Heavy metals and litter decomposition in coniferous forests.

The dynamics of Mn, Zn, Fe, Cu, Pb, and Cd, during litter decomposition was studied to understand how litter and soil metal concentrations influence the accumulation and/or the release of metal. Three types of leaf litters (green and brown leaves of aspen, green leaves of white birch) and four types of needle litters (green and brown needles of Scots pine, brown needles of lodgepole pine and stone pine), differing for heavy metal concentrations, were incubated at two unpolluted coniferous forest sites: a temperate silver fir forest at M. Taburno, Southern Italy, and a boreal Scots pine forest at Jädraås, Sweden. The pH of the humus layer was 6 in the silver fir forest and 4 in the Scots pine forest; the humus of the silver fir forest had higher concentrations of available Cu and Cd and a lower Fe concentration compared to the Scots pine forest. After a period of about 900 days all litter types were more decomposed at M. Taburno than at Jädraås. Zn, Fe, Pb and Cd concentrations in leaf and needle litters increased at both sites; Cu concentrations increased only in the Scots pine forest; Mn concentrations decreased in the litter richest in Mn and increased in the litters poorest in Mn. Litter decomposition in an early phase (0 up to 528/565 days) was significantly and positively correlated to initial Cu and Cd concentrations in litter. In a late phase (528/565 up to 918/929 days) decomposition was correlated significantly and negatively with litter Zn and Cu, and significantly and positively with litter Mn concentrations at the start of the period. At both sites increases in absolute amounts of Fe, Cd, and Pb were observed in all litters; however stone pine, that showed the highest initial concentration of Pb, released Pb during decomposition. All litters released Cu at Jädraås and accumulated Cu at M. Taburno. Zn was released at both sites from all leaf litters and from the needle litter of lodgepole pine. Mn was released at both sites from all litters except aspen litters that accumulated Mn at Jädraås. The results indicate that heavy metal accumulation or release may depend on the gradient of metal concentration between litter and soil, as well as on the capacity of litter to bind metal; atmospheric deposition could account at least partly for the increase of absolute amounts of Pb, Cd, and Cu.