Phosphate-enhanced Cd stabilization in soil by sulfur-doped biochar: Reducing Cd phytoavailability and accumulation in Brassica chinensis L. and shaping the microbial community.

To explore the potential of livestock manure-derived biochar for the remediation of Cd-contaminated soil, a pot experiment was conducted to explore the stabilization efficiency of cattle manure biochar (T2, BC), sulfur-doped biochar (T3, SBC), and SBC combined with phosphate (T4, SBC-PF) on Cd in contaminated soil and their effects on Cd accumulation in Chinese cabbage (Brassica chinensis L.) and soil microorganisms. The results showed that soil available phosphorus (AP), available potassium (AK), and organic matter (OM) significantly increased in T3 and T4, and the biomass of Chinese cabbage also increased from 0.46 g/pot to 0.57 and 1.05 g/pot, respectively. The DTPA-extractable Cd in T3 and T4 dramatically reduced by 78.6% and 91.4% (p < 0.05); the acid-soluble Cd decreased by 11.3% and 13.2%; and the residual Cd increased by 30.0% and 10.0%. Most importantly, the Cd contents in T2, T3, and T4 decreased by 2.2%, 89.7%, and 93.1% in the shoots of Chinese cabbage and 21.3%, 82.2%, and 86.2% in the roots of Chinese cabbage, respectively. Moreover, SBC-PF obviously changed the bacterial community and enhanced the interactions among microbes in the soil. Structural equation modeling revealed that microbial interspecific mutualistic relationships were the key factor in the pathway for reducing Cd phytoavailability. Mantel tests and random forest analyses further revealed that biochar enhanced the interactions among microorganisms by increasing the AP content in the soil. These findings demonstrated that SBC combined with phosphate is appropriate for stabilizing Cd and improving soil quality.