Remediation of Cadmium-Contaminated Paddy Soils by Phosphorite Magnetic Biochar: A New Insight Into Soil Microbial Responses

Abstract

Cadmium (Cd)-contaminated rice paddies have been a primary environmental concern worldwide. Biochar or modified biochar application favors reducing Cd availability in paddy soil, which has been demonstrated in physical and chemical terms. However, we know little about soil microbial responses after biochar application. In this study, we revealed the immobilization effect and mechanism of Cd in paddy fields by a novel phosphorite magnetic biochar (PMCB) from the perspective of soil microbial response. The results suggested that PMCB application facilitated the conversion of highly active Cd to less active Cd in soil and reduced Cd uptake by rice by improving soil physicochemical properties and functional bacterial and gene abundance. Compared with the control, acetic acid extractable Cd and reducible Cd contents in soil and Cd content in brown rice decreased by 7.4%–18.4%, 16.2%–19.9%, and 44.0%–47.9%, respectively, and residual Cd content increased by 16.3%–25.8% under PMCB treatment. A decrease in sqr and fccB gene abundances (downregulated by 6.3% and 12.4%, respectively) inhibited CdS oxidative dissolution after 0.2% PMCB application. Conversely, other treatments stimulated the abundant proliferation of functional microbes (e.g., Anaeromyxobacter, Geobacter, and Thiobacillus) and these genes while suppressing sulfide-producing gene abundance (sreA and phsA decreased by 30.5%–73.4% and 3.3%–11.3%, respectively). This result implies that Fe(III) mineral reduction and CdS oxidation lead to a limited secondary release of Cd. Additionally, aqueous-phase analysis of the immobilization mechanism revealed that PMCB immobilized Cd mainly by coprecipitation with PO43− and CO32− (34.5%) and ion exchange (32.2%), followed by surface complexation (18.7%) and cation π-bonding (13.1%).