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Abstract: Microbe–mineral interactions play an important role in affecting geochemical transformations of heavy metals in the soil environment. The formation of metal sulfide, which is mediated by sulfate-reducing bacteria (SRB) through contributing to sulfate reduction is an important pathway for heavy metal stabilization in anoxic soil. In oxic rice rhizospheres, there are abundant sulfur oxidizing bacteria (SOB) which can enhance sulfur oxidation and hence the availability of heavy metals, resulting in the uptake of such metals by the plant and a potential risk to human health. In this study, the potential existence of SRB in oxic rice rhizospheres, their contribution to sulfate reduction, and potential to reduce the availability of heavy metal was investigated. PCR-DGGE fingerprinting and real-time PCR results showed increasing numbers of SRB with Pb addition, which corresponded with increases in soil pH and reduction in Eh, suggesting the enhancement of sulfur reduction and SRB activity. Sulfur K-edge XANES, which characterized sulfur speciation in situ, revealed reduced states of sulfur. The SRB mediated the sulfate reduction and contributed to the formation of reduced sulfur which interacted with Pb, leading to the formation of stable metal sulfide and reduction of Pb availability. In return, acclimated SRB populations developed in Pb-polluted conditions. Hence stabilization of reduced sulfur by Pb enhanced the activity of SRB and sulfate reduction in rice rhizosphere. [Copyright &y& Elsevier]

Lead contamination in soil has become a worldwide threat on food security and human health. To assess the Pb bioavailability and evaluate the safe use of low Pb polluted soil for food production, the speciation of Pb in 19 types of paddy soil were investigated by chemical extraction and X-ray absorption near-edge structure (XANES), and the uptake and accumulation characteristics of Pb in different soil-rice systems were investigated. Moreover, an empirical model was established to predict the content of Pb in rice grain, and field validation was conduct to evaluate model performance. Results showed that the proportion of available Pb in different soil satisfied normal distribution N (0.47, 0.23). Pb(CH 3 COO) 2 , GSH-Pb, PbO, PbHPO 4 and Pb 3 (PO 4) 2 performed well in characterizing the speciation of Pb in different rhizosphere soils, and PbHPO 4 accounted for more than 70%. The exceedance of Pb in grain in CK, 0.5X and 1X treatment were 10.5%, 36.1% and 42.1%, respectively, and the accumulation of Pb in grain was significantly related with Pb content in root. Carbonate and organic bound Pb in rhizosphere soil were two major Pb species that influenced the accumulation of Pb in rice. Moreover, content of total Pb, clay and SOM performed well in predicting the Pb content in grain, both for pot and field samples. Above all, our predicting model worked well in evaluating Pb accumulation in rice grain among low polluted paddy farmland (Total Pb < 300 mg/kg). Image 1 • Edible Pb content was positively related to the carbonate and organic bounded Pb in rhizosphere soil. • The specific speciation of Pb in different rhizosphere soils was clarified using XANES. • The content of total Pb, clay and SOM performed well in predicting the Pb content in rice grain. • Our model worked well in evaluating Pb content in rice grain among low polluted farmland. [ABSTRACT FROM AUTHOR]

Abstract: A pot experiment was carried out to study the sulfur (S) and lead (Pb) interaction and its impact on soil microbial community composition in rice rhizosphere soil under flooded conditions. Paddy soil was treated with a Pb gradient with and without thiosulfate addition and then planted with rice. The increasing addition of Pb resulted in plant yield reduction and high, phytotoxic concentrations of Pb in roots with relatively low concentration of Pb in shoots. Under the impact of thiosulfate, Pb uptake in plants and NH4OAc extractable Pb did not increase dramatically. PCR-DGGE experiment suggested that S action led to new bands. Specific clones (T3 and T6) found in S addition soils had high similarity to Thiobacillus, which indicated relatively high rates of potential S oxidation. S addition did not affect the availability Pb and the composition of soil microbial community. S addition is not a suitable amendatory tool of phytoremediation for Pb polluted soil. [Copyright &y& Elsevier]