Effects and mechanisms on the reduction of lead accumulation in rice grains through lime amendment

The influence of lime amendment on the absorption and accumulation of lead (Pb) in the grains of three rice cultivars ((Dong Lian 5 (DL-5, Indica), TeYou 009 (TY-009, Hybrid Indica), and YiYou 673 (YY-673, Hybrid Indica)) was investigated and the associated mechanisms were explored. Pot experiment was conducted in heavily Pb-polluted paddy soil. The transformation of Pb species in soil and roots under different levels of lime application was studied by Pb L3-edge X-ray absorption spectroscopy (Pb L3-XAS) and BCR sequential extraction. The results showed that lime amendment significantly increased soil pH and decreased DTPA-extractable Pb in soil. When the additional ratio of Liming was lower than 32 g kg^-1, the Pb accumulation in rice plants, particularly in the grains was remarkably reduced. Moreover the biomass of rice was not significantly affected. Among the three cultivars, the lowest Pb content in rice grains occurred in YY-673. The cultivation of rice cultivar with low Pb accumulation will reduce the Pb content in the grains. Liming significantly inhibited the translocation of Pb from the roots to the aboveground parts, and from the stems and leaves to grains. Pb L3-XAS and BCR s sequential extraction method analysis revealed that lime amendment increased Pb²⁺ association with Fe oxides (Pb-Ferr), the precipitation of 2PbCO₃·Pb(OH)₂, the residual fraction of paddy soil, and also promoted the transformation of Pb(NO₃)₂ and Pb-pectin to 2PbCO₃·Pb(OH)₂ and Pb-Ferr in rice roots. The suitable lime addition (> 4 g kg^-1, under this pot experiment) could effectively reduce Pb accumulation in rice grains.