| The rapid development of industrialization and urbanization in China causes the growing problem of heavy metal pollution of soil,threatening ecological environment and human health.The suburban farmland ecosystem carries heavy metals which from industry,transportation and our life.In addition,application of fertilizer and pesticide and waste water irrigation are main Heavy metal input pathway.Therefore,prevention and management of heavy metal pollution in suburban farmland ecosystem become particularly important.Critical loads of heavy metals are an important management tool,and the assessment of critical loads of metals in soil can be used as an important tool for evaluation and for risk precaution of future inputs of metal in order to avoid the occurrence of heavy metal pollution and its long-term risks for people.This paper is based on a large amount of measured data,studying the process mechanism of migration and transformation for heavy metals in soil-crop system,and carry out the mapping of critical loads for heavy metals in soil,to provide scientific basis for heavy metals management.The measured data in this paper mainly include the content of heavy metals in soil,wheat,rice,and water.In addition,the atmospheric heavy metal deposition flux also has been estimated.The basic data of this paper mainly include land use data of the study area,agricultural planting pattern,water environmental standards,and the health standards of children and adults.The process mechanism is the absorption,migration and accumulation process of heavy metals elements in soil-crop system.We have established heavy metal uptake model to improve and perfect the critical load model,and to improve the accuracy of the critical load model ultimately.We have calculated and mapped the critical load of heavy metals in soil based on mass balance effects,children health risk effects,adult health risk effects,and water quality effects.We compared the critical loads on different effects with the input flux of heavy metals in cropland to calculate the exceedances of critical load,and to put forward the management strategy of heavy metal input.Critical loads of heavy metals in different effects of this study can serve as a reference to control the emissions of heavy metals effectively and to prevent the heavy metal pollution,and achieve the purpose of protecting the ecosystem and human health ultimately.The conclusions are as follows:?1?The total content of different elements in soil was significantly different.The mean values of Cd?Pb?Cu and Zn are 0.6 mg·k9-1?36.3 mg·kg-1?22.1 mg·kg-1?99.2 mg·kg-1,respectively.The total content of the same element is also different.The difference between maximum and minimum of Pb and Zn are the most obvious,the difference are 134.2 mg·kg-1 and 399.7 mg·kg-1.The difference between maximum and minimum of Cd and Cu are smaller,the difference are 2.6 mg·kg-1.and 35.1 mg·kg-1.In addition,the spatial distribution of heavy metals in soil was also different.Higher values of Cd mainly occurred in the southern part of the study area.The value of the southern region is larger than that of the north.Higher values of Pb mainly occurred in the southeast of construction land.Higher values of Cu are mainly concentrated in the northeast of the city.The values of Zn are descending from the southeast to the northwest.In addition,a small portion of the middle and high values are distributed in the northeastern part of the study area.?2?Different parts of wheat had different heavy metal absorption capacity,the absorption of the stems and grains was weak,followed by leaves,and the absorption capacity of the roots was the strongest.The mean content of Cd,Pb,Cu and Ni in roots are 1 mg·kg-1,16.6 mg·kg-1,16.2 mg·kg-1,and 9.1 mg·kg-1 respectively.The mean content of Cd,Pb,Cu and Ni in grains are 0.15 mg·kg-1,0.54 mg·kg-1,5.4 mg·kg-1,and 1.3 mg·kg-1 respectively.The total content of different elements in same part of wheat was significantly different too.The content of heavy metals in roots and leaves show a trend of Pb>Cu>Ni>Cd.Different parts of rice had different heavy metal absorption capacity.For Cd,Pb,and Cu,the absorption capacity of the roots was strong,and the absorption of the stems and grains was weak.For Zn,the absorption capacity of the stems is stronger than roots,and the absorption of the grains was weak.The mean content of Zn in stem and grain are 186.2 mg·kg-1 and 38.6 mg·kg-1.?3?Critical load of same element on different effects was significantly different in paddy field.The minimum and maximum of Cd was occurred on mass balance effects and water quality effects,and the values are 0.6 g·ha-1·a-1 and 17.9 g·ha-1·a-1,respectively.The mean value of critical load was the largest on adult health risk effects,followed by children health risk effects and water quality effects,and the mean value on mass balance effects was the smallest.The minimum and maximum of Cu was occurred on mass balance effects and adult health risk effects.The mean critical load of Cu show a trend of adult health risk effects>children health risk effects>water quality effects>mass balance effects.The mean value of critical load for Pb on water quality effects was larger than values on children health risk effects.The minimum and maximum of Cd was occurred on mass balance effects and adult health risk effects in dry areas.?4?The mean value of heavy metal input flux in cropland show a trend of Pb>Cu>Cd,and the mean value are 14.98 mg·m-2·a-1,10.2 mg·m-2·a-1,and 0.36 mg·m-2·a-1.The heavy metal input flux of Cd was small,and the values are between 0.009 mg·m-2·a-1and 1.17 mg·m-2·a-1.The values of heavy metal input flux for Cu are between 2.84 mg·m-2·-1 and 31.21 mg·m-2·a-1.The values of heavy metal input flux for Pb are between 4.68mg·m-2·a-1 and 31.68 mg·m·a-1 and 31.69mg·m-3·a-1.?5?For Cd,39%and 72%of areas occurred exceedances in paddy field and dry areas under mass balance effects.26%?48%and 82%of the area exceeded the critical loads under children health risk effects in paddy field,dry areas and non-farming areas.18%?40%and 26%of the area exceeded the critical loads under adult health risk effects and 26%?32%and 17%of the area exceeded the critical loads under water quality effects in paddy field,dry areas and non-farming areas,respectively.For Cu,exceedances mainly occurred in mass balance effects and water quality effects.57%and 28%of areas occurred exceedances in paddy field and dry areas under mass balance effects.50%?28%and 70%of the area exceeded the critical loads under water quality effects in paddy field,dry areas and non-farming areas.For Pb,the proportion of exceedances under children health risk effects,adult health risk effects and water quality effecs are the same. |
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