Innovative use of modified biochar for cadmium remediation in saline soils: Integrating biological and physicochemical strategies with implications for Cd uptake in garden cress (Lepidium sativum)

Abstract

Cadmium (Cd) contamination in saline soils poses a serious challenge to agricultural productivity and food safety. Although biochar and earthworms have been widely studied as separate remediation agents, their combined performance and mechanisms under saline conditions remain poorly understood. This study evaluated the effectiveness of ball-milled biochar (MBC) and Fe-Mn modified biochar (FMBC), alone and in combination with the earthworm Eisenia fetida, for Cd stabilization at two contamination levels (4 and 8 mg kg⁻¹). Lepidium sativum (garden cress) was used as an indicator plant to assess the implications of Cd stabilization for crop safety. Results showed that FMBC reduced the exchangeable Cd fraction by up to 72% and increased the stable carbonate- and Fe-Mn oxide-bound forms by 64% compared with the control. The co-application of FMBC and E. fetida further enhanced Cd immobilization efficiency by approximately 25% relative to FMBC alone. Earthworm activity increased the organic-bound Cd fraction through bioturbation and stimulation of microbial processes, while FMBC improved microbial colonization and pH buffering in the rhizosphere. Despite the negative influence of salinity, FMBC (2%) effectively mitigated Cd mobility and lowered plant Cd uptake by 79%. Risk assessment code (RAC) analysis confirmed that FMBC combined with earthworms reduced the environmental Cd risk from “very high” to “moderate.” These findings highlight the potential of integrating modified biochar with E. fetida as a synergistic and sustainable strategy to mitigate Cd bioavailability and promote safer crop production in saline-contaminated soils.