Dissolved organic carbon and decreasing pH mobilize cadmium and copper in soil

When limed farmland is converted to forestry cadmium (Cd), copper (Cu) and other heavy metals can become mobile because of acidification and increased concentration of dissolved humic substances. The influence of pH and dissolved organic C on amounts and rates of Cd and Cu release was investigated in a cultivated soil by extraction with ～ 1 mm hydrochloric acid at pH 3 with and without dissolved organic C in the batch mode with weekly replacement of the extraction solution. After 88 weeks, 35–50% of aqua regia‐extractable Cd was extracted; addition of 10 mm dissolved organic C had no effect on the amount dissolved, but it increased the initial rate of release because the organic matter buffered the suspension at a lower pH. The solubility and release rate of Cd decreased as the geochemically active fraction was depleted. This suggests that Cd occurs in the soil in a continuum of binding strengths ranging from readily available to strongly bound forms. The repeated extractions resulted in distribution coefficients (K_{d dis}) that have log‐linear relationships with pH. This allows prediction of Cd solubility during acidification of soil. Dissolved organic C enhanced the release of Cu from less than 8% (without) to more than 20% (with) of aqua regia‐extractable Cu. Total contents of Cd and Cu cannot be used as measures of the metals' availability during acidification of former limed farmland. Predictions of availability should be based on the solubility as a function of pH and the degree of Cd or Cu depletion from the geochemically active fraction in soil.