Formation and dissolution of vivianite in paddy field soil

We present visual evidence of vivianite formation based on rice root samples from the plow layer soil of an ordinary paddy field. Vivianite crystal aggregates were colorless when sampled but turned blue within a few hours under an optical microscope. The modal size of the crystal aggregates was approximately 0.05 mm in diameter. Their X-ray microdiffraction pattern was identical to that reported previously, and the elemental composition was very similar to that of vivianite according to energy dispersive X-ray analysis. Furthermore, we found that the amount of vivianite on the roots can be estimated on the basis of the difference in the amount of P extractable from the rice roots in 0.1 mol L^-1 HCl-1 mol L^-1 acetic acid (AcOH) before and after heating at 378 K for 48 h. Rice roots sampled from the flooded and reduced paddy field soil contained 2.0 to 4.5 g P kg^-1 as vivianite on an oven-dry basis. We also found that vivianite content decreased significantly after water drainage from the rice field in fall. This study suggests that the formation and dissolution of vivianite crystals could be repeated each year depending on the water management system employed in the plow layer soil of paddy fields.