| The farmland soil environment was worsened increasingly because of industrial waste and the irrationality use of chemical fertilizer and pesticide.And the study on soil heavy metal availability was attracting more and more attention.Rice-wheat rotation system is the typical paddy-upland rotation system in paddyfield in south china,but the character of heavy metal movement and accumulation in soil-plant system of the planting system was seldom studied.Therefore,based on the analysis of the total content and various forms content of heavy metals(Cd,Cr,Pb,Cu and Zn) in soil in core part in Chengdu plain,as well as the content of heavy metals in the grains of wheat-rice from the same sampling sites,the spatial distribution of heavy metals(Cd,Cr,Pb,Cu and Zn) and the statistical characteristics of heavy metals in various forms were studied in this paper,as well as the heavy metals accumulation difference in the grains of wheat-rice and their influence factors.And through statistical analyses,spatial interpolation and multiple linear stepwise regressions,the relation between heavy metals in various forms and the physical and chemical characters of soil was also discussed,as well as the effect of heavy metals in various forms on the biology validity of wheat-rice grains.The results indicated:Under the paddy-upland rotation system in Chengdu plain,in the five heavy metals (Cd,Cr,Pb,Cu and Zn),the content of Cr was the highest,and the content of Cd was the lowest.The variation coefficient of Cd,Pb and Cr was higher,and the variation coefficient of Cu and Zn was lower relatively.Except Cd,the other heavy metals were dominated by residual state(over 70%) and exchangeable contents were lowest(under 5%).Cd in soil mainly existed in Fe-Mn oxides form(32.12%),but other forms of Cd were distributed well.Regarding the labile fractions(sum of all fractions excluding the residue),the activity of Cd was the highest,while that for Cr was the lowest in those paddy soils,the order of labile fractions of heavy metals as Cd＞Pb＞Zn＞Cu＞Cr.There was different accumulation influence of rice-wheat grains for different heavy metals.Pb and Cd had the significant accumulation effect,the contents of Pb in the seeds exceeded the upper limit by 17.40%in wheat seed and 10.90%in rice seed,for Cd there were 14.71%in wheat seed and 8.70%in rice seed.According to the analysis of heavy metal accumulation in rice-wheat grains and the distribution characteristics of heavy metals in various forms in soil,the accumulation effect of heavy metals in dry farming (wheat) was easier than water farming(rice),and the ratio of labile fractions content/total content in soil was less,the difference of accumulation effect in paddy-upland rotation was more significant.The influences to the content of the different labile heavy metals were different by the physical-chemical quality in the soils.The content of Exchangeable and Bound to Carbonates were mainly affected by organic matters and pH.And a few elements were affected by bulk density,medium fine silt and CEC.The content of Bound to Fe-Mn Oxides and Bound to Organic Matter were mainly affected by organic matter and bulk density,and a few elements were affected by pH,clay,sand and CEC.Taking it as a whole, the determination influence factors to the labile content were organic matter,pH and bulk density,but the influence to the content of soil in active patterns by the content of clay and CEC which were related to the absorption capability was not markedly to all the heavy metals in the paddy soils.Except Zn,the significant or very significant multiple linear regression equations was founded between the content of heavy metals in the wheat-rice seeds and correspondence heavy metals' form in soil.The regression coefficients showed that great difference existed.The content of Cd in seeds of both rice and wheat were positively related to the contents of all labile fractions of Cd.The content of Cr in the seeds of wheat was highly correlated to that of the exchangeable Cr,but that in rice seeds was mainly affected by the content of Bound to Carbonates Cr.The content of Cu in the seeds of rice and wheat was affected obviously by the exchangeable Cu,but the correlation between the content of Pb in the seeds and each labile fraction was weak and no difference was found among various fractions. |
