Variation of Cd and As accumulation in crops under oilseed rape–rice rotation system in response to different contaminated rice straw-return methods

Background Attention needs to be paid to the environmental risks that may arise when returning contaminated straw to the soil. This study considers the potential effects of rice straw incorporation on cadmium (Cd) and arsenic (As) transformation and uptake in soil–rape–rice systems. Methods Pot experiment was conducted with five straw-return methods: CK (control: no amendment), S (straw return), SD (straw return with straw-decomposing microbial inoculants), SC (straw with biochar), and SC Fe (straw associated with iron-modified biochar). Results The results showed that the returning of straw significantly decreased soil pH during the rape growing season, while soil pH increased during the rice growing season. As a result, contaminated straw return resulted in remarkable increases of the DTPA-extracted concentrations (DTPA Cd and DTPA As ) in rape soil, as well as the DTPA As concentrations in rice soil. Treatments S, SD, and SC significantly enhanced the Cd accumulation in rape seeds, in contrast to CK, while the subsequent Cd accumulation in rice grains decreased. Moreover, the inorganic arsenic content in rice grains in both the SD and SC treatments was much lower than that in the CK and S treatments. After the rape growing season, both SC and SC Fe have a positive effect on decreasing rice soil Cd and As bioavailability, compared with the S treatment. Conclusions Collectively, a rape–rice rotation system could reduce the Cd uptake in after-reap rice grains with the incorporation of contaminated straw return, but the risk of As accumulation should be treated with caution. Graphical Abstract