Origin and distribution of clay minerals in the three climatic regions of eastern Golestan Province

Shoordareh, Aghsu and Narmab watersheds in eastern Golestan Province important with respect to the soil erosion and floodings, were selected to investigate their soil physico-chemical properties and also their clay mineralogy. Soil parent material is mainly composed of loess deposits in all the three regions. They show however, differences in climatic parameters (precipitation, evapotranspiration and temperature), and vegetation. Precipitation increases and temperature and evapotranspiration decrease from Shoordarreh to Aghsu and eventually to Narmab and natural vegetative cover shifts from scattered pasture in Shoordarreh to dense forest in Narmab. The results revealed that all the studied soil properties were significantly different in the three regions. Soil organic carbon, clay, fine silt, saturation percentage, total nitrogene, and soil available phosphorous increased by the favorable soil forming conditions in Narmab. pH, EC, sand, lime and available K significantly decreased as a result of P/ET increase in Narmab. Clay mineralogical analyses showed that there were significant differences in type and abundance of clay minerals for the three regions. Illite, chlorite and kaolinite were the major clay minerals of the loess parent material. By increasing P/ET and subsequent change in other soil properties, illite and chlorite content decreased while smectite, vermiculite and illite-vermiculite interstratified minerals increased significantly. The highest occurrence of smectite was observed in Aghsu which was probably formed from transformation of illite as a result of the leaching of K from interlayers. Vermiculite was observed in Aghsu and Narmab but with a higher abundance in Narmab mainly due to its higher stability under lower pH and alkalinity in Narmab region.