长期不同耕作方式对紫色水稻土重金属含量及有效性的影响

通过长期定位试验,研究了常规平作、水旱轮作、免耕冬水、垄作免耕和厢作免耕5种耕作方式对紫色水稻土剖面重金属(Fe、Mn、Cu、Zn、Pb、Cd)总量、有效量及水稻根、茎叶和糙米重金属含量的影响.结果表明,经过22 a的耕作,5种耕作方式对紫色水稻土剖面Fe、Cu、Zn、Pb、Cd总量的剖面分布状况影响不显著,而对土壤Mn的剖面分布状况影响显著,常规平作、水旱轮作及免耕冬水均导致表层土壤Mn向下层淋失.5种耕作方式下土壤Fe、Cu、Zn、Pb和Cd的有效量均随土层深度的增加呈降低趋势,而常规平作、免耕冬水、垄作免耕及厢作免耕土壤Mn有效量则表现出随土层深度的增加而增高的趋势.5种耕作方式下,表层土壤Fe、Mn有效量以水旱轮作最高,Zn、Pb有效量以常规平作最高,而Cu有效量受耕作方式的影响不显著.相关分析结果表明,表层土壤有效Fe与pH呈极显著的负相关关系,与有机质呈显著的负相关关系,有效Mn与pH和有机质均呈极显著的负相关关系,有效Zn与总Zn呈显著正相关关系.水旱轮作、垄作免耕及厢作免耕条件下水稻根部Fe、Mn含量,茎叶Fe、Mn、Cu、Cd含量,糙米Cu含量均高于常规平作和免耕冬水,水旱轮作能明显降低Cd向糙米的迁移系数,对降低糙米Cd含量有明显的效果,且水旱轮作条件下糙米Cd含量低于国家食品卫生标准.水稻根部Fe含量与pH呈极显著负相关关系,与有效Fe呈极显著正相关关系,根部Mn含量与pH呈极显著负相关关系,与有效Mn呈极显著正相关关系,茎叶中Mn含量与pH呈显著的负相关关系,与总Mn及有效Mn呈显著的正相关关系,茎叶和糙米Cu含量与pH均呈极显著的负相关关系,糙米中Zn含量与pH呈显著负相关关系,与CEC呈极显著正相关关系.结果表明,耕作方式主要通过影响土壤pH而影响土壤重金属的有效量及水稻重金属的含量,水旱轮作可提高表层土壤Fe、Mn活性,降低土壤Zn、Pb和Cd活性,并能在一定程度上降低糙米Pb、Cd含量,但也应注意长期的水旱轮作可能会导致表层土壤Mn过度向下层淋溶.

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 straw and 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 (<0.2 mg·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, paddy-upland 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.