Effectiveness of Biochar on Cd Migration and Bioaccumulation in a Multi-Species Alkaline Fluvo-Aquic Soil System

Cadmium (Cd) contamination in alkaline soils threatens wheat safety in northern China. This study evaluates biochar’s dual role in Cd remediation and ecological trade-offs using a multi-species soil system (wheat–earthworm–soil). Biochar (Pennisetum hydridum) was applied to Cd-contaminated alkaline fluvo-aquic soils under controlled conditions. The results revealed that biochar increased soil pH (8.6–9.6) and reduced CaCl2-extractable Cd by 30–45% in the topsoil (0–20 cm), lowering shoot Cd accumulation in wheat by 42–47%. However, alkaline stress from biochar suppressed wheat biomass by 42%, while earthworm Cd concentrations rose 30–45%, correlating with reduced survival (75% vs. 85–87% in controls). Structural equation modeling identified pH-driven chemisorption as the primary Cd immobilization mechanism, yet biochar amplified ecotoxicity to soil fauna. These findings highlight the need for balanced strategies to optimize biochar’s benefits in alkaline agroecosystems.