Factors controlling soil organic carbon sequestration under exotic tree plantations: A case study using the CO2Fix model in southwestern Ethiopia

Models are important research tools for predicting the build-up of soil organic carbon (SOC), because they provide an increased insight into factors that are involved in the build-up process. The CO2Fix (v. 3.1) model was used to examine the influence of litter production, litter quality and microclimate on differences in SOC accretion under exotic tree species established on farmland in southwestern Ethiopia. The SOC storage under Cupressus lusitanica was larger than that under the other two investigated species (Pinus patula and Eucalyptus grandis). This was mainly because of the higher total litter input and higher proportion of fine woody litter (branches and coarse roots) in the Cupressus stand. SOC accretion was greater under Pinus than under Eucalyptus. However, the total litter input in the Pinus and Eucalyptus stands was nearly the same. The difference between the Pinus and Eucalyptus stands was best explained by the fact that Pinus produced more fine woody litter than did Eucalyptus. Litter quality and microclimate only accounted for a minor part of the differences in SOC storage in the Cupressus, Pinus and Eucalyptus stands. Therefore, the results suggested that total litter input and the proportion of fine woody litter were the main factors that accounted for the inter-specific differences in SOC accretion.