Kaolinite neoformation from palygorskite in the rhizosphere of citrus trees in semi-arid regions

Very limited information is available on fibrous clay minerals transformation in the rhizosphere of agricultural crops; especially that of citrus trees (orange, Citrus sinensis and lemon, Citrus limon) extensively cultivated worldwide, despite a large number of investigations evaluated the environmental conditions for the formation and stability of these minerals. In this study, palygorskite changes within the rhizosphere and bulk soil of selected citrus species of different ages and the ability of these plants to extract Mg from palygorskite were investigated. Orange orchards aged 5, 10 and 20 years and lemon orchards aged 5, 20 and 50 years were selected in Fars Province, Iran. The ratio of 0.71 to 0.64 nm XRD peak area (indicator of the relative quantities of neoformed kaolinite) and different forms of soil Mg were determined. The 0.71/0.64 ratio showed the highest relative quantity of neoformed kaolinite in the rhizosphere of orange trees as compared to the bulk soil, which was consistent with available Mg. Such variation trend of these soil parameters was slightly different in lemon orchards. A significant change in available Mg and kaolinite neoformation rate in older age groups of orange trees were observed because of continuous cultivation, higher Mg demand and root uptake. Furthermore, hexagonal particles of neoformed kaolinite were observed in the rhizosphere of orange trees using TEM. Nonexchangeable Mg was higher in the rhizosphere of 50-year-old lemon orchard compared to the bulk soil, which was consistent with the relative quantity of neoformed kaolinite. In addition, the trend of nonexchangeable Mg variation was in line with that of kaolinite neoformation rate in the rhizosphere and bulk soils of lemon orchards. Leaf analysis showed that palygorskite weathering could meet orchards' physiological Mg demand and palygorskite kaolinitization might play a role in agricultural soils of arid to semi-arid regions with no dolomitic sources of Mg.