| With the rapid economic development of modern society,the discharge of the environmental contamination is increasing steadily.The environmental pollution and ecological damage become more and more serious,which threatens the human life.The heavy metals which could not been degraded and finally disappear in natural environment will accumulate into the crops and other plants by he way of removing and transforming.So the accumulated heavy metals would threaten people' s health through food cycle. Characteristics of heavy metals in soils-crops of regional system are the hotspot of research at present, which are widespread,however,indistinct,surface-agglomerated,and non-reversible and so on.Heavy metals in plants originate from two sources:one is adsorbing from contaminated soil and the other from air. The degree of the pollutions is influenced by species of plants with their various internal structure and external form of organs.In addition,the adsorbing mechanism and accumulating capacity differ from different plants.All of these mentioned aspects make the researches on heavy metals pollution and prevention so difficult.Based on surveying the content of heavy metals(Cu,Zn,Pb,Cd,Ni,Hg,As,Cr,Fe, and Mn)in soil -crops in Chongqing,this paper discussed their relationships between soils and planting pattern,the forms of heavy metals in crops and in soils,and how these heavy metals accumulating into the crops.From these discussions,the characteristics of the heavy metals in regional soil - crop ecosystems can be revealed and further work on heavy metal pollution and prevention can be made.1 Heavy metals in soil- paddy systemThe average heavy metals' holding capacity in soils planted with paddy is Fe＞Mn＞Zn＞Cr＞Pb＞Cu＞Cd.For the above heavy metals,the coefficient of variation of Mn is the maximal,and that of Cr is the minimal.The capacities of other heavy metals are in keeping with skew distribution except Cr and Mn. The heavy metals can be classified into 3 groups by the method of clustering:the first group composed of Zn,Pb,Cu,and Cd,the second group of Fe,Mn,and Cr belongs to the third group.The correlation coefficient was clearly positive among all heavy metals in soils planted with the paddy.Comparisiond with the second national standards of soil environment quality,Cd exceeds 16%in soil samples.This result illustrated that Cd is the main pollutant in soils planted with paddy in Chongqing.The average content of heavy metals in paddy is Fe＞Mn＞Zn＞Cu＞Cr＞Pb＞Cd.The coefficient of variation of Fe is the maximal,and that of Zn is the minimal.The capacity of other heavy metals is in keeping with skew distribution except Zn with a normal distribution.This result shows there is no obvious influence on the content of Zn whether it is abundant or scarce if one person only eats one kind of cereal crops for a long time.There are more positive correlations than negative ones among heavy metals in plants. The relationship between Mn and other heavy metals presents remarkable positive correlation,so the characteristics of heavy metals can be analyzed by the principal component method.The comparability between Cd and Mn in paddy provides a new idea for further research on the behavior of Cd.Comparisiond with the national food hygiene criteria,the content of Pb and Cr in paddy seriously exceed the criteria.The linear regression analysis shows a clear linear relationship of Cd content in paddy and in soils.That indicates Cd in paddy roots in soil.2 Heavy metals in soil-vegetable systemIn the soil with the vegetation plantation,the average heavy metals' contents of Cr,Ni,Cu,Zn,As,Cd, Hg,and Pb are 52.12mg/kg,33.44mg/kg,23.04mg/kg,82.27mg/kg,6.757mg/kg,0.291mg/kg,0.056mg/kg, and 37.04mg/kg,respectively.For these heavy metals,the coefficients of variation for Cr,Ni,Zn,and Pb are the minimal.The contents of heavy metals vary with the different soil horizon.For example,there is remarkable difference in Cd and Hg content in the soil horizon of 0-20cm and of 20-40cm;however,other horizons vary quite little.The coefficient of variation of Cr,Ni,Cu,and As are higher than that of Cd,Hg, and Pb.Apart from Cd and As,the average content of other heavy metals in the yellow soil and yellow brown soil is more than that in the purple soil.That is due to the poor stickiness of the purple soil comparisiond with yellow soil and yellow brown soil,which also leads to the poor ability to absorb heavy metals.Heavy metals in different vegetable plantation soils show superiorer correlation,and the correlation in the yellow soil is the highest.The correlation between different types of heavy,metals in the yellow soil and yellow brown soil shows positive correlation.Ni,Cd in the purple soil show negative correlation to Hg. Hg shows inferior correlation to other heavy metals due to its easy release into air,and the stickiness of the purple soil is poor and with high porosity.Cu,Ni,Cr accompany each other strongly in all types of soil. The analysis showed notable diversity in other heavy metals except for Hg,Cr,and Cd in different area of vegetable plantation(polluted zone and non-polluted zone).That illustrates vegetable soil suffers relatively less from air pollution because of its vegetation coverage.The correlations are all positive between different types of heavy metals in polluted zoning;however those in non-polluted zone show either positive or negative.The correlations between Hg,Cd,and other heavy metals in non-polluted zone are much inferior to in polluted zone.The main reason is that the soil in non-polluted zone is purple soil with a poor absorption,which makes Hg volatilizes into air and Cd go to deeper soil.Ni,Cu,and Cr showed strongly accompaniment in different part of vegetable plantation.Comparisiond with Chongqing's soil background value,the content of Cd,Hg,and Pb is higher,and that of Cr,N,Cu,and Zn,As are almost the same.So Cd, Hg,and Pb accumulate fairly more in vegetable plantation of Chongqing,and human actions obviously affect the content of heavy metals.Comparisiond to the second level of national standards of soil environment quality,the content of Cd,Hg,Cu,Cr,Zn,and Ni exceed seriously,with the values of 19.0%, 0.8%,1.8%,0.3%,0.3%,and 7.0%,respectively.All this indicated vegetable plantations in Chongqing were seriously contaminated by heavy metals,and Cd should appeal to higher concern particularly.The average content of heavy metals in vegetable for Cr,Ni,Cu,Zn,As,Cd,Hg,and Pb are 0.14mg/kg,0.22mg/kg,0.77mg/kg,3.52mg/kg,0.020mg/kg,0.024mg/kg,0.0019mg/kg,and 0.10mg/kg, respectively.The coefficient of variation of Cd is the maximal,and that of Cu is the minimal.The mean and the coefficient of variation of heavy metals' content show remarkably distinctness in different types of vegetable.The average content of heavy metals is higher in amaranth,asparagus,lettuce and onion.While the variation coefficient of cabbage,capsicum,and onion are higher.Correlation analysis among heavy metals in the main types of vegetables indicated that there are clear difference on absorption in heavy metals by different sorts of vegetables at the process of growth.And cluster analysis among heavy metals in different types of vegetables indicated that there was more superior correlation between the content of heavy metals and physiologic morphologic character.Research on characteristics of different kinds of vegetables(leaf vegetables,fruit vegetables)indicated that leaf vegetables are affected by more factors and on higher degree.There is markedly difference on the behaviors of heavy metals in all kinds of vegetables as well as accompanying relationship.The quantity of the accompanying heavy metals is the most in the leaf vegetables,and that is because the interface of leaf vegetables is larger which makes serious air pollution on the leaf vegetables.The analysis of geo-statistics showed the distributions of heavy metals such as Cr,Ni,Zn,As,Cd,Hg,Pb and Cu in the vegetable soil is isotropy and the semi variance function of Hg and Cu coincides with Gaussian model and the other heavy metals coincides with Spherical Model.The evolution trend of the content of Cr is distributed in the east of Chongqing rather than in the west and the higher content region appears in the suburb area,Beibei,Banan,Yubei,Changshou,Nanchuan,Wangsheng.No apparent changes of evolution trend of the content of Ni is detected and the higher content region appear in the suburb area,Hechuang,Wangsheng,Qijiang,Nanshuan.And the concentrations of Ni in these region are 35～49mg/kg.No apparent regularities of evolution trend of the Zn contents are detected and higher contents region appear in the southeast areas including the suburb area,Banan,Nanchuan,Wangsheng and the junction between Hechuang and Tongnan.The concentrations of Ni in these region are 88～140mg/kg. The evolution trend of the contents of As is distributed highly in the north and southeast of Chongqing.The concentrations of As in these region are 10～25mg/kg and the contents of As in the study area are relatively low.The evolution trend of the contents of Cd was highly distributed in the north and south of Chongqing. The concentrations of Cd in these regions are 0.30～0.44mg/kg.The contents of Cd appear in zonal distribution.The evolution trend of the contents of Hg is diffused in circular point style and higher contents region appear in the north areas including the suburb area,Beibei,Banan,Liangtong,Dazhu.The concentrations of Hg in these regions are 0.05～0.13mg/kg.The evolution trend of the contents of Pb is distributed in the east of Chongqing rather than in the west and the higher contents region appear in the suburb area,Beibei,Yubei,Changshou,Nanchuan,Banan,Wangsheng,Qijiang.The concentrations of Pb in these region are 39～54mg/kg.No apparent changes of evolution trend of the contents of Cu are detected and higher contents region appear in the suburb area,Nanshuan,Wangsheng.The concentrations of Cu in these region are 32～64mg/kg.3 Heavy metals in soil-citrus systemThe mean contents of Cu,As and Cr in the yellow soil of the Citrus field are 10%more than those in purple soil.While the mean contents of Zn,Pb,Cd,Ni and Hg have not obvious diversity.Except for Hg in the purple soil,the coefficient of variation of all other heavy metals in the yellow soil and the purple soil is lower than 30%.The coefficient of variation of all heavy metals in purple soil is higher than that in the yellow soil,which indicates that the heavy metals in purple soil differ greatly in the degree of ageing. Comparisiond with the soil background level of Chongqing,the contents of As and Cr in the yellow soil and that of Cd and Cr in the purple soil have increased in certain extent but the difference isn't obvious,and the contents of the other metals have little change.That explains the contents of heavy metals in the Citrus field in Chongqing are relatively steady and hardly influenced by the exogenous pollution.There are significant positive correlations between Zn and Ni,Cu,the same between Cr and Ni,Pb,and also Cd and Pb,As in the purple soil of the citrus field;Zn with Cr,As,Cd with Hg,As with Cr,Pb also have significantly positive correlation.The significantly positive correlations among all kinds of heavy metals in the purple soil explain that the purple soil has been greatly influenced by the parent soil materials and the degree of ageing is lower.Meanwhile except that Hg and Zn,Hg and Ni show the negative correlations, there exists the positive correlations between one type of heavy metals and the other,but the negative correlations in yellow soil are more than those in the purple soil.That illustrates the synergistic reaction among the heavy metals in the purple soil is more obvious,while the antagonism in the yellow soil is more obvious,which may be related to the soil formation.The heavy metals(Cd,Hg,As),(Cu,Ni),(Zn,Cr)in the main types of soil in the Citrus field accord with the process of developing and forming of the soil,which may be related to the character of the elements and to the special eco-environment.In the soil horizon from the depth of 0-30cm and of 30-60cm of the yellow soil and the purple soil in the Citrus field, the heavy metals such as Cu,Zn end Ni are the main forms of mineral,but the contents in the form of exchange is lowest.And the contents of Cd in the form of ferro-manganese are the most,organic form and exchange form follows,and the water soluble matters are the least.In the soil horizon from the depth of 0-30cm and of 30-60cm of the yellow soil,the contents of Pd are mainly in the form of ferro-manganese, but in the purple soil,the contents of Pd are mainly in the form of mineral,the form of ferro-manganese follows,and the exchange form is the least.The contents of Cd in the form of exchange form in the purple soil reaches to 14.55%,and the other metals as the available form are few,that may be the main reason of exceedance about Cd in the agricultural products in Chongqing.The content of Cu in the form of the water soluble in the purple is higher than other heavy metals,which may be the main reason of exceedance about Cu in the river of Chongqing with the high flow period.Correlation analysis among all forms of heavy metals in the soil of the Citrus field explains that the biological available forms(the water soluble form, exchange form,and carbonic acid form)in the yellow soil and the purple soil are greatly influenced by the natural environmental factors.In the yellow soil and the purple soil,the content of Cu and Zn in the form of the water soluble and Cu and Ni in the form of ferro-manganese show the markedly positive correlation. The synergistic reaction of Cu and Zn in the soil may be related to the properties of the elements.In the Citrus field,correlation analysis among all forms of heavy metals in the soil of the Citrus field explains that the correlations among all forms of heavy metals in all types of the soil have markedly differences.The significant correlations among all forms of heavy metals in the yellow soil is more inferior,which further demonstrates the purple soil has been greatly influenced by the parent soil materials and the degree of ageing is lower.Analysis of the concentration of metals in different breeds of Citrus(Jincheng Citrus,Navel Citrus, Valencia Citrus)shows the correlations among heavy metals are closely related to the breed and part of the citrus.The heavy metals in the flesh with those in soil,in leaves and in seedcase show the significant linear correlations.There are significant correlations between some heavy metals and forms of heavy metals in soil.For example,Zn in the citrus leaves shows significant negative correlations with Zn in the form of ferro-manganese in the soil,while Ni of the leaves shows significant positive correlations with Ni in the form of carbonic acid in the soil,and Zn of the seedcase shows significant positive correlations with Zn in the form of exchange in the soil as well as in the form of mineral,and Pb of the seedcase shows significant positive correlations with Pb organic form in the soil,and Cd of the seedcase shows significant negative correlations with Cd exchange form in the soil but shows significant positive correlations with Cd in the form of ferro-manganese in the soil,and also Ni of the flesh shows significant positive correlations with Ni carbonic acid form in the soil.The cluster analysis of metals in different parts of all breeds of Citrus indicates that in flesh and leaf,sorts of metals are accordant,no diversity for breed,while sorts of heavy metals in flesh is closely related to the breed.From what we analyzed in this study,we can draw the conclusion that Cd is the main element of heavy metals pollution in soil planted paddy in Chongqing and Cd in paddy mainly comes from soil;the soil of vegetable plantations in Chongqing were seriously contaminated by heavy metals and Cd should appeal to high concern particularly,and there are no obvious spatial distribution characteristics about the heavy metals in the soil-vegetable of regional ecosystem;the contents of heavy metals in the Citrus field are relatively steady and are hardly influenced by the exogenous pollution.The heavy metals in soil planted with paddy show obvious comparability with those in the Citrus field but distinguish with that in the vegetable plantation,which indicates that the heavy metals in soil is seriously influenced by human-induced factors.The contents of heavy metals as Cu,Zn and Cd accumulated into the vegetable as well as the contents of Cr into the Citrus flesh are obviously higher than those into other plants.The heavy metals such as Pb,Cd and Cr in the paddy present the comparability with those in citrus flesh.This paper obtained lots of research data by studying the Characteristic of heavy metals in soil and crop of regional system so as to provide the scientific guidance for the research and prevention work on the heavy metals pollution.However,there are also some problems.And in the future,we should do further research on the behavior characteristics of releasing the heavy metals from all kinds of soils and on the reason for the physico-diversity of the heavy metals in different kinds(breeds)of the crops,and on taking apart the physiological reaction of sorts of crops to the heavy metals,and also on simulating the general circulation of the heavy,metals on the regional ecosystem and the small size circulation on the crops.
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