典型土壤中铬迁移转化规律和污染诊断指标
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摘要
铬是环境中一种重要的重金属污染物,随着工业的发展,铬及其化合物应用越来越多,如印染、电镀、化工等行业。大量含铬的废水、废渣随意排放导致土壤、水体和生物遭到不同程度的污染。铬在自然环境中,通常以Cr(Ⅲ)和Cr(Ⅵ)两种稳定价态存在。铬的存在形态决定了铬在土壤中的化学行为和生物毒性。农业土壤铬污染,会导致作物可食部的铬超标,铬通过食物链在作物中积累、转化,危害人类健康。为防止土壤铬超标,保障农产品安全生产,维护人类健康,制定符合我国主要农业土壤和作物类型的土壤铬污染诊断指标意义重要。本研究以七种典型土壤类型(红壤、黄壤、砖红壤、青紫泥、黑土、潮土和石灰性紫色土)为代表,研究了七种土壤中六价铬的还原特征以及土壤性质对六价铬还原的影响,并通过PCR-DGGE-克隆测序鉴定了影响六价铬还原的耐性菌种类,并研究了水分管理方式(淹水和干湿交替)对土壤-水稻系统铬形态转化和水稻吸收积累铬的影响,此外研究了土壤-小白菜/水稻系统中铬的吸收积累规律和污染诊断指标,取得的主要研究结果如下：
1.土壤因子对六价铬形态转化的影响及相对重要性
本研究采用室内培养试验模拟七种土壤中Cr(Ⅵ)的还原过程,并通过相关分析和逐步回归分析相结合探讨影响Cr(Ⅵ)还原的土壤性质以及影响因子之间的相对重要性。六价铬在土壤中的还原遵循一级动力学模型,还原速率与土壤pH、有机质、可溶性有机质、亚铁、易还原态锰、土壤黏粒含量、微生物多样性Shannon指数显著相关。根据逐步回归模型,Cr(Ⅵ)在土壤中的还原主要受可溶性有机质、亚铁、pH、土壤颗粒组成等土壤性质的联合作用,模型的决定系数高达95.5%。
2.主要农业土壤中铬耐性菌种类及对六价铬还原的影响
本试验比较灭菌与非灭菌土壤中六价铬的还原情况,探讨土壤微生物在六价铬还原过程中的作用,并通过PCR-DGGE-克隆测序鉴定影响六价铬还原的耐性菌种类主要为芽孢杆菌Bacillus sp.、埃希氏菌Escherichia sp.、异常球菌Deinococcus sp.、小单胞菌Micromonospora sp.、甲基杆菌Methylobacterium sp.、马赛菌Massilia sp.、酸杆菌Acidobacterium sp.、丛毛单胞菌Comamonas sp.、慢生根瘤菌Bradyrhizobium sp和节杆菌Arthrobacter sp.,并证明了它们能显著促进土壤中六价铬的还原。铬对微生物群落的影响主要表现为在铬的选择作用下,铬耐性菌成为土壤中的优势菌群。土壤性质,特别是亚铁和土壤颗粒组成,影响了微生物在六价铬还原过程中的贡献率。
3.水分管理方式对土壤-水稻系统铬形态转化的影响
在铬污染水稻田中,铬主要以Cr(Ⅲ)和Cr(VⅥ)两种形态存在于水-土环境中,且能在一定的氧化还原电位范围内发生相互转化。干湿交替的农田系统随着周期性的淹水与落干的交替变化,使其氧化还原环境发生了剧烈的变化,从而影响到体系内铬元素的迁移转化。本试验选用红壤和黑土两种土壤,研究水分管理方式对土壤-水稻体系中土壤氧化还原电位、铬迁移转化和水稻积累铬的影响。淹水条件下的土壤氧化还原电位随淹水时间不断下降,在水稻收获时降到-370～263mv,而干湿交替条件下,土壤中水和气相互消长,导致氧化还原电位周期性变化,红壤氧化还原电位在-291和142mv之间反复波动,黑土氧化还原电位在-254和189mv之间反复波动,土壤环境在氧化状态和还原状态之间交替变化。干湿交替条件下六价铬浓度随着氧化还原电位的波动而上下波动,土壤中有效态铬的浓度和重金属铬的迁移能力显著高于淹水处理,从而导致稻米铬含量的增加。红壤和黑土干湿交替处理下水稻铬浓度均显著(P<0.05)高于淹水处理,其中黑土中水稻籽粒各部分铬浓度在干湿交替处理下高于淹水处理19.8～26.5%,红壤高于淹水处理18.4～29.8%。因此,建议在铬污染土壤中,采用淹水这种水分管理方式,降低土壤中铬的生物有效性和水稻籽粒中的铬浓度。
4.土壤.水稻系统中铬迁移转化规律和污染诊断指标
目前土壤铬质量标准是以总铬来作为衡量指标的,仅考虑了pH的影响,但是总铬含量并不能真正代表铬污染对环境的危害程度,并且不同土壤采用统一的标准并不科学合理。因此,在确定土壤铬元素的临界含量时,应综合研究不同的土壤类型以及土壤-作物系统。本试验通过盆栽试验研究我国主要土壤类型红壤、黄壤、青紫泥、黑土、潮土、石灰性紫色土中重要农作物小白菜和水稻重金属铬污染主控影响因子,提出主要土壤类型重金属铬污染食物链安全诊断指标,为国家完善铬污染土壤质量标准提供理论依据。根据逐步回归模型,土壤-水稻系统中水稻精米中铬的浓度主要受土壤总铬、有效态铬、六价铬、有机质、亚铁和黏粒含量控制,决定系数高达96%。根据美国国家科学院(1989)提出的人均安全的每日铬食用限量200μg以及土壤铬含量与水稻铬含量之间回归方程,计算土壤-水稻系统中铬污染的诊断指标体系。供试土壤中总铬的污染诊断指标依次为红壤(90.6mg kg-1)>青紫泥(81.1mg kg-1)>石灰性紫色土(80.8mgkg-1)>黑土(78.2mg kg-1)>黄壤(56.6mgkg-1)>潮土(51.8mgkg-1)。黄壤、红壤、黑土和潮土中水稻精米铬浓度与土壤中有效态铬浓度相关系数最高,因此这四种土壤选择有效态铬作为土壤铬污染诊断指标,且分别为1.54、0.56、0.42和2.18mgkg-1,而石灰性紫色土和青紫泥中水稻精米铬浓度与土壤中六价铬浓度相关系数最高,这两种土壤选择六价铬作为土壤铬污染诊断指标,且分别为0.68和0.84mg kg-1。
5.土壤-小白菜系统中铬迁移转化规律和污染诊断指标
根据逐步回归模型,土壤-小白菜系统中小白菜地上部铬浓度主要受土壤总铬、有效态铬、有机质和亚铁浓度控制,模型的决定系数高达99.6%。土壤-小白菜系统中铬污染的诊断指标也通过美国国家科学院(1989)提出的人均每日铬食用限量200μg以及土壤铬含量与小白菜铬含量之间回归方程计算。土壤中总铬的污染诊断指标依次为红壤(443mgkg-1)>青紫泥(352mgkg-1)>石灰性紫色土(274mgkg-1)>黑土(272mgkg-1)>黄壤(226mgkg-1)>潮土(204mgkg-1)。黄壤、红壤、黑土和潮土中小白菜铬浓度与土壤中有效态铬浓度相关系数最高,因此这四种土壤选择有效态铬作为土壤铬污染诊断指标,且分别为20.7、15.8、21.2和20.4mg kg-1,而石灰性紫色土和青紫泥中小白菜铬浓度与土壤中六价铬浓度相关系数最高,这两种土壤选择六价铬作为土壤铬污染诊断指标,且分别为26.5和28.0mg kg-1。
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(Ⅲ) and Cr(Ⅵ). 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(Ⅵ) reduction;2) to identify the species of Cr-resistant bacteria in representative agricultural soils, and investigate the effect of Cr-resistant bacteria on Cr(Ⅵ) 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(Ⅵ) 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(Ⅵ) reduction is not fully understood due to lack of systematic studies. A microcosmos incubation experiment was conducted to investigate the reduction kinetics of Cr(Ⅵ) in seven soils and its relationships with soil properties. The results indicate that the reduction of Cr(Ⅵ) can be described by a first-order reaction. The reduction rates of Cr(Ⅵ) 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(Ⅵ) 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(Ⅵ) in soil could be quantitatively predicted by the measurement of dissolved organic matter, Fe(Ⅱ) content, pH, and soil particle size distribution, with the fitting level of95.5%. The results indicated that the reduction of Cr(Ⅵ) in natural soils is not controlled by a single soil property but the result of the combined effects of dissolved organic matter, Fe(Ⅱ), 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(Ⅵ) 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(Ⅵ) reduction pathway, which was mainly attributed to Cr-resistant bacteria. Soil properties, especially Fe(Ⅱ) and soil particle distribution, affected the microbially mediated Cr(Ⅵ) 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 of200μg 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(Ⅲ) 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.
1.土壤因子对六价铬形态转化的影响及相对重要性
本研究采用室内培养试验模拟七种土壤中Cr(Ⅵ)的还原过程,并通过相关分析和逐步回归分析相结合探讨影响Cr(Ⅵ)还原的土壤性质以及影响因子之间的相对重要性。六价铬在土壤中的还原遵循一级动力学模型,还原速率与土壤pH、有机质、可溶性有机质、亚铁、易还原态锰、土壤黏粒含量、微生物多样性Shannon指数显著相关。根据逐步回归模型,Cr(Ⅵ)在土壤中的还原主要受可溶性有机质、亚铁、pH、土壤颗粒组成等土壤性质的联合作用,模型的决定系数高达95.5%。
2.主要农业土壤中铬耐性菌种类及对六价铬还原的影响
本试验比较灭菌与非灭菌土壤中六价铬的还原情况,探讨土壤微生物在六价铬还原过程中的作用,并通过PCR-DGGE-克隆测序鉴定影响六价铬还原的耐性菌种类主要为芽孢杆菌Bacillus sp.、埃希氏菌Escherichia sp.、异常球菌Deinococcus sp.、小单胞菌Micromonospora sp.、甲基杆菌Methylobacterium sp.、马赛菌Massilia sp.、酸杆菌Acidobacterium sp.、丛毛单胞菌Comamonas sp.、慢生根瘤菌Bradyrhizobium sp和节杆菌Arthrobacter sp.,并证明了它们能显著促进土壤中六价铬的还原。铬对微生物群落的影响主要表现为在铬的选择作用下,铬耐性菌成为土壤中的优势菌群。土壤性质,特别是亚铁和土壤颗粒组成,影响了微生物在六价铬还原过程中的贡献率。
3.水分管理方式对土壤-水稻系统铬形态转化的影响
在铬污染水稻田中,铬主要以Cr(Ⅲ)和Cr(VⅥ)两种形态存在于水-土环境中,且能在一定的氧化还原电位范围内发生相互转化。干湿交替的农田系统随着周期性的淹水与落干的交替变化,使其氧化还原环境发生了剧烈的变化,从而影响到体系内铬元素的迁移转化。本试验选用红壤和黑土两种土壤,研究水分管理方式对土壤-水稻体系中土壤氧化还原电位、铬迁移转化和水稻积累铬的影响。淹水条件下的土壤氧化还原电位随淹水时间不断下降,在水稻收获时降到-370～263mv,而干湿交替条件下,土壤中水和气相互消长,导致氧化还原电位周期性变化,红壤氧化还原电位在-291和142mv之间反复波动,黑土氧化还原电位在-254和189mv之间反复波动,土壤环境在氧化状态和还原状态之间交替变化。干湿交替条件下六价铬浓度随着氧化还原电位的波动而上下波动,土壤中有效态铬的浓度和重金属铬的迁移能力显著高于淹水处理,从而导致稻米铬含量的增加。红壤和黑土干湿交替处理下水稻铬浓度均显著(P<0.05)高于淹水处理,其中黑土中水稻籽粒各部分铬浓度在干湿交替处理下高于淹水处理19.8～26.5%,红壤高于淹水处理18.4～29.8%。因此,建议在铬污染土壤中,采用淹水这种水分管理方式,降低土壤中铬的生物有效性和水稻籽粒中的铬浓度。
4.土壤.水稻系统中铬迁移转化规律和污染诊断指标
目前土壤铬质量标准是以总铬来作为衡量指标的,仅考虑了pH的影响,但是总铬含量并不能真正代表铬污染对环境的危害程度,并且不同土壤采用统一的标准并不科学合理。因此,在确定土壤铬元素的临界含量时,应综合研究不同的土壤类型以及土壤-作物系统。本试验通过盆栽试验研究我国主要土壤类型红壤、黄壤、青紫泥、黑土、潮土、石灰性紫色土中重要农作物小白菜和水稻重金属铬污染主控影响因子,提出主要土壤类型重金属铬污染食物链安全诊断指标,为国家完善铬污染土壤质量标准提供理论依据。根据逐步回归模型,土壤-水稻系统中水稻精米中铬的浓度主要受土壤总铬、有效态铬、六价铬、有机质、亚铁和黏粒含量控制,决定系数高达96%。根据美国国家科学院(1989)提出的人均安全的每日铬食用限量200μg以及土壤铬含量与水稻铬含量之间回归方程,计算土壤-水稻系统中铬污染的诊断指标体系。供试土壤中总铬的污染诊断指标依次为红壤(90.6mg kg-1)>青紫泥(81.1mg kg-1)>石灰性紫色土(80.8mgkg-1)>黑土(78.2mg kg-1)>黄壤(56.6mgkg-1)>潮土(51.8mgkg-1)。黄壤、红壤、黑土和潮土中水稻精米铬浓度与土壤中有效态铬浓度相关系数最高,因此这四种土壤选择有效态铬作为土壤铬污染诊断指标,且分别为1.54、0.56、0.42和2.18mgkg-1,而石灰性紫色土和青紫泥中水稻精米铬浓度与土壤中六价铬浓度相关系数最高,这两种土壤选择六价铬作为土壤铬污染诊断指标,且分别为0.68和0.84mg kg-1。
5.土壤-小白菜系统中铬迁移转化规律和污染诊断指标
根据逐步回归模型,土壤-小白菜系统中小白菜地上部铬浓度主要受土壤总铬、有效态铬、有机质和亚铁浓度控制,模型的决定系数高达99.6%。土壤-小白菜系统中铬污染的诊断指标也通过美国国家科学院(1989)提出的人均每日铬食用限量200μg以及土壤铬含量与小白菜铬含量之间回归方程计算。土壤中总铬的污染诊断指标依次为红壤(443mgkg-1)>青紫泥(352mgkg-1)>石灰性紫色土(274mgkg-1)>黑土(272mgkg-1)>黄壤(226mgkg-1)>潮土(204mgkg-1)。黄壤、红壤、黑土和潮土中小白菜铬浓度与土壤中有效态铬浓度相关系数最高,因此这四种土壤选择有效态铬作为土壤铬污染诊断指标,且分别为20.7、15.8、21.2和20.4mg kg-1,而石灰性紫色土和青紫泥中小白菜铬浓度与土壤中六价铬浓度相关系数最高,这两种土壤选择六价铬作为土壤铬污染诊断指标,且分别为26.5和28.0mg kg-1。
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(Ⅲ) and Cr(Ⅵ). 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(Ⅵ) reduction;2) to identify the species of Cr-resistant bacteria in representative agricultural soils, and investigate the effect of Cr-resistant bacteria on Cr(Ⅵ) 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(Ⅵ) 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(Ⅵ) reduction is not fully understood due to lack of systematic studies. A microcosmos incubation experiment was conducted to investigate the reduction kinetics of Cr(Ⅵ) in seven soils and its relationships with soil properties. The results indicate that the reduction of Cr(Ⅵ) can be described by a first-order reaction. The reduction rates of Cr(Ⅵ) 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(Ⅵ) 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(Ⅵ) in soil could be quantitatively predicted by the measurement of dissolved organic matter, Fe(Ⅱ) content, pH, and soil particle size distribution, with the fitting level of95.5%. The results indicated that the reduction of Cr(Ⅵ) in natural soils is not controlled by a single soil property but the result of the combined effects of dissolved organic matter, Fe(Ⅱ), 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(Ⅵ) 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(Ⅵ) reduction pathway, which was mainly attributed to Cr-resistant bacteria. Soil properties, especially Fe(Ⅱ) and soil particle distribution, affected the microbially mediated Cr(Ⅵ) 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 of200μg 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(Ⅲ) 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.
引文
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