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 green house 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 was to investigate the effect of LUC and slope positions on SOC stocks in Kitabi watershed. Lands at 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 top 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 sampling was done at two depths 0-20 cm and 20-50 cm in each land use category using a soil auger. To make a composite soil mixture, five samples were taken from 10 m x 10 m plots in each land use category.SOC laboratory analysis was done using Loss on ignition (LOI). One-Way ANOVA and Games Howell tests were used to analyze the data. Results indicate that LU types significantly affect SOC stocks (p < 0.001) while slope position does not (p =.491). The interaction of LU and slope position has no significant effect on SOC stocks across the three land use types (p = .061). Good management practices in community tea and soil erosion control measures in annual crops are recommended.