Thermodynamic Stability of Clay Minerals in Boreal Forest Soils and Its Relationship to the Properties of Soil Organic Matter

This paper assesses the thermodynamic stability of clay minerals in the upper organo-mineral horizon of podzolic soil, as well as in the rhizosphere of Norway spruce (Picea abies (L.) H. Karst.) and Norway maple (Acer platanoides L.). Moreover, it determines the impact of soil organic matter on thermodynamic stability of clay minerals. Calculations of ΔGf and the saturation index (SI) for clay minerals in laboratory experiments simulating soil conditions without soil moisture outflow allowed us to find out that the thermodynamic stability of clay minerals decreased in the series: kaolinite > illite > vermiculite > chlorite. The spruce rhizosphere contained kaolinite, vermiculite and illite in the lowest thermodynamic stability, while the soil under maple showed the highest thermodynamic stability of clay minerals due to the differences in the properties of soil organic matter in the rhizospheres of different tree species. Laboratory experiments on the sorption of soil humic acid (HA) on clay minerals demonstrated that sorbed HA decreased the thermodynamic stability of biotite and increased the thermodynamic stability of kaolinite and muscovite. Thermodynamic stability of clay minerals decreased with increased proportion of sorbed thermolabile organic matter.