Organic matter addition promotes Cd immobilization in alkaline paddy soils

Straw incorporation into the soil is a common agricultural practice, but its effect on soil cadmium (Cd) mobility is not well understood. We added 0–20 g kg organic matters (OMs) with different C/N ratios to three spiked alkaline paddy soils that contained a realistically low concentration of total Cd (0.94 mg kg), and then investigated soil Cd solubility in alternate watering conditions. As current physical and chemical methods have difficulties in accurately determining the distribution and speciation of Cd in soil at a low concentration, we measured multiple soil properties to identify key factors regulating dissolved Cd concentration. For all three soils, pH and dissolved Cd concentration both decreased after flooding and increased after subsequent drying. OM addition significantly reduced soil Cd solubility at both flooding and drying stages. Random forest and linear regressions further confirmed that soil total organic carbon, rather than pH, dissolved organic carbon, or total inorganic carbon as previously suggested, was the primary predictor of Cd solubility. OMs with different C/N ratios had similar effects on soil Cd solubility, whereas the effect of OM addition rate depended on soil type. The results demonstrated the potential of straw incorporation for the remediation of Cd-contaminated alkaline paddy soils, through mechanisms that differ from those reported for acid soils. Graphical Abstract: [Figure not available: see fulltext.]