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 metals. Three types of leaf litters (green and brown leaves of Populus tremula L., green leaves of Betula pubescens Ehrh.) and four types of needle litters (green and brown needles of Pinus sylvestris L., brown needles of Pinus contorta L. and Pinus pinea L.), differing in heavy metal concentrations were incubated at two unpolluted coniferous forest sites: a temperate silver fir (Abies alba Mill.) forest at site Monte Taburno, southern Italy, and a boreal Scots pine (Pinus sylvestris L.) forest at site Jadraas, Sweden. Compared to the Scots pine forest, the humic surface horizon of the silver fir forest had a higher pH, higher concentrations of available Cu and Cd, and lower Fe concentration. After a period of about 900 days, all litter types were more decomposed at M. Taburno than at Jadraas. Zn, Fe, Pb and Cd concentrations in leaf and needle litters increased at both sites; Cu concentrations increased only in the silver fir forest; Mn concentration 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 concentration at the start (528/565 days) of the period. At both sites, increases in absolute amounts of Fe, Cd, and Pb were observed in all litters; however, P. pinea, which showed the highest initial concentration of Pb, released Pb during decomposition. All litters released Cu at Jadraas and accumulated Cu at M. Taburno. Zn was released at both sites from all leaf litters and from the needle litter of P. contorta. Mn was released at both sites from all litters, except the litter of P. tremula, which accumulated Mn at Jadraas. The results indicate that heavy metal accumulation or release may depend on the gradient of metal concentration between litter and soil, on the pH of the soil, and on the capacity of litter to bind metal; atmospheric deposition could account at least partly for the increase of absolute amounts of Pb, Fe, and Cu at M. Taburno.