Analysis of soil carbon and income over Acacia decurrens and Eucalyptus globulus land uses in the highlands of Ethiopia

Poverty alleviation and the realities of adapting to climate change have contributed to a rapid expansion of exotic tree plantations in the Ethiopian highlands. In this region, improving income and soil carbon content became the driving forces for planting non-native trees, among which Acacia decurrens and Eucalyptus globulus, are prominent. However, farmer decision-making on land-use selection is not well informed by local evidence-based best practices. This study aims to analyze the comparative advantages of Acacia decurrens and Eucalyptus globulus land uses on income and soil organic carbon (OC) content in the highlands. Socio-economic data and farm-plot-based soil data were collected for the analysis of income and soil OC content (%), respectively. For the socio-economic survey, a total of 120 sample household farmers were selected through a stratified and systematic random sampling. The farmers were evenly split among those who planted Acacia decurrens (60) and those who opted for Eucalyptus globulus (60). A total of 48 (6 replications x 2 kebeles x 2 plantation type x 2 soil depths) composite soil samples were collected for soil analysis. The collected socio-economic and soil datasets were subjected to descriptive, one-way ANOVA and regression analyses. A correlation matrix was also employed to analyze the effects of plantation types on organic carbon and other selected soil properties. In the 6 years observed, the local farmers generated a net profit of 46,587.6 ETB (1 USD =43.9 ETB) per hectare from Acacia decurrens plantation and 825,042.8 ETB from Eucalyptus globulus. The net income generated from Eucalyptus globulus was found to be approximately 17 times more than that of Acacia decurrens. The average OC percentage varied from 4.1% in the Acacia decurrens to 3.5% in the Eucalyptus plantations at the soil depth of 0-15 cm. The value of OC was found to be significantly different (p < 0.001) in the two land uses. Income and soil carbon content were found to have no significant association with the choice of farmers between the two land uses. A higher OC value was observed in the Acacia decurrens plantations, which can improve land productivity. Meanwhile, Eucalyptus was found more advantageous for improving income and quicker return on farmer investment. Considering the differing competitive advantages of the two land uses, mixed plantations in separate plots could be more appropriate to harmonize interests in increasing income and improving soil carbon content.