Exploring the dual mechanisms of organic fertilizers in regulating Cd bioavailability in soil: Abiotic immobilization and microbially driven processes

Cadmium (Cd) contamination in agricultural soils threatens global food safety. Organic fertilizers (OFs) are renewable soil amendments with considerable potential for regulating Cd bioavailability. However, how abiotic reactions interact with microbially mediated processes under different OF properties and soil conditions remains insufficiently understood. This review focuses on OF sources and composition, their effects on soil physicochemical and biological properties, and the pathways that modulate Cd bioavailability. Key advances include: (1) OFs abiotically immobilize Cd via adsorption, ion exchange, complexation, and precipitation. Indirectly, they alter Cd partitioning by elevating soil pH and cation exchange capacity (CEC), modulating redox potential (Eh), and improving aggregate stability. (2) OF composition influences microbial activity by controlling carbon and nutrient availability, thereby affecting extracellular polymeric substance production, sulfate reduction, phosphate mobilization, carbonate precipitation, and Fe cycling. These microbial processes further modify Cd partitioning through changes in Cd binding, mineral formation, and mineral dissolution. (3) Cd regulation by OFs is dynamic and bidirectional. Continuous feedbacks among soil pH, Eh, dissolved organic matter, and microbial activity can either sustain Cd retention or increase the risk of Cd remobilization during progressive OF decomposition. Overall, this review proposes an integrated framework linking source-dependent OF chemical “fingerprints” to abiotic–microbial interactions governing Cd retention and remobilization in soils. This framework provides a mechanistic basis for selecting and applying OFs to reduce Cd bioavailability while accounting for potential remobilization risks in contaminated agricultural soils.