Erosion Control and Carbon Sequestration by Bench Terraces and Agroforestry in the West Usambara Mountains, Tanzania

Abstract

Soil erosion by water on steep slope areas is causing a land degradation problem in The West Usambara Mountains in Tanzania resulting in some land becoming unproductive. Traditional agroforestry (AF) practices seemed to have the potential to neutralize these effects maintaining the productivity of the fields and working as a biomass carbon pool, over others soil and water conservation (SWC) measures that have been promoted in the area by the government and private projects for decades like bench terraces. The aim of this thesis is to make a comparison between bench terraces, and traditional AF systems, taking into account three aspects a) effectiveness of soil erosion control, b) carbon sequestration rates, and c) net economic profit. In this study, nine agricultural plots located in the Lushoto district were used as experimental fields. The ACED method was used to quantifies soil loss from sites based on field observations and measurements of visual erosion features. The hillslope version of the revised Morgan, Morgan and Finney water erosion model (hMMF) was used to establish the water erosion patterns. C sequestration was estimated on the base of the biomass carbon and the soil organic carbon (SOC). The biomass was estimated using destructive methods and allometric equations, and the Walkley and Black method was used to determine SOC. Finally, socioeconomic information was collected to characterize its influence on the use of a specific SWC and to estimate the net operating profit of the fields. The results showed that traditional AF and terraced fields are equally effective to control soil erosion, but important differences in terms of carbon sequestration and economic benefits are evident between these two SWC measures. AF fields sequestrate six times more carbon than terraced fields, while terraced fields generate net incomes 2.6 higher than AF fields due to the differences in crop market prices and the inadequate management of AF fields.