Distribution with depth of protozoa, bacteria and fungi in soil profiles from three Danish forest sites

The numbers and biomass of protozoa, bacteria and fungi were measured at various depths (1.5–122.5 cm) in the unsaturated zone of three contrasting pristine Danish forest site profiles: a dry beech (Fagus silvatica) forest on mor, a wet peaty spruce (Picea abies)/birch (Betula pubescens) forest and a dry spruce (P. abies) forest on mor. All sites were situated on a Weichel moraine. Except for a bacterial peak at 42.5 cm in the peat profile, the general tendency was a decrease in biomass with increasing depth for all groups examined. Protozoa decreased more rapidly with increasing depth than the other two groups of organisms examined. An evaluation of the bacterial–protozoan relationship by a simple mathematical model indicated that the subsurface protozoan populations are active and not accidental percolated cysts. The low protozoan numbers found in shallow subsurface sites contrast markedly with the results from contaminated sites where much larger protozoan populations have been reported even at considerable depths. Consequently, the results suggest that protozoa are good indicators of organic pollution in subsurface soils; however, more work involving the comparison of polluted and unpolluted soils is needed to confirm this suggestion.