Effect of feedstock, organic matter content and pH on Cd, Zn and Mn availability in farm compost based on bark and wood chips

We aimed to determine the effect of feedstock composition on Cd, Zn and Mn concentrations and availability during the composting process for small-scale composting. The effect of using wood chips instead of tree bark on compost properties was determined in a compost trial with leek residues and hay of grass included in the feedstock. Supplementary experiments included sequential extraction for determining the easily exchangeable, reducible, oxidizable and residual fractions for Cd, Mn, and Zn in composts. We determined the effect of decreasing the pH or the organic matter content and adding easily available Zn and Mn on the fractionation and availability of Cd, Mn and Zn in composts.
Use of bark or wood chips resulted in elevated Cd, Zn or Mn concentrations in the compost. Cd concentrations in the compost may exceed legal criteria, mainly due to the use of wood chips or bark of tree species with high Cd uptake. Based on sequential extractions, we measured that Cd and Zn were mainly bound in the reducible fraction, while Mn was found in the easily available fraction. Even at higher total Cd and Zn concentrations in the composts, the results of the sequential extractions showed a small fraction which is present in the exchangeable fraction, which indicates a low environmental risk when applying these composts. Higher Cd concentrations in compost due to the use of bark or wood chips were not found to result in higher risk for Cd leaching. The results of pH-stat experiments illustrated that only a strong pH decline in the compost resulted in higher availability of Cd, Zn and Mn. The amount of soil in the feedstock material or unintentionally added during the composting process affected total and available concentrations for Mn.