[Effects of Straw Incorporation on Cadmium Accumulation and Subcellular Distribution in Rice]

Cadmium (Cd) is classified as a Group-1 human carcinogen and rice consumption constitutes a major source of dietary intake of Cd for populations whose staple food is rice. Straw incorporation is widely performed in Cd-contaminated paddy fields, which may significantly affect the bioavailability of Cd in soil and the distribution of Cd in rice plants, consequently altering Cd accumulation in rice grains. In this study, both pot and field trials were conducted to investigate the effects of different amounts of straw incorporation (0.0%, 1.0%, 2.5%, and 5.0%) on Cd sub-cellular distribution in rice plants and Cd accumulation in rice grains. The results showed that Cd was mainly sequestered in cell wall, accounting for 86%-95% and 30%-51% of total cadmium in root and shoot cells, respectively. In shoot cells, about 35%-61% of Cd was distributed in cellular soluble fractions. When rice straw was incorporated at 1.0% and 2.5% levels, Cd sequestration in the cell wall significantly increased and Cd translocation from roots to shoots significantly decreased. However, when rice straw was incorporated at the 5% level, Cd sequestration in root cell walls significantly decreased and Cd translocation from roots to shoots significantly increased at the tillering stage. At the filling stage, 5% rice straw incorporation still significantly increased Cd sequestration in root cell walls and Cd translocation from roots to shoots did not significantly change. The rice straw and rape straw used for the field trail contained high concentrations of Cd (0.49 and 0.67 mg·kg^-1, respectively). Rape straw incorporation alone or together with lime did not significantly affect Cd accumulation in brown rice or rice straw. Rice straw incorporation alone did not significantly affect Cd accumulation in brown rice or rice straw, while incorporation with lime significantly decreased Cd accumulation in both brown rice and rice straw. Biochar application can also significantly reduce Cd accumulation in rice and when biochar was added together with lime, the reduction in Cd accumulation in rice was more significant. Therefore, at Cd-contaminated paddy fields, rice straw or rape straw is not suggested to be returned directly; incorporation with lime would be better for reducing Cd accumulation in rice grains. The results of this study will provide theoretical and practical guidance for the safe production of rice and for straw recycling at Cd-polluted paddy fields.