Microbial activity and biomass in four Finnish coniferous forest soils — spatial variability and effect of heavy metals

In the long-term monitoring of forested areas, there is a need for reference values for soil microbiological variables in different types of forest ecosystem under different pollution loads. Owing to the high spatial variation in microbial flora in the soil and to the uncertainty in the measurements, several replicate samples are needed to give a representative estimate of a study site. Four different soil microbiological variables, soil respiration rate, ATP content, ergosterol content and acid phosphatase activity, were measured in four monitoring areas belonging to the Finnish Integrated Monitoring programme. The inhibitory effect of Cd, Cu, and Ni on the soil respiration rate was investigated in a laboratory experiment. The biomass-dependent variables correlated very well with each other, but differed from the variables representing microbial activities. The number of replicate humus samples needed to accurately describe the microbiological status of a study area is clearly site dependent. In general, less than 15 replicates were sufficient to give results with 20% precision at the 5% significance level. The soil respiration rate clearly decreased in response to the experimental heavy metal addition at all four sites. The EC20 values of soil respiration for Cd, Cu, and Ni were 100–1070, 100–700 and 20–510 mg kg–1 , respectively.