| Food chain contamination by cadmium (Cd) and lead (Pb) is globally a serious health concern resulting in chronic abnormalities. The lack of appropriate disposal facilities has led to severe heavy metal pollution in soils and waters throughout the world. Accumulation of Cd and Pb in soils has increased the potential uptake of these metals by plants, thus posing a great threat to human health through the food chain. To ensure the food safety and environmental quality of soils, guidelines for permissible concentrations of Cd and Pb in agricultural soils need to be established. Seven Chinese soils, Udic Ferrisols, Typic Haplustalf, Periudic Argosols, Calcaric Regosols, Stagnic Anthrosols, Mollisols and Ustic Cambosols were selected for this study. The objectives of this study were:1) to establish Cd and Pb thresholds for representative agricultural soils based potential dietary toxicity.2) to develop empirical models to explain the combined effects of soil properties and extractable soil Cd and Pb content on the phytoavailability 3) to investigate the effects of Cd and lead contamination on Microbial biomass, Microbial community pattern and Dehydrogenase activity with the growth of rice in different soils. The main results are summarized as following;1. In this study cadmium concentrations in polished rice grain were best related to total Cd content in Mollisols and Udic Ferrisols with threshold levels of 0.77 and 0.32 mg kg-1, respectively. Contrastingly, Mehlich-3-extractable Cd thresholds were more suitable for Calcaric Regosols, Stagnic Anthrosols, Ustic Cambosols, Typic Haplustalfs and Periudic Argosols with thresholds values of 0.36,0.22,0.17,0.08 and 0.03 mg kg-1, respectively. Stepwise multiple regression analysis indicated that phytoavailability of Cd to rice grain was strongly correlated with Mehlich-3-extractable Cd and soil pH. The empirical model developed in this study explains the combined effects of soil properties and extractable soil Cd content on the phytoavailability of Cd to polished rice grain. This study indicates that accumulation of Cd in rice is influenced greatly by soil type, which should be considered in assessment of soil safety for Cd contamination in rice. This investigation concluded that the selection of proper soil type for food crop production can help us to avoid the toxicity of Cd in our daily diet.2. The purpose of this study was to establish soil Cd thresholds of representative Chinese soils based on dietary toxicity to humans and develop a model to evaluate the phytoavailability of Cd to Pak choi (Brassica chinensis L.) based on soil properties. Mehlich-3-extractable Cd thresholds were more suitable for Stagnic Anthrosols, Calcaric Regosols, Ustic Cambosols, Typic Haplustalfs, Udic Ferrisols and Periudic Argosols with values of 0.30,0.25,0.18,0.16,0.15 and 0.03 mg kg-1, respectively, while total Cd is adequate threshold for Mollisols with a value of 0.86 mg kg-1. A stepwise regression model indicated that Cd phytoavailability to Pak choi was significantly influenced by soil pH, organic matter, total Zinc and Cd concentrations in soil. Therefore, since Cd accumulation in Pak choi varied with soil characteristics, they should be considered while assessing the environmental quality of soils to ensure the hygienically safe food production.3. Food chain contamination by lead (Pb) is globally a serious health concern resulting in chronic abnormalities. Rice is a major staple food of the majority world population; therefore, it is imperative to understand the relationship between the bioavailability of Pb in soils and its accumulation in rice grain. Objectives of this study were to establish environment quality standards for seven different textured soils based on human dietary toxicity, total Pb content in soils and bioavailable portion of Pb in soils. Lead concentrations in polished rice grain were best related to Mehlich-3 Pb of soils of Calcaric Regosols, Mollisols, Ustic Cambosols, Typic Haplustalfs, Stagnic Anthrosols and Periudic Argosols, with food safety thresholds values of 15.26,14.17,14.10,13.84,13.15,12.62 and 12.22 mg kg-1, respectively. Contrastingly, total Pb thresholds were more suitable for Udic Ferrisols with threshold level of 99.31 mg kg"1. The stepwise regression model indicated that extractable Pb contents in soil, pH, organic matter and cation exchange capacity played an important role to predict the phytoavailability of Pb to rice. The accumulation of Pb in rice is influenced greatly by soil type, which should be considered in risk assessment of Pb contamination in rice and the selection of proper soil type for rice production can help us to avoid the toxicity of Pb in our daily diet.4. This study investigated Pb accumulation in Pak choi (Brassica chinensis L.) grown on seven different agricultural soils of China. Lead concentration in the shoots (edible parts) of Pak choi generally increased with soil Pb concentrations following the order:Udic Ferrisols> Periudic Argosols> Typic Haplustalfs>Ustic Cambosols> Stagnic Anthrosols> Mollisols> Calcaric Regosols. Regression correlation analysis indicated that Pb concentration in Pak choi was significantly correlated with the total Pb and Mehlich-3 extractable Pb. Lead concentrations in Pak choi shoots were better correlated to total Pb content in Periudic Argosols and Typic Haplustalfs with the threshold levels of 103.07 and 119.43 mg kg-1, respectively. Contrastingly, Mehlich-3 extractable Pb thresholds were more suitable for Mollisols, Stagnic Anthrosols, Ustic Cambosols, Udic Ferrisols and Calcaric Regosols with thresholds values of 35.50,27.03 20.83,9.02 and 37.01, respectively. Stepwise regression model demonstrated that the phytoavailability of Pb to Pak choi was significantly correlated with Mehlich-3 extractable Pb, pH, organic matter, and CEC. The accumulation of Pb in Pak choi is greatly influenced by soil type, which should be considered in risk assessment of soil Pb contamination. This investigation concluded that the selection of proper soil type for vegetable production can help us to avoid the toxicity of Pb in our daily diet.5. The contamination of Cd and Pb at lower concentrations resulted in a slight increase in biomass carbon whereas added Cd (>4 mg kg-1) concentrations and Pb (>400 mg kg-1) caused a significant decline in MBC. However, the degree of impact on Cmic by these metals was related to the pH and organic matter in soils. The indicators of PLFA showed that the soil microbial community pattern varied between the two soil types, two metals and different treatments. Decreases in fatty acid indicators for bacteria and actinomycetes and gradual increases in the indicators of were related with increasing metal concentration. In short, microbial biomass carbon and community pattern may be potential indicators demonstrating environmental stress of Cd and Pb pollution in soil- rice system. The iso:anteiso was significantly increased at higher levels of Cd and Pb in both soils indicating a microenvironmental stress for soil microbiota. |
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