Regional Variability of Cd, Hg, Pb and C Concentrations in Different Horizons of Swedish Forest Soils

Contents of cadmium (Cd), mercury (Hg), lead (Pb) and carbon(C) in the O, B and C horizons of podzolized forest soils in Sweden were surveyed. Concentrations and storage of Cd, Hg and Pb in the O and B horizons were high in southern Sweden and gradually decreased towards the north, though with considerable local variability. This pattern reflects the influence of anthropogenic emissions of these metals, as well as the effects of soil-forming processes. Parent till material, as represented by the C horizon concentration of the respective metal, accounted for little of the variation in metal concentration in the O horizon. For Cd and Pb, the correlations were not significant or slightly negative (R2 = 0.12 and 0.09 respectively) depending on region, while for Hg the correlation was not significant or slightly positive (R2 = 0.03 and 0.08). Furthermore, parent till material accounted for more of the variation in metal concentrations in the B horizons in the northern part of Sweden than in the middle and southernmost parts, where the concentration of total carbon had more influence. The correlation between the metal concentrations in the B and C horizon was strongest for Pb (R2 = 0.63 and 0.36 in the two northernmost regions), lower for Cd (R2 = 0.19 and 0.16) and not significant for Hg. For all soil horizons, total C concentration accounted for much of the variation in Hg concentration in particular (O-horizon R2 = 0.15-0.69, B horizon R2 = 0.36-0.50, C horizon R2 = 0.23-0.50 and ns in one region). Ratios of metal concentrations between the B and C horizons were highest for Hg(maximum value of 30), indicating a relatively larger addition or retention of Hg compared to Cd and Pb (maximum value of 10)in the B horizon. This study indicate that factors other than parent material account for the large scale variation in O horizon concentrations of metals but patterns correspond well with those of atmospheric deposition of heavy metals and acidifying substances. Furthermore, the study highlights the importance of soil organic matter and the intensity of pedogenic processes for the content and distribution of metals throughout the soil horizons