Temporal changes in heavy metal concentration and composition in forest soils of southwestern Germany

The emissions of heavy metals have peaked in the second half of the 20th century in central Europe, and so have their depositions. Moreover, soil properties have changed through acidification, which is expected to impact the mobility and availability of trace elements. Thus, the aim of this study was to explore the effects of these changing anthropogenic deposition rates and to understand the long-term behavior of heavy metals in a context of soil acidification. For this, temporal changes of the elements Zn, Co, Cu, Sr, Cr, Ni, Cd and Pb were analyzed using archived and recent soil samples. Their occurrence and mobility at four forest sites in southwestern Germany were studied. This was done using techniques of HNO3 digestion and NH4OAc extraction. Finally, several soil properties were measured and tested for correlation with the heavy metals in order to derive information regarding potential interactions. The obtained results suggested diverse changes in soil properties for the individual sites, ranging from a slight worsening to a considerable recovery from acidification. Overall, a strong influence of the heavy metal’s origin on their distribution and availability in the soil was observed. Pb was the only element that was almost purely anthropogenic. Many heavy metals had a mixed origin (Zn, Co, Cu, Cr, Ni), while Sr showed the strongest geogenic contribution. Cd was totally absent or only detected in trace concentrations, which was not enough to study this compound. Only for the case of Sr, a strong and significant influence of the soil properties was detected, suggesting that a higher pH increases this compound’s mobility. For the other elements, the influence of soil properties was less important. Instead, their vertical distribution is mainly determined by the input, long-term deposition dynamics, and transportation rates. The site diversity allowed to study various situations, but restricted sometimes the feasibility of general conclusions. Moreover, challenges regarding the use of soil archives were encountered, which appeals for a precise documentation of stored samples in order to make them usable for future research.