Effect of land use change and slope position on soil organic carbon in Kitabi Watershed ,Rwanda

Abstract

Soil organic carbon (SOC) is widely used as a proxy for soil health and soil fertility. Land use change (LUC) has been implicated as one of the factors leading to the loss of SOC and increased release of greenhouse gases (GHGs) to the atmosphere hence contributing to global climate change (GCC). Rwanda is a mountainous country and has faced a challenge of land use inter-conversions between forests, perennial crops and annual crops over the years. The impact of LUC on SOC stocks is poorly documented. The aim of this study is to investigate the effect of LUC and slope positions on SOC stocks in Kitabi watershed. Landsat TM-5 images of 2008 and 1986 were used to make a land use change map using Maximum Likelihood Classification (MLC) algorithm in Erdas. Slope positions were derived from the DEM using Topographic Position Index (TPI) tool and the output raster was overlaid with the LUC map to identify sample strata. Soil samples were obtained using stratified topo-sequential random sampling from annual crops, community tea and factory tea. A total of 134 soil samples were taken from the three LU categories along each of the three slope positions: upslope, middle slope and down slope. Soil samples were taken from 10 m x 10 m plots in each of the three land use categories at two depths 0- 20 cm and 20-50 cm using a soil auger. A total of 62 farmers from the three LU types were interviewed about the farm management practices in the study area. SOC laboratory analysis was done using Loss on ignition (LOI). One-Way ANOVA and Games Howell tests were used to analyse the data. Results indicate that 17.1% of land was converted from annual crops to tea plantations. Significant differences in SOC stocks exist across the three LU types (p <.001). Converting land from annual crops to factory tea significantly affects SOC stocks (<.001) while LU conversion from annual crops to community tea has no significant effect on SOC stocks (p >.05). Slope position has a significant effect on SOC stocks in annual (p <.05) but it has no significant effect on SOC stocks in tea plantations (p >.05). The interaction of LU and slope positions has no significant effect on SOC stocks across the three land use types (p = .061). Soil erosion control measures in annual crops to reduce loss of SOC and soil nutrients are recommended.