EFFECT OF LAND USE LAND COVER ON SELECTED SOIL PROPERTIES AND SOIL ORGANIC CARBON STOCK IN GUDER WATERSHED, NORTH WESTERN ETHIOPIA

Abstract

Continuous conversion of land for different agricultural production purpose has resulted in degradation of soil resources. The decline of soil fertility and soil organic carbon stock become notable phenomena in the highlands of Ethiopia. The present study was conducted to analyse and quantify the land use/land cover changes over the periods of 28 years and to investigate the influence of A. decurrens tree-based farming system on selected soil properties and soil organic carbon stock in Guder watershed. Data for this study were obtained from field surveys, satellite images and laboratory analyses. Satellite images of Landsat TM for 1990, ETM for 2015 and OLI/TIRS for 2018 were obtained and analysed using ERDAS Imagine 2014. The maximum likelihood algorithm of supervised classification was used to generate land use/land cover maps. A total of 90 soil samples (5 land uses × 2 soil depths × 3 slope positions× 3 replications) were collected and analysed for selected soil physicochemical properties. Soil organic carbon stock was estimated following the standard procedure. Soil data were subjected to three-way ANOVA. The result revealed that major land use/land cover types, namely grassland, forest land, and settlement area were expanded at a rate of 0.2, 1.9 and 2.8% per year, respectively. On the other hand, cropland declined by 1.01% per year. The highest clay (54.6%) content and bulk density (1.24 g cm-3) were recorded from A. decurrens tree farmland and cropland soils, respectively. The maximum soil pH was recorded from natural forest (5.99), subsurface soil layer (5.56) and lower slope (5.57). The lowest soil organic carbon (1.91%) and total nitrogen (0.11 %) contents were observed in cropland. Higher available phosphorus (13.58) mg kg-1 was obtained in the topsoil depth of A. decurrens tree farmland at the lower slope. Higher (229 Mg ha-1) soil organic carbon stock was recorded in the topsoil layers of the natural forest land. Generally, A. decurrens tree farmland contributed about 33% higher soil organic carbon stock than cropland, indicating that planting of trees on degraded agricultural land was able to restore 90 Mg ha-1 CO2e from the atmosphere. The semivariogram of soil organic carbon stock was best fitted to the exponential model with a nugget/sill value of between 0.25 and 0.75. The predicted map showed that soil organic carbon stock ranged from 25 to 438 Mg C ha-1 in the upper 50 cm soil depth of Guder watershed. Among the studied soil quality indicators total nitrogen, cation exchange capacity, and pH were strongly and positively correlated with soil organic carbon stock. Overall, it can be concluded that land conversion from cropland into A. decurrens tree farmland improves soil health. Therefore, the present study indicated that integration of A. decurrens tree in the agricultural landscape with appropriate land use and integrated soil fertility management approach can restore soil organic carbon and reverse soil quality deterioration that meets the goals of climate-smart agriculture and demands for basic human needs.