| The remediation of heavy metals contaminated agricultural soils is one of the problems in the remediation of contaminated soils fields. In China, the remediation for large-scale of contaminated agricultural soils with low to moderate levels of heavy metals must be performed, in which, agricultural crop production activities are still taking. Although, phytoextraction using hyperaccumulator plants to remove heavy metals from soils has been proposed because of environmental friendly, low-cost technology and others, farmers will not undertake to plant hyperaccumulators with stopping crops in farmlands due to no harvest of grain. Intercropping and rotation systems by assigned reasonably might remedy this defect, with little affecting crop production and economic benefits. Therefore, according to previous investigations of our research team, field experiments were conducted in lower reaches of Minjiang River, southwest of China. Four typical cropping systems including S1:ginger-lettuce-lettuce, S2:maize+sweet potato-chinese cabbage, S3:maize+ginger-stem mustard, and S4:rice were selected and cultivated from February in 2009 to January in 2010. With affected by four cropping systems, the variations of Pb,Zn,Cd metals in soils and the accumulation and transfer characteristics of Pb,Zn,Cd metals from soils to the crops were investigated. In meantime, the security of edible parts of agricultural products and risk of harm to human health by feeding agricultural products from cropping systems were brief analysis. GRATS (grey relational analysis with time series) was used to evaluate comprehensive ecological remediation efficiencies of four systems, including scavenging ability of cropping systems for Pb, Zn, Cd, and extraction capacity of inedible parts and security of crops for Pb, Zn, Cd and so on. The results were as follows:(1) With affected by four cropping systems, the variation of Pb content in the soils decreased significantly then flat in S1 to S3 (decreased to 31.54%,10.71%, and 12.20%, respectively), and decreased significantly in S4 (decreased to 14.17%). And, which of Zn content in the soils decreased then increased significantly in S1 to S3, and decreased significantly in S4. In the meantime, fall down was observed about Cd content in the soils of S1 to S3, and decreased significantly in S4, in which, the ratios of decrease were 24.77%,47.13%,56.84%, and 19.00%, respectively. These variations might be resulted by crop composition and types of each system, the difference of heavy metals accumulated by crops, the influence from soil fertility, and the migration of heavy metals with soil water, and so on. The variation about Zn content might be related to agricultural fertilization measure from phosphorus and nitrogen fertilizer to poultry manure.(2) The distribution consistency of Pb in same type crop organs were observed, as follows:cereals (maize and rice) for leaves>stems> seeds, root crops (ginger and sweet potato) for leaves>stems>roots, vegetables (lettuce, cabbage, and mustard) for roots>leaves and stems. The accumulation of Pb in inedible parts of crops exceeded 1.00 to 50.70 times than that in edible parts. And the distribution consistencies of Zn in same type crop organs were not observed. Cd was found that mainly accumulated in stems and leaves of crops, which distribution characteristics in cereals and root crops were similar to Pb, but reserved in vegetables. The difference of Pb, Zn, and Cd accumulated by same crop organs from different cropping systems were due to soil texture and pollution levels, crop compositions in different cropping systems, and influence from fertilization, and so on.(3) The results about application analysis of bio-accumulation factors (BAF) showed, compared with BAF in inedible parts of crops, the BAF in edible parts exceeded 12.78 to 50.70 times of Pb,1.76 to 7.54 times of Zn, and 5.46 to 19.38 of Cd in cereals,2.62 to 12.85 times of Pb,1.93 to 4.08 times of Zn, and 1.25 to 3.75 times of Cd in root crops, 1.00 to 2.81 times of Pb,0.90 to 1.49 times of Zn,0.62 to 0.97 times of Cd in vegetables. According to appraise security of edible parts of crops, cereals were belonged to pollution-safe cultivars (PSCs), with lower heavy metals content accumulated in edible parts of crops than relevant standard limits or only polluted lightly. In addition to Zn content in ginger, sweet potatoes, and lettuce, and Cd content in S3 ginger were security or polluted slight, heavy metals in root crops and vegetables were more serious, and there are certain risks consumed by human direct, but security as pigs, poultry feed. Moreover, a certain avoidance capacity and tolerance were observed in corn, rice and crops from S1 systems.(4) According to the estimated daily intake of heavy metals through foods from four cropping systems, the daily intake amount of Pb,Zn,Cd metals per day for an adult through crop productions were 2.86 mg/d,16.33 mg/d, and 0.34 mg/d, respectively, and exceeded 6.67,1.09,4.72 times of corresponding permit maximum limit of standards by WHO/FAO, mainly due to the heavy metals concentration in edible parts of vegetables went beyond related standard limit. But, the risk of heavy metals content in crop productions for human body has reduced effectively, because of machine processing measures before food, tolerance and cleanup mechanism of human body for heavy metals, sweet potato, vegetables were used as pig feedstuff by farmers, market transaction of agricultural products, and diversification foods in daily and others. In conclusion, based on the above measures, there was certain ensure for security of foods from four cropping systems, with decreasing significant of heavy metals content in the soils as affected by four systems and others.(5) GRATS was used to evaluate comprehensive ecological remediation efficiencies of four systems, and showed the order as:S3>S2>S4>S1. Considering with economic benefits, decrease ratio of heavy metals contamination in the soils, security of foods from systems and others, we inferred the S3 system would be most popular by local farmers in four systems, resulting in promotion and application to achieve the sustainability and large-scale remediation for the local heavy metals contaminated agricultural soils. Although, comprehensive ecological remediation efficiency was relatively poor, S1 system has been proposed in agricultural soils contaminated seriously by Zn and can not remediated immediately, because of avoidance capacity and tolerance of crops for and economic benefits of ginger. |
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