This study was conducted to evaluate the effects of three land use management (natural forest, crop land and natural fallow) and soil depth on soil organic carbon fractions. The study was carried out in Toungo and Jada, Adamawa State, Nigeria. Three transects were cut 100 m apart in each of the study sites and four sampling plots of 20 m × 20 m in dimension were laid in alternate positions along each transect at 50 m interval. Soil samples were collected at depth of 0-15, 15-30, 30-45 and 45-60 cm and at three points along the diagonal of each plot using a 3 cm diameter soil auger. Soil contents of mineral-associated organic carbon fraction were higher than the particulate organic carbon pool in all land use management systems. Surface soil (0-15 cm) total organic carbon (TOC) content was highest under the natural forest (1.94%) and lowest in the cropland (1.46%). From the results it was noted that, natural forest had capacity for increasing soil organic carbon to an appreciable concentration. Fallow contribution to soil organic carbon was minimal. This could be attributed to several factors such as wind erosion, grazing, bush burning and cutting of vegetation for fuel wood. The contents of total, particulate and mineral-associated soil organic carbon was significantly influenced by land use management and soil depth. All the different land use types showed highest accumulation of the various carbon fractions in the surface layer (0-15cm). This high level of organic carbon stock in the surface layer could be due to the slow of mixing of the soil.
Key words: Land use, carbon fraction, forest, mineral, particulate organic carbon.
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