural forests (p<0.05). Farmlands and natural forests had significantly higher mean soil total nitrogen than the pine, eucalyptus and wattle forests (p<0.05). However, the wattle forests had a higher mean TN (1.16%) than the pine and eucalyptus forests (0.13%). Liming and application of phosphate-based fertilisers are recommended when farmers use the harvested exotic tree farms for food crop production.