Changes in Soil Organic Matter Associated with Land Use of Arenosols from Southern Botswana

The effect of land use on sandy soils of southern Botswana was carried out by comparing the composition and properties of soil organic matter. Non-disturbed and disturbed soils were sampled from savanna ecosystems (Central District and Kweneng District). The biodegradability of organic matter was evaluated by incubation in the laboratory. Humic fractions were quantified and humic acids were analyzed by visible and infrared spectroscopy. The results indicate that continued disturbance, whether due to grazing or subsistence farming, has resulted in small yet significant changes in the concentration of available nutrients and organic matter in the soil. Nevertheless, substantial changes could be established in the soil C/N ratio, in the humic acid/fulvic acid ratio, and in the biodegradability of soil organic matter and the structural characteristics of humic acids. The increased aromaticity of humic acid (visible and IR spectroscopies) following disturbance suggests increased biogeochemical activity and/or the impact of abiotic processes (such as periodic fires) selectively removing aliphatic constituents. The overall results indicate low potential soil fertility, the sustainable preservation of which depends more on features related to quality than on the total amount of the soil organic matter, which shows aromatization parallel to its degree of association with the mineral fraction.