Effects of climate on distribution of soil secondary minerals in volcanic regions of Tanzania

In the volcanic regions of Tanzania, including Mt. Kilimanjaro, Mt. Meru, and Mt. Kieyo, soils are formed on relatively recent tephra deposits (<0.45 Ma). Because of the diversity of climate caused by the volcanic terrain in these regions and different parent material compositions, various soil secondary minerals were expected to form. The objectives of this study were to determine the distribution of secondary minerals in soils formed on tephras in the volcanic regions of Tanzania and to estimate the effects of climate and parent material composition on the distribution of secondary minerals. A total of 30 soil samples from the B-horizon from different parent materials and under different climatic conditions (540-2800 m altitude) were collected. The soils were classified into five groups depending on location and considering the total elemental contents. Selective extraction methods, X-ray diffraction, and differential thermal analyses were applied to determine the composition of the secondary minerals. The composition of the soil water extract was determined to assess the stability of the minerals. Excess precipitation (EP), as an index to evaluate the soil water regime, was calculated by subtracting potential evapotranspiration from precipitation. The soils in Mt. Kilimanjaro and Mt. Meru were rich in Fe or Mg, whereas soils in Mt. Kieyo were rich in Si, indicating mafic and felsic parent materials, respectively. At the low moisture sites (EP < 500 mm), Alo + 1/2Feo (the sum of acid oxalate soluble Al and Fe) were low and increased with EP. Meanwhile, the values of Alo + 1/2Feo negatively correlated with mean average temperature (MAT) for all sites. At low moisture sites, low EP would be favorable for the crystallization of nanocrystalline Al and Fe. At the sites with sufficient moisture, the temperature was the primary factor controlling the distribution of nanocrystalline minerals. Furthermore, gibbsite was found at the sites with high EP (EP > 500 mm) with low H4SiO40 activities. More kaolinite was detected at the low elevation sites with high H4SiO40 activities in soil solutions. Soil moisture and temperature are critical factors influencing the distribution of secondary minerals, whereas parent material compositions have less effect on soils in the volcanic regions of Tanzania.