Geochemistry as a Control on Dissolved Organic Carbon and Dissolved Nitrogen in African Tropical Forest Soils

Abstract

Tropical African soils contain some of the largest worldwide soil organic carbon (SOC) stocks within the first meter of soil, making them relevant to the global carbon cycle. Here, the effect of these tropical carbon stocks on dissolved organic carbon (DOC) and dissolved nitrogen (DN) concentrations of tropical forest plateau and mid-slope soils are quantified within three geochemically distinct regions (felsic, mixed sedimentary rocks and mafic), located in the eastern part of the Democratic Republic of the Congo (DRC), Rwanda and Uganda. Additionally, the potential connection between DOC concentration and carbon dioxide (CO2) respiration is examined. DOC concentration was found to vary significantly depending on the geochemical region and increased significantly with depth. DN also varied significantly depending on the geochemical region yet did not show any significant depth-dependent trend. The largest concentration of DOC and DN were found in soils of mafic geochemistry, intermediate amounts in soils of mixed sedimentary rocks geochemistry and the lowest amounts in soils of felsic geochemistry. These differences were attributed to the different amounts of clay minerals, aluminum and iron pedogenic oxides in the soils of the different geochemical regions that stabilize dissolved organic matter (DOM) through sorptive processes. No overall significant correlation between DOC and CO2-C respiration was found. However, a positive and significant correlation between DOC and CO2-C exists within the 10 cm depth increments. Further analyses are required to confirm if the findings of this thesis accurately represent DOC and DN concentrations under field conditions.