Bioavailability and safety issues of heavy metals in paddy soil-rice continuum in Korea

There is an increasing concern over heavy metal contamination of agricultural soils and the subsequent translocation of metals to rice in Korea. Rice is one of the most important crops in Korea. Therefore, it is very important to monitor the status and long-term trend of heavy metal contamination in paddy soils and rice, periodically. It is also important to verify the bioavailability of heavy metals in paddy soils released from mine tailings, which is major source of contamination in Korea, and to make a prediction model of heavy metal uptake in the rice. The average concentrations of As(arsenic), Cd(cadmium), Cu(copper), Ni(nickel), Pb(lead), and Zn(zinc) in paddy soils nationwide were 0.66, 0.08, 3.83, 0.59, 4.82 and 4.33 mg kg-1, respectively, which were within the threshold level for soil contamination designated by the Soil Environmental Conservation Act in Korea. However, heavy metal contents in paddy soils from 94 sites nearby the abandoned/closed mines and 43 sites nearby industrial complexes exceeded the Korean regulations. The respective average concentrations of As, Cd, Cu, Ni, Pb, and Zn in brown rice collected at the non-contaminated paddy fields in 1999 were 0.07, 0.035, 2.74, 0.29, 0.28 and 18.3 mg kg-1, respectively. Heavy metal phyto-availability in paddy soils near five abandoned mining areas was analyzed by sequential extraction method. Cd in soil was consisted with 25.1% of exchangeable form, 32.9% of oxide and carbonate form, 37.4% of sulfide and residual form, and 4.6% of organically bound form. The exchangeable forms of Cd were negatively correlated with soil pH. While oxide, carbonate, and sulfide forms of Cd was positively correlated with soil pH. All kinds of Cd fractionation except organically bound form were highly positively correlated with Cd content in brown rice. Two-year lysimeter experiments showed that Cd contents in the mine tailings were the limiting factor for Cd levels in rice, which were higher in the fine textured soil than in the coarse textured soils. Major species of Cd in leachate were Cd2+, CdCl+, and CdSO4 (aq), but these species were not positively correlated with Cd contents in rice. Rice variety was more important factor regulating Cd translocation to grain than soil physico-chemical properties and agricultural management practices. Prediction of Cd uptake by rice was delineated by the model [log (Rice Cd) = -1.60 + 1.11 log (total soil Cd) - 0.014 (soil OM, %) + 0.18(pH) - 0.03(soil clay, %) (R2=0.404***)] using 108 soils and rice grains collected from the closed mine areas.