Rhizosphere-induced weathering of minerals in loess-derived soils of Golestan Province, Iran

Weathering of minerals is the major source of nutrients to plants. The rhizosphere can play a central role in the weathering of minerals through the release of acid exudates that drive hydrolysis of minerals. Although many studies have examined the weathering of pure natural industrial minerals in rhizosphere, information on the weathering of clay and silt minerals from soils is limited. This study examined the weathering and K release of minerals in clay and silt fraction in the rhizosphere of corn. Growth medium was a mixture of quartz sand as filling materials and clay and silt fraction of four dominant soils formed on loess parent material with different characteristics (i.e., Aridisols, Alfisols, Inceptisols and Mollisols). During 100 days of the experiment, plants were irrigated with distilled water and complete and K-free nutrient solution as needed. At the end of cultivation, plants were harvested and their K uptake was measured. Clay and silt fractions were separated from quartz sand and analyzed by X-ray diffraction (XRD) and XRF both before and after the experiment. Intense weathering of mica and chlorite was observed in silt fraction of Aridisols and Mollisols. The high acidity of the rhizosphere induced by root exudates has resulted in the release of structural K from mineral lattice. In contrast to the silt fraction, the X-ray patterns of the clay fraction showed minor changes following plant growth such as the formation of mixed layer mica-vermiculite and vermiculite. The K content extracted by corn was significantly greater for silt than the clay sizes. The highly weatherable silt sized minerals therefore, could be regarded as the major source of nutrients to plants.