Content, distribution, and solubility of cadmium in arable and forest soils

Afforestation of former farmland decreases soil pH and thus increases the solubility of Cd in the soil, which may cause Cd leaching to streams and groundwater. The Cd concentration in soil and soil solution were determined in 11 pairs of Danish arable and forest soil profiles representing three different texture classes (sand, loamy sand, and sandy loam). The soil pH did not change or decrease with depth through the arable profiles, but did increase with depth in the forest profiles. Significantly higher, Cd contents were found in the upper 30 cm of the arable soil compared with that of the forest soil. The total soil Cd concentrations correlated with the effective cation-exchange capacity (ECEC), clay content, and organic matter content, but not the soil pH. The soil solution pH was unchanged or decreasing downwards through the arable profiles, but increasing with depth in the forest profiles. The soil solution concentration of Cd was significantly higher in the forest soils than in the arable soils. The Cd concentration in the soil solution decreased. as pH increased. Both total soil and soil solution Cd concentrations were higher in the sandy loam soils compared with the loamy sand and sand soils. It is concluded that afforestation may lead to higher soil solution concentrations of Cd as decreasing pH and ECEC diminish Cd retention and reduces Cd concentrations in the forest topsoils.