Remediation potential of rice with high cadmium accumulation to cadmium contaminated farmland

Abstract: Phytoremediation is a typical environmentally friendly and economical method to in situ natural remediation of soil pollution. Therefore, it can widely be expected to serve as a very feasible way to remediate the contaminated rice paddy fields with high cadmium accumulation, particularly without changing the property and structure of farmland. This study aims to explore the remediation potential of high Cadmium (Cd) accumulated rice varieties (Yangdao 6 and Yuzhenxiang) in the contaminated farmland soil. A field plot experiment was conducted to investigate the accumulation regularity in rice, removal time and removal height of rice varieties with high cadmium accumulation. The content of Cd was also determined in 5 parts (root, stem 0-10 cm, stem 10-20 cm, stem above 20 cm and grain) of rice straw at 6 different growth stages (seedling, tillering, booting, full heading, ripening and full ripening). The results showed that the content of Cd in different parts of rice (Yangdao 6 and Yuzhenxiang) increased with the prolongation of the growth stage ranging from the booting to the mature stage, where the maximum was observed. Besides, the Cd distribution in different parts of rice decreased with the plant height in the same period. At the mature stage, the Cd content in roots, 0-10 cm stem, 10-20 cm stem, above 20 cm stem, and grain of rice varieties Yangdao 6 and Yuzhenxiang were 19.3, 11.8, 9.4, 8.1, and 3.9 mg/kg, and 19.5, 16.3, 14.3, 9.7 and 3.7 mg/kg, respectively. The Cd enrichment coefficients were also all greater than 1, indicating high Cd accumulation. The removal content of cumulative cadmium was ranked in order of the whole plant, the whole ground part, 10 cm above the ground, 20 cm above the ground, harvested rice stalk of Yangdao 6 and Yuzhenxiang during the whole growth period. Specifically, the removal contents of cumulative cadmium in the rice stalk of Yangdao 6 and Yuzhenxiang were up to 1 299.90 and 1 278.33 μg/plant, respectively, when the whole shoot was removed at the full maturity stage. The removal contents of cumulative cadmium in the rice straw of Yangdao 6 and Yuzhenxiang were up to 1 652.11 and 1 547.70 μg/plant, respectively, when the whole plant was removed at the full maturity stage. The removal efficiency of soil Cd was 9.1% and 8.5%, respectively, when the rice varieties Yangdao 6 and Yuzhenxiang were planted once a year and the whole plant was removed. By contrast, the removal efficiency of soil Cd was 7.2% and 7.1%, respectively, when the whole shoot was removed. It infers that the Cd polluted rice roots were difficult to be removed. Therefore, it was strongly recommended to remove the rice stalk by harvesting the whole ground part after the rice was fully ripe, particularly considering the remediation and operability. As such, the remediation of Cd contaminated rice paddy fields can remain the soil structure and local planting pattern in a long term, compared with dryland phytoremediation of Cd polluted farmland. The finding can provide a new insightful idea for the phytoremediation in cadmium-contaminated paddy fields.