Cobalt sorption in aged compost-amended soils

This study investigates the long-term effects of compost amendments on cobalt (Co) retention in Mediterranean soils, with a specific focus on Co sorption–desorption behavior. The long-term effects of compost amendment aging on Co sorption, particularly several years after the cessation of compost application, is a topic largely unexplored. Three soils were compared: an untreated control (US) and two compost-amended soils, C5 and C10, where compost application ceased 5 and 10 years earlier, respectively. Compost addition significantly increased soil organic matter (SOM) content and shifted soil pH toward neutrality, resulting in enhanced sorptive capacity. Langmuir isotherm modelling showed that the maximum Co sorption capacity (Q m ) was higher in compost-amended soils (C5: 222 mmol kg −1 ; C10: 196 mmol kg −1 ) than in the untreated soil (US: 141 mmol kg −1 ). Desorption experiments with 0.5 M CaCl₂ confirmed greater Co retention in compost-treated soils with respect to the untreated soil. These results indicate that strong sorption mechanisms dominate in amended soils, particularly at low Co concentrations. Overall, the findings demonstrate that compost amendments enhance the long-term immobilization of cobalt in soil. Such effects persist years after the cessation of compost application, highlighting the role of compost in sustainable soil management and in mitigating trace metal bioavailability.