Shoot two birds with one stone: chemical and biological mechanisms of calcium-phosphorus modification for enhancing carbon sequestration and heavy metal stabilization of biochar.

Developing biochar amendments that couple heavy-metal stabilization with soil carbon preservation is essential for restoring contaminated agricultural soils and upgrading crop residues. Here, calcium-phosphorus co-modified straw biochar (CPBC) was fabricated via impregnation-assisted secondary pyrolysis to realize individual immobilization of Cu2+ and Pb2+ in separate contaminated soil systems, together with short-term soil organic carbon preservation. CPBC developed a porous surface enriched with Ca- and P-containing species, including pyrophosphate-related crystalline phases. In contaminated soils, CPBC increased the residual fractions of Cu and Pb by 25 percentage points and reduced their acid-soluble fractions, indicating suppressed metal mobility. Meanwhile, CPBC decreased soil organic carbon mineralization rates from 1.25 to 0.70 mg g-1 d-1 in Cu-contaminated soil and from 1.31 to 0.95 mg g-1 d-1 in Pb-contaminated soil over 40 days. Spectroscopic analyses supported phosphate-associated precipitation and Ca-associated cation-exchange/coordination pathways for heavy-metal immobilization, whereas Fukui-function calculations revealed site-dependent carbon reactivity across the modeled biochar configurations. Microbial analysis further revealed treatment-related shifts in Proteobacteria, Firmicutes, and Gemmatimonadota, together with statistical associations between these taxa, soil properties, and metal fractions. Small-scale qualitative pot observations confirmed that CPBC could alleviate visible Cu and Pb phytotoxicity to alfalfa seedlings. This work provides a mechanistically supported crop-residue-derived amendment for integrated heavy-metal stabilization and soil carbon preservation.