稻麦轮作下水稻土重金属形态特征及其生物有效性

The speciation and bioavailability of heavy metals in paddy soils under the rice-wheat cultivation rotation

在成都平原稻麦轮作下86个土样中重金属全量和各形态含量分析的基础上,采集分析相应点位上种植的主要粮食作物小麦和水稻籽粒的重金属含量,通过描述性统计分析、相关性分析及线性回归方程的模拟,研究了土壤中重金属的形态分布特征及其生物有效性。统计分析结果表明:土壤中Cr、Cu、Pb、Zn各形态含量的分配顺序为:残渣态>有机物结合态>铁锰氧化物结合态>碳酸盐结合态>可交换态,而Cd各形态含量的分配顺序为:铁锰氧化物结合态>残渣态>碳酸盐结合态>有机物结合态>可交换态。若以活性态(非残渣态含量)而论,Cd的活性最高,Cr的活性最低。除Cr外,各元素全量与各活性形态之间的关系较为密切。土壤中全Cd含量与4种活性形态Cd含量间的相关性均达极显著水平;Cu、Zn全量分别与其铁锰氧化物和有机物结合态含量,Pb全量分别与其可交换态和铁锰氧化物含量间的相关系数达显著或极显著水平。用卫生部颁布的食品中重金属限量卫生标准评价稻麦籽粒重金属累积情况,其结果表明,小麦籽粒(旱作)中的Cd、Cr、Cu、Pb分别超标14.71%、8.70%、6.50%和17.40%,而水稻籽粒(水作)仅有Pb和Cd含量超标10.90%和8.70%。不同形态重金属对水稻和小麦籽粒中重金属累积的影响效应不同,从多元线性回归方程的回归系数来看:稻-麦籽粒中Cd含量受到土壤中各活性形态Cd含量的影响。除铁锰氧化态Cd和有机物结合态Cd含量分别对小麦和水稻籽粒中Cd的累积表现出负效应外,其余均为正效应。可交换态Cu含量对稻-麦籽粒Cu累积的效应最大,其次受碳酸盐结合态Cu含量的影响。稻-麦籽粒Pb含量的累积受土壤各活性态Pb含量影响效应的差异不甚明显。稻-麦籽粒Zn含量与土壤各活性态Zn含量间无显著线性关系。

The speciations of the heavy metals （Cu, Cd, Cr, Pb and Zn） in paddy soils under the rice-wheat rotation cultivation in Chengdu Plain were determined with the chemical fractionation method. The contents of heavy metals in seeds of both rice and wheat grown in these soils at the same sampling sites were also determined to estimate the bioavailability of these heavy metal fractions. Statistical analyses including correlation analysis and multiple linear regression revealed that concentrations of Cr, Cu, Pb, and Zn in all the paddy soils decreased in the following order： Residue 〉 Bound to Organic Matter 〉 Bound to Iron and Manganese Oxides 〉 Bound to Carbonates 〉 Exchangeable, while that for Cd decreased in a different order： Bound to Iron and Manganese Oxides 〉 Residue 〉 Bound to Carbonates 〉 Bound to Organic Matter 〉 Exchangeable. Regarding the labile fractions （sum of all fractions excluding the residue）, the content of Cd is the highest, while that for Cr the lowest in those paddy soils. Except for Cr, the contents of the labile fractions of all the heavy metals studied were correlated with their total contents. This is especially true for Cd with the correlation between the total content of Cd and that of all fractions significant at a 99% confidence level. Significant correlations at 95% to 99% confidence levels were also found between the content of the fraction ＂Bound to Iron and Manganese Oxides＂ or ＂Bound to Organic Matter＂ and the total contents for Cu and Zn, and between the content of ＂ Exchangeable＂ or ＂ Bound to Iron and Manganese Oxides＂ fraction and the total content for Pb. Relative to the national standards on heavy metals in foods by the Chinese Ministry of Health, the contents of Cd, Cr, Cu and Pb in the wheat seeds exceed the upper limit by 14.71% 8.70% ,6.50% and 17.40%, respectively. In the rice seeds, however, only the contents of Pb and Cd were over the upper limits （ by 10.90% and 8.70%, respectively）. The speciation of these heavy metals in the paddy soils apparently affected the accumulation of the different heavy metals in the seeds of wheat and rice, but the exact effect varied for different elements. For example, regarding the regression coefficients in these multiple linear regression equations, the content of Cd in seeds of both rice and wheat were positively related to the contents of all labile fractions of Cd except two fractions ＂Bound to Iron and Manganese Oxides＂ and ＂Bound to Organic Matter＂, while a negative correlation was found between the Cd contents in seeds and these two fractions. The content of Cu in the seeds of rice and wheat was highly correlated to that of the Exchangeable Cu, but also related to that of the Cu fraction bound to carbonate as well. The correlation between the content of Pb in the seeds and any labile fractions was weak and no difference was found between the various fractions in this regard. Finally, there was no significant relationship between the content of Zn in the seeds and that of any fractions of Zn in the paddy soils.