Identification of vivianite formed on the roots of paddy rice grown in pots

Abstract

An understanding of the phosphate (P) dynamics in paddy rice fields is the basis for improving P fertilizer efficiency and reducing P loss from paddy fields. During the ripening stage of rice plants cultivated in pots, we identified vivianite on the roots. We placed 3 kg of air-dried soil in a pot with coated urea (1 g N), coated potassium sulfate (1 g K2O) and granular superphosphate (1 g P2O5) as basal fertilizers. Three rice seedlings were transplanted into each pot and grown until the ripening stage under submergence outdoor conditions. The bulk soil showed a black color indicating the formation of amorphous FeS. According to the soil analysis data, the oxalate-extractable Fe content was much greater than the labile S and P contents, indicating that enough Fe(II) can be supplied to the S and P for the reaction. Bluish vivianite particles were observed on the roots using an optical microscope. Scanning electron microscopy revealed that the vivianite was an aggregate of platy crystals, and an energy dispersive X-ray analysis showed that Fe and P were the major elements in the crystal aggregates. The diffraction peak positions by the X-ray microdiffractometer were very close to the reported pattern for vivianite. Future research on the dynamics of P is expected based on vivianite formation in paddy field soils. © 2010 Japanese Society of Soil Science and Plant Nutrition.