The comparison of biochar-based amendments in reducing both Cd and As absorption and transfer in spring and autumn rice

The safe utilization of paddy soils contaminated with both Cd and As is considered a major and urgent problem in South China. This study compared the effects of four carbon-based amendments on the absorption, enrichment, and transportation of Cd and As in spring and autumn rice plants through field experiments to screen suitable amendments for the safe utilization of polluted farmland and provide a scientific basis for the safe utilization of polluted soil. Rice varieties ‘Ruanhuayou 6100’ (spring rice) and ‘Yexiangyou 9’ (autumn rice), commonly grown in Guangdong Province, were used as the test rice plants. Four biochar-based amendments were investigated: BO (biochar∶organic fertilizer=1∶1), BS (biochar∶sepiolite=1∶1), BB (biochar∶bentonite=1∶1), and BA (biochar∶alkali residue=1∶1). The measured parameters included grain yield and both Cd and As contents in the major organs of rice. The biological accumulation (BCF) and transfer factors (TF) were calculated. The results showed that biochar-based amendments significantly increased the spring rice yield by 26.87%−48.93%, whereas only the BO treatment significantly increased the autumn rice yield by 26.16%. Biochar-based amendments induced a large reduction in Cd content in both spring and autumn rices by 40.23%−67.89% and 7.64%−32.91%, respectively. However, the amendments increased the As content in spring rice by 44.09%−67.73%, although with little effect on autumn rice. Generally, BS amendment showed consistent performance in spring and autumn rices, with the greatest reduction in Cd but an increase in As. Soil available Cd was significantly and negatively correlated with soil pH and cation exchange capacity, whereas soil available As was not significantly correlated with soil physicochemical properties. The Cd content was significantly and positively correlated with the soil available Cd in brawn rice, and rice straws and roots. There were significantly negative correlations between the As content in brawn rice and the Zn content in brown rice, rice husks and straws. It is evident that different methods based on the absorption and transportation mechanisms of different heavy metals must be implemented to control both Cd and As contents in rice. Our results provide a theoretical basis for controlling Cd and As content in rice in South China.