Geochemically targeted valorization of rice straw decouples selenium recovery from cadmium transfer with climate co-benefits.

Abstract

Recycling crop residues is central to sustainable agriculture, yet residues co-enriched in beneficial trace elements and toxic metals create a fundamental trade-off between resource recovery and contaminant re-entry. Here, guided by regional evidence of selenium (Se) - cadmium (Cd) co-occurrence in paddy soils and rice straw, we developed a geochemically targeted valorization strategy that converts co-enriched rice straw into hydroxyapatite-engineered biochar (PBC). Laboratory experiments showed that PBC promoted Se mobilization while suppressing Cd release, consistent with increased Se availability and decreased Cd availability in soil. These contrasting responses were also observed in grain Se and Cd concentrations in field trials, where PBC reduced grain Cd concentrations by 14.2-20.6% and increased grain Se concentrations by 12.9-33.7% relative to the unamended control (CK). These outcomes were consistent with changes in transporter-related gene expression and stress-associated metabolic pathways. Life-cycle accounting indicated a net carbon reduction of 85.3 t CO2-eq ha⁻1 and a net economic benefit of approximately US$9,072 ha⁻1 per growing season relative to CK, with the latter comprising grain revenues based on yield and quality, coproduct revenues, and monetized avoided climate damage, minus associated costs. These findings demonstrate that geochemically targeted residue valorization can decouple micronutrient recovery from toxic metal transfer, providing a field-validated framework for sustainable crop residue recycling.