Control of climate on soil charge characteristics through organic matter and clay mineral distributions in volcanic soils of Mt. Kilimanjaro, Tanzania

The soil cation exchange capacity (CEC), which is contributed by constant and variable charges, is a vital indicator of soil fertility and affects elemental cycling. Climate is expected to affect soil CEC by controlling soil secondary minerals and organic matter distributions. The objective of this study was to clarify the effects of climate on the charge characteristics of the soil in Mt. Kilimanjaro under diverse climatic conditions. The soils of 15 soil profiles from southeast and northwest slopes of Mt. Kilimanjaro along the elevation gradient (910-2800 m asl.) were collected. The soil physicochemical and mineralogical properties, including constant and variable charge densities, were determined. Climate data for the study site was obtained from the WorldClim database. The results showed that the contribution of the variable charge to CEC was high at the humid sites of the elevated area of the southeast slope. At the humid sites, the high content of organic matter, which was considered to be stabilized as organo-mineral complexes by nanocrystalline minerals, enlarged the soil CEC, being more evident at cool high elevation sites. On the other hand, at the dry sites on the northwest slope and low elevations of the southeast slope, the constant charge and variable charge contributed equally to the CEC. The variable charge from organo-mineral complexes was smaller due to the progressive crystallization of nanocrystalline minerals under the dry condition, and the formed crystalline clay minerals would contribute to constant charge density. Our study indicates the importance of moisture in addition to temperature as factors that influence the distribution of organic matter and crystalline clay minerals, which in turn control the soil charge characteristics. Thus, the constant charge from crystalline clay minerals should be considered in addition to the variable charge when studying the weakly weathered volcanic soils in the relatively dry regions.

Automated summary

The study showed that the contribution of variable charge to CEC was high in humid sites on the southeast slope, while constant and variable charge contributed equally in dry sites on the northwest slope and low elevations of the southeast slope. Moisture and temperature both play important roles in influencing organic matter distribution and crystalline clay minerals, which in turn impact soil charge characteristics in weakly weathered volcanic soils in relatively dry regions.