| In order to find out the bio-availability of heavy metals(Cd, As, Pb, Cu, Ni, Co, Cr and Zn) in the soil- crop system around coal gangue heap of Zhongma village, Jiaozuo City, Henan Province,the improved BCR three steps sequential extraction procedure was used to investigate the fraction of heavy metals in agricultural soils, then the risk assessment and pollution sources of heavy metals were conducted based on that. The relationships between the fraction of heavy metals and soil organic matter, soil enzymes were analyzed. The study also analyzed the distribution of heavy metals in wheat plants with time, and discussed the differences between wheat plants and maize plants in heavy metal enrichment ability and effect. In the end, the study assessed the health risks of crop seeds and the suitability of straws returning. The results indicated that:(1)The area of farmland soils were weakly alkaline and mineral. Cd occurred mainly as exchangeable and residual forms, while the rest of the heavy metals(As, Pb,Cu, Ni, Co, Cr and Zn) were dominant in residual state. The order of respective heavy metal total content(unit: mg·kg-1) was Zn( 81.43±7.34) >Cr( 48.58±3.68) >Pb( 33.59±3.98) >Ni( 29.31±1.42) >Cu( 24.32±2.06) >As( 18.02±1.73) >Co(12.85±1.64)>Cd(0.48±0.12), and the order of their respective bio-availability content(unit: mg·kg-1) was Zn(18.46±5.82)>Pb(15.70±4.05)>Ni(7.48±0.60)>Co(5.49±1.93)>As(3.07±1.10)≈ Cr(2.89±0.52)>Cu(1.38±0.34)>Cd(0.29±0.06).(2) The heavy metals(Cd, As, Pb, Cu, Ni, Co, Zn) in soils of this area were accumulated, and the accumulation of heavy metal Pb, Cu, Ni, Co and Zn were mainly caused by the coal gangue heap, the monsoon played an important role in the process of coal gangue heap releasing of heavy metals to the surrounding environment. The soils were contaminated with moderate to strength grade of Cd pollution, and the comprehensive ecological risk of eight kinds of heavy metals was in medium level. The content of Cd met the requirement of Grade Ⅲ(GB156-1995), and the respective content of As, Pb, Cu, Ni, Co, Cr and Zn met the GradeⅡ.(3)Relationships between heavy metals and organic matter as well as relationships between heavy metals and soil enzymes were complex and various. Organic mattercould increased bio-availability of As and Ni, but could suppressed that of Pb and Zn.Accumulation of heavy metals could inhibited the activities of soil invertase and amylase, the tolerance mechanism of soil enzymes could affected the bio-availability of heavy metals at the same time.(4) Wheat plants selectively absorbed heavy metals, specifically, the roots were more likely to enrich Cd and Cr, and the straws more likely to enrich Cd and Zn, the grains more likely to enrich Zn and Cu, yet enrichment effects differed by heavy metal species and crop organs. Heavy metal contents in plants had orders with stable regularity, specifically, the order for roots was Zn ≈Cr >Cu ≈Ni >Pb >Co ≈As >Cd, the order for straws was Zn >Cr >Cu ≈Ni ≈Pb >As >Cd >Co, and the order for grains was Zn >Cu >Ni ≈Pb ≈Cr ≈Cd ≈As >Co. Heavy metal enrichment capabilities of plants also had orders with stable regularity, specifically, the order for roots was Cd >Cr >Zn ≈Cu≈Ni >Pb >Co >As, the order for straws was Cd >Zn >Ni >Cu >Cr ≈Pb >As >Co, and the order for grains was Zn >Cu >Cd >Pb ≈Ni ≈Cr >As >Co.(5)The heavy metal enrichment regulation separately remained consistent with it’s distribution of content in the growth period of wheat plants, specifically, the regulation for Cd, As, Pb, Ni, Co, Cr were roots > straws > grains, the regulation for Cu was roots > grains > straws, but the distribution of Zn was uniform. The contents of heavy metals in plants were unstable in the whole growth period of wheat plants, and the time variation characteristics of heavy metal contents in organs kept pace with time variation characteristics of enrichment of heavy metal in organs. Heavy metal contents of wheat roots had significant or extremely significant positive correlation with that of wheat straws at elongation stage. Cd showed significant positive correlation between the roots and straws in the whole growth period of wheat plants.(6) The order of heavy metal contents in maize roots was Zn >Cu >Cr >Ni >Pb >Co >As >Cd, order for straws was Zn >Cu >Ni >Cr >Pb >Cd≈As >Co, and order for grain was Zn >Cu >Pb ≈Ni ≈Cr >As >Cd ≈Co. The order of enrichment capability in roots was Cd >Cu >Zn ≈Ni ≈Cr >Co >Pb >As, order for straws was Cd ≈Cu >Zn >Ni >Cr ≈Pb >Co ≈As, order for grains was Zn >Cu >Pb ≈Ni >Cd≈Cr >Co ≈As. The heavy metal enrichment regulation also separately remainedconsistent with it’s distribution of content in maize plants, the regulations of Cd, Pb, Cu,Ni were roots ≈ straws >grains, the regulations of As, Cr, Cu were roots >straws >grains,the regulation of Zn was grains ≈straws >roots.(7) wheat plants in common with maize plants in the following points: the content of Zn was the highest in all parts of plants, the contents of Cd, As, Co were very low;vegetative organs had very strong enrichment ability of Cd and grains had very strong enrichment ability of Zn and Cu; their roots had the similar enrichment ability of Cu,straws and grains had the similar enrichment ability of Ni. Differences included: maize straws had stronger enrichment ability of Cu than that of wheat, Pb and Cr in grains had the same point; the content of Zn in wheat straws was higher than that of maize even though their straws had similar enrichment ability of Zn; the content of Co in maize roots was higher than that of wheat even though the latter had stronger enrichment;wheat organs had higher enrichment and content than maize organs in other cases.(8) Wheat grains produced by this area were safe. The content of Pb in maize grains was generally higher than the limited value of GB2762-2012, but met the limited value of NY861-2004, the rest heavy metals(Cd, As, Cu, Cr and Zn)in maize grains were at the safe levels. There were no non-carcinogens health risks caused by heavy metals(Pb, Cu, Ni, Cr, Co, Zn) through dietary intake for adults and children, and there were no carcinogenic health risks caused by Cd and As for adults. There were no carcinogenic health risks caused by Cd and As for children by intaking of maizes,bue the carcinogenic health risks caused by Cd and As for children by intaking of wheat were higher than the value recommended by America Environmental Protection Agency.Whether intaking of wheats or maizes, the children faced far greater potential health risks than adults. Whether children or adults, the potential health risk caused by intaking of wheats were higher than that of maizes. Wheat straws returning were not suitable and maize straws returning was not recommended for this area. |
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