Biochar assists phytoremediation of cadmium by regulation of rhizosphere microbiome in paddy fields

Cadmium (Cd) pollution in paddy fields is becoming a great threat to the ecosystem. Phytoremediation with hyper-accumulating plants is an economic strategy to attenuate the hazards of Cd in the paddy fields. However, its Cd-removing efficiency remains to be improved. In this study, we prepared the moss-derived biochars by heating the dried moss bodies and investigated their effect on plant growth, Cd removal, and rhizosphere microbiome compositions. The biochars promoted the growth of the two remediation plants, i.e., Solanum nigrum and Echinochloa crus-galli, and improved Cd removal from paddy fields, leading to the increase in Cd-removing rate from 11 % to 15 % to 23–35 % after 60 days. Furthermore, the biochars increased the number of rhizosphere bacteria from 2.26 to 2.76×10⁶ cell/g soil to 3.89–4.78×10⁶ cell/g soil, together with soil organic matters from 18.3 mg/kg to 24.5 mg/kg for Solanum nigrum, and from 18.0 mg/kg to 21.1 mg/kg for Echinochloa crus-galli on day 30. Consistently, the activity of urease, acid phosphatase, and catalase increased to 1.18–1.29 times, 1.26–1.32 times and 1.13–1.18 times, respectively. Bacterial community analysis further showed the biochars increased both the abundance-based coverage estimator index and the Simpson index, implying the increased bacterial diversity induced by the biochars. Moreover, biochars strongly changes the bacterial compositions, leading to the increased abundance in Microbacteriaceae, Alcaligenaceae, Acidimicrobiales, Rhizobiales, etc., most of which are involved in stress tolerance and biofilm formation. This study provides new information on the positive effect of biochars on phytoremediation and implies the important role of rhizosphere microbiome remodeling in pollutant removal.