Inhibition of arsenic accumulation in Japanese rice by the application of iron and silicate materials

There are many rice paddy fields with relatively high arsenic (As) concentrations in areas that have not been subjected to countermeasures because the As level was below the criterion value established by the Agricultural Land Soil Contamination Prevention Law in Japan. To reduce the risk of excessive As in rice paddy fields using agronomic methods applicable to common farming practices, we examined the effect of the application of iron (Fe) and silicate materials on As uptake by rice plants in bottomless concrete frames filled with soil collected from an area surrounding a former As-polluted region. Following the application of calcium silicate slag at a rate of 0.5 kg m^-2, no significant effect was observed on the As concentration in rice grain or straw, but the available silicate in the soil was increased. The application of Fe materials at a rate of 0.5 kg m^-2 resulted in a significant reduction of the As concentration in grain and straw, as shown by an analysis of variance. The lowest concentrations of As in both grain and straw were obtained with the application of a metal Fe powder (EM), followed by that with an Fe oxide material (FB), a converter furnace slag (FM), and the control. The EM and FB applications both significantly reduced the 1M HCl-soluble As concentration in soil compared to the control. The levels of both acid ammonium oxalate extractable Fe (Fe-_ox) and citrate-bicarbonate-dithionite extractable Fe (Fe-_CDB) in the soil were significantly increased by the application of EM and that of FB compared to the FM and the control. A negative significant correlation was obtained between the amount of 1M HCl-soluble As and two types of free Fe oxide. These results suggest that the available As in soil was strongly fixed with the Fe oxides that were increased by the application of Fe materials, and that the decrease of available As in the soil induced a decrease in the As uptake by rice plants. The application of Fe materials will thus be effective to lower or prevent the risk of the rice cultivation in paddy fields with relatively high levels of As.