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

Abstract

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.

The influence of humic acids on the accumulation of lead (Pb) and cadmium (Cd) in tabacco leaves grown in different soils

“…In soils with similar Cd levels, the pH played a vital role in metal uptake. This finding is similar to results in the literature (Strobel et al, 2005;Meers et al, 2005;McBride, 2002;Gray et al, 1999) showing that Cd availability to plants in soils is influenced by soil properties. Recent studies even proposed that Cd uptake by crops could be estimated from the total soil Cd, soil pH, and % humus (McBride, 2002;Hough et al, 2003).…”

Copper content of soils and river sediments in a winegrowing area, and its distribution among soil or sediment components

“…Although the threat is held at bay by most of the vineyard Cu being bound to organic matter, and therefore not immediately available for release into the soil solution (Bolan and Duraisamy, 2003;Bolan et al, 2003a), it would be made available by a sufficient rise in pH (Bolan et al, 2003b); in the present case, results on the dependence of copper desorption on soil pH (Arias et al, 2006) suggest that it would suffice to increase pH to above 6.5 for copper release. Copper would also be released into the soil solution by a sufficient reduction in pH (in these soils, to values below pH 4.5; see Arias et al, 2006), especially in abandoned vineyards (Hesterberg, 1993;Strobel et al, 2005).…”

Changes in copper content and distribution in young, old and abandoned vineyard acid soils due to land use changes

“…The new forest plantations established in abandoned vineyard soils might acidify the soil and increase the risk of Cu mobilisation, as suggested by Strobel et al (2005). In order to reduce such risks, it is advisable to develop a forestation plan for these areas including deciduous species-the acidifying potential of which is lower than that of coniferous species.…”