Effect of bone chars on phosphorus‐cadmium‐interactions as evaluated by three extraction procedures

Foreseen P shortage and contamination problems have stimulated the search for renewable and contaminant‐free P‐fertilizers and amendments that immobilize Cd. We investigated the P‐dissolution and Cd‐immobilizing effect of bone char (pyrolyzed de‐fatted bone chips; BC) and bone char with added reduced S compounds (BC^plus). Five soils varying in pH and low to high Cd‐contamination were incubated with slow‐release P‐fertilizers (BC and BC^plus) and the fast P‐release diammonium phosphate (DAP), and extracted with NH₄NO₃‐, NaHCO₃‐solutions, and H₂O. The P‐concentrations obtained by the three extractants were well correlated and NH₄NO₃ well suited to simultaneously assess the P‐ and Cd‐solubility. The addition of BC increased pH in all soils whereas BC^plus and DAP lowered the pH in soils with pH > 5. Similar trends for NH₄NO₃‐P differences between treatments and control were observed for BC and BC^plus during the incubation period, although BC^plus resulted in much larger P‐concentrations. The highest Cd‐immobilization efficiency was obtained in BC‐treated soils. The addition of BC^plus and DAP decreased the Cd‐concentrations until 34 d of incubation in all soils and remained effective in Cd‐immobilizing in soils that showed a pH raise over 145 d of incubation. Thus, the results indicate that surface modification of BC may promote the P‐dissolution along with a concomitant Cd‐immobilization largely through its pH‐effect but this must be confirmed in studies under non‐equilibrium conditions.