Soil Humic Substances and Root Exudates: Key Factors Modulating Cadmium Mobility in Paddy Soils Under Organic Fertilization

ABSTRACT Cadmium (Cd) contamination in paddy fields poses a high potential risk of environmental exposure. Organic fertilizer application can promote the efflux of rice root exudates, increase soil organic matter, and induce changes in Cd activity in rice fields. However, the coupled effects of organic inputs and rhizosphere processes on Cd speciation at the rice root–soil interface (RSI) remain unclear. Here, the results show that organic fertilization reorganizes Cd partitioning primarily between acid‐extractable and oxidizable pools, and the direction and magnitude of change are set by rhizosphere regulation rather than bulk‐soil responses. Rhizosphere soil pH was consistently lower than that of non‐rhizosphere soil (by 0.05–1.35 units), coinciding with elevated acid‐extractable Cd during key growth stages, indicating that root‐driven acidification can enhance Cd activity. Structural equation modeling further identified contrasting roles of soil organic phases. Humic acid reduced Cd bioavailability, whereas root‐derived low‐molecular‐weight organic acids and fulvic acid favored it. This finding reveals that Cd risk in paddy fields is not simply determined by the application of organic fertilizers, but rather depends on the fertilizer type and the relative strength of organic matter sequestration and rhizosphere processes at the RSI. Overall, organic fertilization exerts dual effects on the RSI, with the regulation of rice root activity often playing a more important role than Cd chelation by organic matter. These processes constitute a balanced mechanism governing Cd migration in the RSI. These results bridge the critical knowledge gap regarding the factors influencing Cd bioavailability in rice paddies in the rhizosphere under organic fertilizer application.