Surface Co-application of Dolomitic Lime with Either Biochar or Compost Changes the Fractionation of Cd in the Soil and Its Uptake by Cacao Seedlings

The purpose of this study was to determine the effects of the application of compost or biochar on the mobility of soil-applied dolomite lime in the soil and its impact on Cd bioavailability, as well as its uptake by cacao seedlings. The experiment was conducted in a greenhouse for 120 days. Dolomite lime was applied superficially without incorporation, individually or in co-application with commercial compost or corn straw-derived biochar. Soil samples were collected at four depths (0-10, 10-20, 20-30, and 30-40 cm). Subsequently, pH, electrical conductivity, and dissolved organic carbon were measured. Bioavailable Cd and geochemical fractionation of Cd were measured at each soil depth. Root- and leaf-Cd concentrations in cacao seedlings were also determined. The results illustrate that the application of either compost or biochar has a pronounced impact on enhancing the mobility of dolomite lime within the soil (up to 40 cm). Notably, heightened alkalinity penetration was observed when dolomite lime and biochar were jointly applied. Concurrently applying biochar with dolomite lime led to a significant elevation in soil pH and a marked reduction in the concentration of bioavailable Cd. Furthermore, the concentration of leaf-Cd underwent a reduction exceeding 50% (+/- 2%) due to the application of dolomite lime and biochar. Potential mechanisms connected to Cd immobilization may encompass ion exchange and the formation of co-precipitates. Overall, the simultaneous use of dolomite lime and biochar was effective in decreasing Cd concentrations in cacao seedlings.