The potential of magnetic biochar prepared by a solvent-free method as a soil amendment: Metal immobilization performance, soil microbial co-occurrence network and community assembly mechanism

Abstract

Magnetic biochar (MBC) is widely recognized as an effective soil amendment for the immobilization of heavy metals in soils. However, the most commonly used methods currently for manufacturing MBC necessitate the use of iron salt solutions, leading to energy consumption for dehydration and potential environmental risks. Here, we fabricated a magnetic biochar based on the solvent-free method and applied it as a soil passivator at a rate of 5 % (w/w) for 90 days. The metal immobilization performance of MBC was investigated, together with its soil micro-ecological effects including the impacts of MBC application on soil microbial community diversity, composition, co-occurrence network pattern and assembly mechanism. The results showed that MBC treatment significantly decreased the DTPA-extractable Cd and Zn by 54.1 % and 59.5 % (P<0.05) after the 90-day incubation, respectively, higher than that of biochar (BC) treatment (25.6 % for Cd and 50.4 % for Zn). The transformation of Cd, Zn and Pb from labile fractions to residual fraction was facilitated in MBC treatment. Besides, MBC application significantly increased soil pH, soil organic matter content and cation exchange capacity (P<0.05). The addition of MBC considerably altered the soil bacterial community diversity and composition. Both soil environmental variables and bioavailability of metals shaped microbial communities, while the latter explained more. Compared to CK, the complexity and stability of the co-occurrence network in MBC-amended soil were enhanced. Null model analysis implied that the microbial community assembly was governed by the stochastic process in MBC treatment, while the deterministic process was the dominant mechanism initially. Overall, our findings provide insight into the ecological mechanisms and interactions of soil microbial communities in response to MBC application and demonstrate that MBC is a promising amendment for metal-contaminated soil remediation.