Effects of time and rate of application of phosphate rock on the availability of cadmium in soil

摘要

Among the important factors affecting the availability of added Cd from Cd-contaminated phosphate rock (PR) to soil are Cd solubility and content, and the application rate of PR. More Cd is presumably released from PR particles in soil as PR solubility increases with time. This greenhouse study examined effects of time and rate of application of PR containing 40mg(Cd)·kg<'-1> on soil Cd availability measured by DTPA, CaCl<'2> extraction and Cd uptake by lettuce (Lactuca sativa L.). Repeat PR applications at low rates over 2.5-year period did not increase Cd extractability in the soil by CaCl<'2> or DTPA, compared with one-time application at high rates. Within this study period, soil Cd extractability by CaCl<'2> and DTPA was affected primarily by the amount of PR (or Cd) applied and soil pH regardless if the PR was one-time or repeatedly applied. The repeat applications of PR Cd did not increase Cd uptake by lettuce compared with the one-time application. With the amount of DTPA- Cd in the PR-amended soil (25.7%) being the same as the Cd associated with PR particles<0.15mm (25.8%), the availability of the PR Cd within the study period was dominated by these small PR particles. The effect of the PR on soil Cd availability and Cd uptake by lettuce was cumulative. Further research is needed to determine how much longer than 2.5 years of equilibration the PR with the soil is needed for the Cd in the larger PR particles to become available to plants. The information enables the long-term impact of the application of the PR to be better assessed.