Phosphorus-iron modified biochar reduces soil salinity and enhances carbon sequestration via calcium-sodium exchange and the reverse stimulation of iron-driven functional microbial taxa.

This study hypothesized that phosphorus-iron (P-Fe) modification improves the pore structure and surface chemical properties of biochar, thereby enhancing its adsorption capacity for salt ions and CO2. When incorporated into saline-alkali soil, this modification improves soil fertility and enriches beneficial microbiota, ultimately directly and indirectly reducing soil salinity and alkalinity while enhancing carbon sequestration capacity. Hence, three types of P-Fe-modified biochar (P1-Fe@WBC, P2-Fe@WBC, and P3-Fe@WBC) were prepared using wheat straw biochar modified with three phosphate and iron salts, and the effects of P-Fe-modified biochar on saline-alkali soil remediation and carbon sequestration were systematically investigated. The results suggest that P-Fe modification substantially improved the pore structure of biochar, forming FeO and POC functional groups, which enhanced CO2 sequestration performance by 8.49-25.05%. Through Ca-Na exchange mechanisms and surface immobilization effects, P-Fe-modified biochar reduced soil exchangeable sodium percentage (ESP) by 21.97-61.54%. Additionally, it increased soil organic carbon (SOC) by 1.40-2.65-fold and soil inorganic carbon (SIC) by 0.31-1.33-fold. Furthermore, P-Fe-modified biochar enriched beneficial bacterial species such as Burkholderia-Caballeronia-Paraburkholderia and Mesorhizobium, thereby enhancing salt excretion potential mediated by ABC transport proteins. More importantly, iron incorporation may promot soil carbon transformation through enrichment of Acidiferrimicrobium, suggesting potential for stable carbon sequestration. These findings provide a feasible and effective approach for ecological restoration and carbon sequestration in saline-alkali soils.