| Tillage methods significantly affect physiochemical properties of soil, this gives rise to changes in bioavailability of heavy metals. Relative researches mostly focus on upland field, little information on the movement and accumulation of heavy metal in paddy soil-plant system was available. Therefore, a long-term experiment, conducted at Southwest University since1990, was used to evaluate the effect of tillage methods on the physiochemical properties, total and available contents of heavy metals in the profile of purple paddy soil and contents of those metals in rape and rice. The experiment included four tillage methods, congventional tillage, paddy-upland rotation, no-tillage and fallow in winter and ridge-no-tillage. The main conclusions are as follows:(1)Tillage methods have obvious influence on surface soil pH. In April2012, plough layer soil pH follows the order of convention tillage>no-tillage and fallow in winter>paddy-upland rotation>ridge-no-tillage, in August2012follows the order of convention tillage> ridge-no-tillage> no-tillage and fallow in winter> paddy-upland rotation. Except for April and August2012, plough layer soil pH follows the order of convention tillage> no-tillage and fallow in winter> ridge-no-tillage> paddy-upland rotation. No-tillage and paddy-upland rotation resulted in soil acidification. The content of soil organic matter was lowest in paddy-upland rotation, while ridge-no-tillage could increase the organic matter in all soil layers. CEC in the soil profile had no significant differences among four treatments.(2)Total concentrations of Cu, Zn, Pb, Cd decreased with increase of soil depth, but there was no obvious trend for the total Fe in soil. Total concentrations of Fe, Cu, Zn, Pb, Cd in the soil profile had no significant difference among four treatments, but it was found that total Mn has a significant decline in0-20cm under conventional tillage, paddy-upland rotation and no-tillage and fallow in winter compared with ridge-no-tillage. Total Mn in plough layer soil was in the order of ridge-no-tillage> convention tillage> paddy-upland rotation> no-tillage and fallow in winter.(3)In the whole year, the availability of Fe, Cu, Zn, Pb and Cd decreased with the increase of soil depth in all treatments, the availability of Mn increased with the increase of soil depth under convention tillage and no-tillage and fallow in winter but decrease with the increase of soil depth under paddy-upland rotation, while under ridge-no-tillage, the availability of Mn decreased with the increase of soil depth in rape season and increased with the increase of soil depth in rice season. The effects of tillage methods on available heavy metals decrease with increase of soil depth, and the contents of available heavy metal changes with the seasons. In0-20cm soil depth, the availability of Fe, Mn were significantly higher under ridge-no-tillage and paddy-upland rotation compared with convention tillage and no-tillage and fallow in winter, the content of available Zn was the highest in ridge-no-tillage, while the content of available Cd was the highest in convention tillage, but tillage treatments had not significant influence to the contents of available Cu and Pb in the rape season. In rice season, the contents of Fe, Mn was the highest under paddy-upland rotation, while the contents of available Zn, Pb, Cd was the highest under conventional tillage, but tillage treatments could not affect available Cu. Compared rice season, available Pb, Cd under paddy-upland rotation, available Fe, Mn, Zn, Pb, Cd under ridge-no tillage were higher in rape season. Correlation analysis showed that available Fe, Mn was significantly negatively related to the pH values, whereas the available Zn, Pb, Cd was significantle positively related to organic matter.(4)In2012, the contents of Fe, Mn, Zn, Pb and Cd in rape stem, the contents of Mn, Zn in pod and rapeseed were higher under ridge-no-tillage than those in paddy-upland rotation, the contents of Fe in stem were significantly positively related to organic matter, the contents of Mn in each part of rape were significantly positively related with soil available Mn, the content of Zn in each part of rape were significantly positively related with soil available Zn and organic matter, the content of Cd in rapeseed was significantly positively related with soil available Cd. In2013, the contents of Fe, Mn, Pb and Cd in rape stem, the content of Mn in pot were higher under ridge-no-tillage compared with paddy-upland rotation, the contents of Fe in stem were significantly positively related to organic matter, the contents of Mn in stem and pod were significantly positively related with soil available Mn, the contents of Zn in stem was significantly positively related with soil available Zn, the contents of Zn in pod was significantly positively related with soil total Zn, the content of Cd in rapeseed was significantly positively related with organic matter.(5)The contents of Fe, Mn in rice root, the contents of Fe, Mn, Cu and Cd in rice straw and Cu in brown rice were higher under paddy-upland rotation and ridge-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 Pb, Cd concentration in brown rice in the paddy-upland rotation was lower than the upper (<0.2mg·kg-1) of the National Standard for Food Hygiene for Pb, Cd concentration. In2012, the content of Fe, Mn, and Cd in root, the content of Mn, Cu and Zn in straw and Cu in brawn rice were significantly negatively related with soil pH, the contents of Fe, Zn in straw, Mn in root were significantly negatively related with soil organic matter, the contents of Fe in root and rice, Mn in root, Zn in root and Cd in brawn rice were significantly positively related with available Fe, Mn, Zn and Cd. In2013, the contents of Fe, Mn in root and straw were significantly negatively related to soil pH and significantly positively related to available Fe and Mn, the content of Cu in brawn rice were significantly negatively related to soil pH, the content of Cd in brawn rice was significantly negatively related to pH and organic matter. |
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