Bioremediation of Ni and Cd Contaminated Soil Using Comamonas testosteroni ZG2 Enhanced by Fe–Mn Modified Spent Mushroom Substrate Biochar

In the present study, a type of biochar was produced from spent mushroom substrates (SMS) and modified with Fe–Mn binary oxides. Spent mushroom substrate biochar (SMSB) and Fe–Mn oxide composite modified spent mushroom substrate biochar (FM-SMSB) combined with Comamonas testosteroni ZG2, with excellent heavy metal resistance and urease production capacity, were used to remediate Cd and Ni contaminated soil. The surface appearance of SMSB and FM-SMSB was analyzed, and the saturated adsorption capacity of Ni and Cd was measured. The distribution of Ni and Cd in the rhizosphere soil and leaves of pakchoi (Brassica chinensis L.) was investigated. Simultaneously, 16S rRNA high-throughput sequencing was used to examine the microbial community composition in soil. The result showed that the modification of Fe–Mn oxide composite could immobilize Ni and Cd on SMSB effectively, and FM-SMSB had a greater saturated adsorption capacity than SMSB. Moreover, SMSB, FM-SMSB, and their respective combination with strain ZG2 could increase the distribution ratio of residual Ni and Cd in the rhizosphere soil of pakchoi, help the transformation of Ni and Cd from high toxicity to low toxicity, and reduce the distribution ratio of Ni and Cd in the organelles of pakchoi leaves. FM-SMSB (or SMSB) with (or without) strain ZG2 could affect the soil microbial community structure. The soil after adding FM-SMSB and strain ZG2 together had a better microbial ecology, indicated by higher abundances of some important beneficial bacteria, including Flavisolibacter, Adhaeribacter, Pseudarthrobacter, Massilia, Devosia, etc., compared with those in other samples. The efficient remediation capacity and associated mechanism for this novel functional biochar, combined with strain ZG2, can provide insights for improved remediation of heavy metal-contaminated soil.