Amounts and associations of heavy metals in paddy soils of the Khorat Basin, Thailand

This study determined the amounts and speciation of heavy metals (HM) in paddy soils at100 sites in the Khorat Basin, Thailand to provide a guide to reducing HM concentrations in rice. The soils were analyzed for HM in crystalline iron oxides (DCB, dithionite citrate bicarbonate extract), amorphous iron-manganese oxides (oxalate soluble) and total element concentrations (AR). The soils have median HM concentrations in both topsoil and subsoil that are mostly lower than the thresholds for concern listed in Thailand's Soil Quality Standard for Habitat and Agriculture but with a few high values of concern. Soils derived from alluvium and wash deposits from sandstone have low concentrations of HM compared to soils on basalt. Manganese, Co and Pb in soils on all parent materials are mostly in the oxalate soluble fraction which is considered to be labile. Much the Fe and V are associated with the crystalline iron oxide fraction.All metals showed similar spatial distributions with relatively lower concentrations of all elements except Si and Zr in the eastern part of the Basin due to the prevalence of sandy soils. Commonly, concentrations of HM in these paddy soils increase with depth reflecting the increase in clay content as the HM are mostly incorporated into or adsorbed onto clay minerals and Fe/Mn oxides.