The Study Of Proflie Distribution Characteristics And Infulence Factors Of Cd,Pb Of Several Typical Paddy Soil In Eastern Hunan

Regarding present problems that a large scale of paddy soil system in eastern Hunan had seriously contaminated by cadmium(Cd)and other heavy metals which are hard to find out the source because the elements that cause the contamination came from many different places,we used soil taxonomic classification method to collect some typical soil profiles of paddy soil derived from 5 soil parent materials(sediment,purple stone area,granite,slate,quaternary red clays,)in Changsha,Zhuzhou,Xiangtan,and some samples of topsoil and rice were collected randomly in those place,then total contents of Cd in the soil and rice,total contents of lead(Pb)and available Pb and soil physical and chemical properties(pH,OM,particle concentration of all level)were determined to investigate the distribution in soil profile and differences of the total contents of Cd and Pb in paddy soil developed by different parent materials,evaluate the contribution rates of parent materials to the contents of Pb and Cd in paddy soil and explore the main influence factor of the difference between different soil developed by different parent materials and the relationship of Cd contents in rice to available Cd in soil.The results showed that:(1)In paddy soil samples derived from 5 different types of parent material,river sand mud has the highest total contents of Cd and available Cd through whole soil profile(total Cd contents was 0.219mg/kg,available Cd contents was 0.049mg/kg);followed by purple mud field and yellow clayey mud soil(contents of average total and available Cd was 0.219mg/kg and 0.084mg/kg,respectively),red clayey mud and alluvial sandy paddy soil has the lowest average contents of total and available Cd,were 0.146 mg/kg and 0.049mg/kg,respectively.Percentage of available Cd to total Cd in paddy soil are between 29%~38%,only river sand mud has total Cd contents which exceeds the limits for agricultural land in the China soil environmental quality standard II(0.3mg/kg).(2)From the view of Cd content distribution in the soil profile,only river sand mud has a different variation characteristic,others are gradually decreased from topsoil to bottom soil.The content of Cd in topsoil and most plough pan profile samples were higher than the standards;the contents of total cadmium and available cadmium in topsoil and plough pan soil in purple mud field,river mud field and yellow mud field were higher than others.Theaverage content of total Cd and available Cd in topsoil and plough pan in those soil were0.539mg/Kg and 0.326mg/kg,0.409mg/kg and 0.264mg/kg,respectively.Followed by red and yellow mud and sandy mud field,the average content of total Cd and available Cd in these topsoil were 0.329mg/kg and 0.183 mg / kg,and for the plough pan soil were 0.205mg/kg and 0.089mg/kg(contents of total Cd was 0.911mg/kg,contents of available Cd was0.254mg/kg),and the total cadmium in the other four kinds of paddy soils were lower than the natural background value(0.2mg/kg).The average contents of Cd was 0.145mg/kg,average available cadmium were 0.039mg/kg which were lower than 0.06mg/kg.There was a significant positive correlation between soil available Cd and total Cd(r = 0.8562 **,n =199).(3)Then we investigated the contribution rate of parent material to the topsoil total Cd contents,the fact that total cadmium content has a higher contribution rate to the top soil total Cd contents than that of available cadmium,and river sand mud has the highest contribution rate of total Cd and available Cd contents of 171.5%,92.7%,respectively.Clayey mud has the lowest contribution rate of total Cd and available Cd contents of 28.9%and 10.7%,respectively.Others were between 32%~38%,14% ~22%,respectively.There was a significant positive correlation between total cadmium(X)and total cadmium(Y),the regression equation was Y = 0.6844 X +0.2926(r = 0.7134 *,n = 28).(4)Correlation analysis showed that soil organic matter and 2-0.002 mm granule content were the main soil factors to determine soil Cd contents.Soil available cadmium contents have a significant positive correlation with total soil cadmium contents and cadmium contents in the plants(rice and rice straw).Significant positive correlation between available cadmium and cadmium was only founded in clayey mud field and sandy mud field,the unitary linear regression equation of clayey mud field and sandy mud field were Y =2.4389X-0.1324(r=0.0.9817**,n=5),Y =3.5239X-0.3302(r=0.9554**,n=6),respectively.(5)The total lead content in different paddy soils was 17.25mg/kg ~ 54.89mg/kg,and the available lead contents was 2.25mg/kg ~ 15.76mg/kg.The content of lead in the soil decreased with the increase of the profile.The average content was 1.27 times of the background value.The results showed that the total contents of lead were not higher than that of natural background(250mg/kg),total contents of lead in sandy muddy field,riversediment field and yellow mud field were higher than natural background(35mg/kg).Soil available lead contents had a significant positive correlation with total lead contents(r =0.4306,n = 67).The contribution rate of lead in the parental material to total lead in topsoil is more than 58%,while that of available lead is less than 58%.The contribution rate of sediment and granite to the contents of total Pb and available Pb were much higher than other parent material as a whole,the contribution rate is greater than 46%.The correlation analysis showed that there was a significant positive correlation between total lead(X)and total lead(Y)in the plough layer,and the regression equation was expressed as Y =1.1058 X + 7.4376(r = 0.6092 **,n = 24).Soil organic matter,pH value,free iron oxide and so on are the main soil factors that determine the soil lead contents.