HUMIC SUBSTANCES AND MINERAL-ASSOCIATED SOIL ORGANIC CARBON AS INFLUENCED BY LAND USE IN A SOUTHERN GUINEA SAVANNA, NIGERIA

Abstract

Land use management has been observed to affect labile, stable and humified soil organic matter fractions, but the magnitude of these changes has not been established in the Southern Guinea savanna of Adamawa State, north eastern Nigeria. This study investigated effects of five land use management systems (undisturbed natural forest, crop land, grazing reserve, forest plantation and fallow land) on the status of humic substances and mineral-associated soil organic carbon. The study was carried out in and around Gumti sector, the northern half of Gashaka-Gumti National Park, covering Toungo local government and Jada local government areas of Adamawa State. Prior to soil sampling, soil profile studies were carried out and the profile morphological characteristics of the dominant soil types were determined. Three transects that are 100 m apart were cut in each of the study site and four sampling plots of 20 m × 20 m in dimension were laid in alternate positions along each transect at 50 m interval. Soil core samples were collected diagonally at a depth of 0-15, 15-30, 30-45 and 45-60 cm and at three points in each plot using a 3cm diameter soil auger. From the results it was noted that, natural forest, plantation and grazing reserve had potentials for storing organic carbon of appreciable concentration. These results showed that both mineral-associated and humified organic matter are better protected under natural forest, plantation and grazing reserve and consequently less vulnerable to mineralization. The study revealed that seven (7) years fallow contribution to soil organic carbon was minimal. This might be attributed to several factors such as livestock grazing, incidence of annual bush burning and cutting of vegetation for fuel wood. The relationship between stable soil organic carbon and various soil nutrients indicates that positive correlation exists. This shows that availability of humic, fulvic acid, exchangeable magnesium, exchangeable calcium, total phosphorus, cation exchange capacity and total nitrogen depend to some extent on the status of mineral-associated soil organic carbon. The content of humic was significantly influenced by land use and soil depth. Highest humic acid content (17.42 g/kg) was recorded under plantation in the soil surface layer (0-15 cm), followed by natural forest (15.16 g/kg), grazing reserve (14.16 g/kg), and fallow land (12.31 g/kg). The fulvic acid content also followed the same pattern as obtained for humic acid. The highest %mineral associated soil organic carbon was recorded for plantation and natural forest at the surface layer (0-15 cm) (1.19 and 1.15 respectively), followed by the grazing reserve (1.12). The crop land and the fallow land recorded 0.95 and 0.87 respectively. There exist differences across land use types in their potentials of storing total, particulate, mineral-associated organic carbon and humic substances in the Southern Guinea Savanna soils of Adamawa State, Nigeria. This result provides valuable information for implementing tillage practices (such as zero tillage) that can favour carbon sequestration and improve soil quality.