Effects Of Rice-rape Rotation On Heavy Metal’s Migration,transformation And Its Activity In Soil

The background value of heavy metals in soils is high in Guizhou and the activity of heavy metals is affected by the main farming behaviors.Based on the field test and indoor simulation test,the enrichment and transport of heavy metals in different growth stages of rape and rice were analyzed,and the effects of the rotation pattern on heavy metal activity were researched,factors were studied under the soil chemical properties,and the main results were as follows:(1)The pH in bottom soil changed more seriously than that of in topsoil.The changing range of Eh was more wider in surface soil than that in bottom soil.The soil was in a moderate reduction state during rape growing period,and in a strength restore state when rice grew.The content of organic matter in the surface soil was higher than that in the ground,but the degree of variation was lower.(2)The variation trend of soil heavy metals content had been different in the topsoil(0-20cm)and the bottom soil(20-40cm).Heavy metal pollution in soil having been assessed depending on single pollution index and comprehensive pollution index Cr,Pb,Cu and Zn were clean,Cd was at moderate pollution level.Cr and Cd composite pollution index of bottom soil were respectively larger than that of topsoil,Pb,Cu and Zn were opposite.(3)The residual state of Cr,Cd,Pb,Cu,Zn was largest,exchanging state smallest.As a whole,the residual proportion of Cr,Cd,Pb and Zn had been decreased in the rotation,the other states proportion of them had been increased in different degrees,the states of Cu had changed differently from other elements.It showed that Cr,Cd,Pb,Zn had been activated when filled water during rice-rape rotation.(4)The enrichment coefficient during the growth of rape was Zn>Cd>Cu>Pb>Cr,the enrichment ability of the same heavy metal was different in rape growing period.The leaves,roots and stems were the main enrichment organs of Zn and Cd in each growth period of rape,stems were the weakest organ for the accumulation of Cu.The root was the main organ of Pb enrichment.The ability of rape to enrichment and transport Cr were weak.Rice mainly absorbed Cd,Cu and Zn,not Cr and Pb.Enrichment ability of heavy metals was weaker in rice seedling and tillering,the enrichment ability of rice to Cd was increasing.Heavy metals were mainly enriched in the roots of rice during each growth period.Compared to the standard of pollutant limits in food,except Pb,other elements were not exceeded.(5)Correlating analysis between heavy metals activity and soil chemical properties showed that there exists closely correlation between heavy metals activity and soil pH,soil Eh and soil organic matter.By fitting the regression equation,it found that the change of soil Eh and soil pH were the main factors to affect the heavy metals activity of surface and bottom soil.There were not fit regression equation between Cd and Zn and soil physical and chemical properties in bottom soil(20-40cm).It showed that the effects of factors to activity were more complicated.(6)Laboratory simulation showed that water was also an important factor to affect activity of heavy metals in soil,Cr and Pb have the largest variation in dry-wet alternating situation(variable coefficient of Cr was 38.01%,variable coefficient of Pb was 42.88%),Cd has the largest variationin 60% field holding water(variable coefficient of Cd was 9.96%),and Zn was in flooded state(variable coefficient of Zn was 43.71%),but the variation degree of Cu activity was not obvious under different water conditions.Besides,in the same water environment,there are great differences in the activity of different heavy metals in different cultivating time.