Long-Term Effects of Tillage Methods on Heavy Metal Accumulation and Availability in Purple Paddy Soil

A long-term experiment,conducted at Southwest University since 1990,was used to evaluate the effect of tillage methods on the total and available contents of heavy metals( Fe,Mn,Cu,Zn,Pb,Cd) in the profile of purple paddy soil and the contents of those metals in root,stem leaf and brown rice. The experiment included five tillage methods: conventional tillage,paddy-upland rotation,no-tillage and fallow in winter,ridge-no-tillage and compartments-no-tillage. The results showed that the total concentrations of Fe,Cu,Zn,Pb and Cd in the soil profile had no significant differences among five treatments,but it was found that total Mn has a significant decline in 0-20 cm under conventional tillage,paddy-upland rotation and no-tillage and fallow in winter compared with ridge-no-tillage and compartments-no-tillage. The availability of Fe,Cu,Zn,Pb and Cd decreased with the increase of soil depth in all treatments,but the availability of Mn was found to be the highest in the 20-40 cm layers except those in the paddy-upland rotation. In the ploughed layer,the contents of available Fe,Mn was the highest in paddy-upland rotation,while the contents of available Znand Pb was the highest in conventional tillage,but tillage treatments had not significant influence to the contents of available Cu. Correlation analysis showed that available Fe was significantly negatively related to the pH values and significantly negatively related to the organic matter of soils,available Mn was significantly negatively related to the pH values and organic matter of soils,whereas the available Zn was significantly positively related to total Zn. The contents of Fe,Mn in rice root,the contents of Fe,Mn,Cu and Cd in rice strawand Cu in brown rice were higher under paddy-upland rotation,ridge-no-tillage and compartments-no-tillage than those in conventional tillage and no-tillage and fallow in winter. Paddy-upland rotation can significantly lower the migration coefficient value of Cd in brown rice,and the Pb,Cd concentration in brown rice in the treatment of paddy-upland rotation was lower than the upper limit( 0. 2mg·kg-1) of the National Standard for Food Hygiene for Cd concentration. The content of Fe in root was significantly and negatively related with soil pH and significantly and positively related with soil available Fe,the content of Mn in root was significantly negatively related with soil pH and significantly positively related with soil available Mn,the content of Mn in straw was significantly negatively related with soil pH,significantly positively related with soil total Mn and significantly positively related with soil available Mn,the content of Cu in straw and brown rice was significantly negatively related with soil pH,the content of Zn was significant related with soil pH and significant related with soil CEC. The content of Fe in root,Mn in root and straw and Cd in straw was positively related with soil available Fe,Mn and Cd,respectively,but was negatively related with pH in plough layer soil,Zn in straw was also negatively related with plough layer soil pH. From the results as above,it is concluded that different tillage methods can change the values of soil pH,alter the availability of heavy metal in soils,consequently affect uptake of heavy metal by rice. Of the tillage methods,paddyupland rotation could increase the availability of Fe and Mn,but decrease the availability of Zn,Pb and Cd in purple paddy soils.Paddy-upland rotation can also increase the contents of Fe,Mn in rice root and straw,but decrease Cd content in brown rice,and could reduce the Pb,Cd contents in brown rice in a certain extent,however,attention should be given to long-term paddy-upland rotation cause of leaching of soil surface Mn.