Silicon-modified lignin biochar outperforms hydrochar in heavy metal remediation: stochastic microbial assembly and...

Heavy metal contamination severely threatened soil health, microbial stability, and food safety worldwide. While lignin-based biochar had shown potential in mitigating metal toxicity, its regulatory effects on soil microbiomes and ecosystem functions remained inadequately understood, especially across different carbonization strategies. A cross-scale remediation framework using silicon-modified lignin-based hydrochar (Si-LHB) and pyrochar (Si-LPB) was developed to restore cadmium (Cd)/zinc (Zn)-contaminated soil ecosystems. In a pot experiment combined with metagenomics and biochemical assays, metal speciation, microbial dynamics, nitrogen cycling, and soil multifunctionality were evaluated. Results showed that both Si-LHB and Si-LPB effectively enhanced metal stabilization and plant growth, yet Si-LPB outperformed Si-LHB by significantly reducing acid-soluble Cd and Zn (by 16.22 % and 48.43 %) and increasing residual fractions up to 72.70 %, primarily via silicate precipitation and microenvironment modulation. Compared to Si-LHB, Si-LPB more effectively restructured bacterial communities, enriching Pseudomonadota and increasing community stochasticity (NST＞50 %). It also selectively upregulated nitrogen transport genes (e.g., narK, nrtP) and suppressed denitrification genes (e.g., nirK, nosZ), enhancing nitrogen retention and carbon fixation. Consequently, Si-LPB improved soil multifunctionality by 72.13 % and reduced Cd/Zn accumulation in plants by up to 99.66 %. This study demonstrates that silicon-enhanced lignin biochars, particularly Si-LPB, restore soil ecological functions through functional redundancy and enrichment of keystone taxa rather than by increasing diversity, offering a promising carbon-silicon-microbe strategy for sustainable remediation.