| Chromium (Cr) has been extensively used in industrial activities such as ore refining, electroplating industry, tanning, paper making, steel production and automobile manufacturing. The lack of appropriate disposal facilities has led to severe Cr pollution in waters and soils throughout the world. In the natural environment, Cr exists in two common oxidation states:Cr(?) and Cr(?). The mobility, toxicity and plant uptake of Cr depend strongly on its oxidation states. Accumulation of Cr in soils increases the potential uptake of the metal by plants, thus posing a great threat to human health through the food chain. The need to protect consumers from Cr toxicity and ensure environmental safety is the scientific motive for establishing guidelines on Cr concentrations in food and acceptable concentrations of Cr in agricultural soils.Seven representative soils were used in this study:Udic Ferrisols, Typic Haplustalf, Periudic Argosols, Calcaric Regosols, Stagnic Anthrosols, Mollisols and Ustic Cambosols. The objectives of this study were:1) to investigate the relative importance of direct and indirect effects of soil physicochemical and biological properties on Cr(?) reduction;2) to identify the species of Cr-resistant bacteria in representative agricultural soils, and investigate the effect of Cr-resistant bacteria on Cr(?) reduction;3) to develop an empirical model to correlate the Cr phytoavailability with common soil properties and with Cr concentrations in soil extractable fractions;4) to establish Cr thresholds for potential dietary toxicity in representative agricultural soils. The main results are summarized in the following text:(1) The reduction of Cr(?) in soil is not controlled by a single soil property but the result of collective effects of many involved factors. However, the influence of soil properties on Cr(?) reduction is not fully understood due to lack of systematic studies. A microcosmos incubation experiment was conducted to investigate the reduction kinetics of Cr(?) in seven soils and its relationships with soil properties. The results indicate that the reduction of Cr(?) can be described by a first-order reaction. The reduction rates of Cr(?) in the seven soils decreased in the orders:Udic Ferrisols> Stagnic Anthrosols> Calcaric Regosols> Mollisols> Typic Haplustalf> Periudic Argosols> Ustic Cambosols. Simple correlation analysis revealed that the reduction of Cr(?) in soils was positively related to organic matter, dissolved organic matter, Fe(II) content, clay fraction and diversity index of bacterial community, but negatively correlated with easily reducible Mn content. Using the stepwise regression, the reduction of Cr(?) in soil could be quantitatively predicted by the measurement of dissolved organic matter, Fe(?) content, pH, and soil particle size distribution, with the fitting level of95.5%. The results indicated that the reduction of Cr(?) in natural soils is not controlled by a single soil property but the result of the combined effects of dissolved organic matter, Fe(?), pH, and soil particle size distribution.(2) In this study, seven representative agricultural soils with different physicochemical properties were used to investigate the importance of microbially mediated Cr(?) reduction and the response of soil microbial community to Cr contamination. Soil microbial community responded to Cr contamination through changes in microbial community structure, with Cr-resistant bacteria becoming dominant species. Bacillus, Escherichia, Deinococcus, Micromonospora, Methylobacterium, Massilia, Acidobacterium, Comamonas, Bradyrhizobium, and Arthrobacter were identified as the Cr-resistant bacteria. Moreover, our results demonstrated that microbial reduction was an important Cr(?) reduction pathway, which was mainly attributed to Cr-resistant bacteria. Soil properties, especially Fe(?) and soil particle distribution, affected the microbially mediated Cr(?) reduction. These results provide useful information for the bioremediation of Cr contaminated soils under a wide range of environmental conditions.(3) Anthropogenic chromium (Cr) pollution in soils poses a great threat to human health through the food chain. A pot experiment was conducted to investigate Cr phytoavailability to rice (Oryza sativa L.), which is a major staple food crop for the largest population of people on Earth, and establish Cr thresholds for potential dietary toxicity in representative agricultural soils. Simple correlation analysis indicated that Cr concentration in polished rice was significantly correlated with total Cr, Mehlich-3extractable Cr, and Cr(VI). Stepwise multiple regression analysis also demonstrated that the Cr phytoavailability was strongly correlated with soil total Cr, Mehlich-3extractable Cr, Cr(VI) concentration, and soil organic matter, Fe(II), and particle size distribution. Critical Cr concentrations were evaluated for rice based on maximum safe level of200?g for Cr daily intake. In soil-rice system, the total Cr thresholds for potential dietary toxicity conformed to the order of Udic Ferrisols> Stagnic Anthrosols> Calcaric Regosols> Mollisols> Periudic Argosols> Ustic Cambosols, and were90.6,81.1,80.8,78.2,56.6, and51.8mg kg-1, respectively. Mehlich-3extractable Cr are most suitable Cr thresholds for Periudic Argosols, Udic Ferrisols, Mollisols, and Ustic Cambosols, with values of1.54,0.56,0.42, and2.18mg kg-1, respectively, while Cr(VI) are adequate thresholds for Calcaric Regosols and Stagnic Anthrosols, with values of0.68and0.84mg kg-1, respectively.(4) As the proportion of vegetables has increased with the improvement of living standards, it is imperative to understand Cr accumulation properties in common vegetables. This study was aimed to investigate Cr accumulation in pak choi (Brassica chinensis L.), and to establish Cr thresholds for potential dietary toxicity in representative agricultural soils. Simple correlation analysis indicated that Cr concentration in pak choi was significantly correlated with the total Cr, Mehlich-3extractable Cr, and Cr(VI). Stepwise multiple regression analysis also demonstrated that the phytoavailability of Cr was strongly correlated with the extractable fraction by Mehlich-3, total Cr concentration, and soil OM, Fe(II). Critical Cr concentrations were evaluated for pak choi based on maximum safe level for daily intake of Cr. In soil-pak choi system, total soil Cr can be used as Cr thresholds for potential dietary toxicity in pak choi, which conformed to the order of Udic Ferrisols (443mg kg-1)> Stagnic Anthrosols (352mg kg-1)> Calcaric Regosols (274mg kg-1)> Mollisols (272mg kg-1)> Periudic Argosols (226mg kg-1)> Ustic Cambosols (204mg kg-1), whereas Mehlich3-extractable Cr are most suitable to be used as Cr thresholds for Periudic Argosols, Udic Ferrisols, Mollisols, and Ustic Cambosols, with values of 20.7,15.8,21.2, and20.4mg kg-1, respectively, while Cr(VI) are most suitable for Calcaric Regosols and Stagnic Anthrosols, with values of26.5and28.0mg kg-1, respectively.(5) Chromium exists in paddy soil as two stable oxidation states, Cr(?) and Cr(VI), and Cr is susceptible to oxidation-reduction reactions when soil redox conditions change. Compared to continuous flooding (CF), alternating wetting and drying (AWD) may lead to the changes of soil redox potential and Cr redox state, thus Cr mobility, toxicity and plant uptake of Cr. This study was aimed to understand the behavior of Cr in intermittently irrigated rice paddies subject to varying soil redox conditions, and to determine the effects of irrigation management on Cr uptake by rice. Under CF irrigation, soil redox potential (SRP) decreased gradually with increasing flooding time, and fell to-370-263mv when rice harvested, while SRP fluctuated from-291to189mv under alternating oxic and anoxic soil conditions imposed by AWD irrigation. Compared to CF practice, soil extractable Cr and Cr mobility was significantly promoted by AWD irrigation, which resulted in higher Cr concentration in rice. AWD irrigation increased Cr concentration in rice by19.8-26.5%and18.4-29.8%, respectively for Mollisols and Udic Ferrisols. Therefore, CF is recommended to be used as irrigation management in Cr polluted soils to lower soil Cr mobility and rice Cr concentration. |
PDF Full Text Request