Clay mineralogy as an evidence of land degradation on loess hillslopes

In order to study the soil clay minerals behavior as affected by land use change and land degradation, a loess hillslope located in Agh-Su watershed, eastern Golestan province was selected. Illite and chlorite are dominant minerals in loess parent material. The major clay minerals in the forest soil and adjacent deforested cultivated area were according to the following orders: illite > smectite> kaolinite > chlorite > vermiculite and illite > chlorite > kaolinite > smectite respectively. Illite, chlorite and kaolinite are mainly of inheritance origin. Deforestation and soil erosion has led to an increase in the amount of these minerals in the cultivated land use due to outcropping of parent material. Origin of smectite is believed to be both inheritance and also transformed in the study area. Illite and chlorite can transform to smectite in the presence of more soil available-moisture mainly due to leaching and release of K⁺ from their structures. Natural forest cover has provided higher moisture and consequently more smectite in forest land use relative to adjacent cropland. Soil loss, absence of permanent vegetation, weak leaching and lower soil moisture are the main reasons for the rare occurrence of smectite in the deforested area. Vermiculite is mainly observed in weak acidic forest soil where agroclimatological index, P/ET, (ratio of mean annual precipitation to mean annual reference crop evapotranspiration) is more than 0.4. Long-term tillage practices and severe soil surface erosion has distributed subsurface calcareous material through out soil profile and the consequent higher soil pH has led to an increase in vermiculite instability. Increase in illite, chlorite and kaolinite content and decrease in smectite content and also absence of vermiculite in degraded lands which were formerly under the same forest ecosystem and also most probably the same clay mineralogy indicate to the loss of topsoil and landscape instability following deforestation. In general, clay minerals and their variations besides other physicochemical and biological soil parameters can be used as a good indicator for land degradation.