CARBON STORAGE AND CYCLING IN AFRICAN DARK EARTHS

The objectives of this dissertation were to identify the properties and processes of soil organic matter (SOM) that contribute to mechanisms that promote carbon (C) storage, and the microbial community form-function relationships that contribute to C cycling in African Dark Earths (AfDE) compared to unamended, adjacent soils (AS). Soil C concentrations and soil mineral specific surface area were used to calculate C-loading and tested against a hypothesized saturation limit to test the degree of C saturation in bulk AfDE and mineral-associated fractions. Chemical composition as a mechanism promoting persistence was tested by quantifying pyrogenic C (pyC) by evolved gas analysis during ramped combustion and validated with benzene polycarboxylic acid (BPCA) analysis. Microbial activity and community composition were used to test C cycling in AfDE and AS to determine the degree to which microbe-SOM form-function relationships contribute to C persistence in AfDE. C-loading in bulk AfDE and mineral-associated fractions was above the hypothesized saturation limit, indicating mineral-associated SOM was over-saturated with C in AfDE, possibly due to the presence of pyrogenic C (pyC). PyC was present in greater quantities in AfDE compared to AS, suggesting that chemical persistence is contributing to the C stabilization in AfDE. PyC appeared to suppress microbial activity, further promoting OM persistence. The recent additions of C as SOM may promote increased microbial community diversity in AfDE, while C persistence may contribute to the lack of abundant distinct microbial communities in AfDE compared to AS. Mineral-association and chemical persistence both contribute to long-term SOM storage in extant AfDE and this dissertation suggests that the phenomenon of chemically persistent mineral-associated OM may be leading to greater C concentrations in AfDE compared to AS.