A micromorphological study on the development of humus profiles in heavy metal polluted and non-polluted forest soils under Scots pine

To evaluate the influence of heavy metal pollution on the long term decomposition process of soil organic matter, the development of two humus profiles in the vicinity of a zinc smelter were compared to two reference humus profiles. Humus profiles reflect the relationships between vegetation type, biological activity and environmental conditions. All sites were located in Scots pine (Pinus sylvestris) forest stands on nutrient poor driftsand deposits in the southeastern part of the Netherlands. The results of the chemical analysis showed that the concentrations of Cd, Zn and Pb in the polluted humus profiles were up to ten times that of the heavy metal concentrations in the non-polluted humus profiles. The development of the humus profiles was studied by micromorphological observations in thin sections and by field observations. All four humus profiles were characterized by an accumulation of soil organic matter on the mineral soil surface. These organic surface layers showed a similar development of distinct horizons, which represented proceeding stages in the decay continuum of soil organic matter. The increased decomposition of soil organic matter with depth in the humus profiles was supported by a relatively decrease of the amount of organic components and a simultaneous increase of excrements in thin sections. Micromorphological study of the excrements revealed that the decomposition of soil organic matter was mainly carried out by two groups of soil fauna: soil arthropods and macrofauna. The excrements of macrofauna were absent in the thin sections of the polluted humus profiles, probably as a result of pollution stress. The different types of soil fauna, however did not result in the development of different humus profiles. It is concluded that the decomposition of soil organic matter was not influenced by elevated heavy metal concentrations in the humus profiles studied, but was mainly determined by the chemical quality of Scots pine litter and the age of the forest stands. (C) 1998 Elsevier Science B.V.