The relationships of soil microbial activity and biomass to the chemical composition of mor

Multivariate methods for modelling microbial parameters, and separating them into factors dependent on organic matter quality and heavy metal content have been developed and evaluated. Organic matter quality has been characterised using near infrared reflection (NIR) elemental analysis, pyrolysis GC/MS, carbon fractionation, humus extraction, and measurement of pH and several physicochemical variables. Mor horizons were studied from forests close to two point sources of heavy metals: the primary smelter in Roennskaer (the largest point source of heavy metals and sulphuric acid in Sweden), and the former chlor-alkali factory at the paper mill in Skutskaer. In the Roennskaer study, the negative effects on the soil microorganisms of the heavy metals dominated in mor with a copper + zinc content higher than 1000 microg-1 dry soil. In mor with lower heavy metal content, the organic matter quality factors (degree of decomposition, and content of K and Mn) were the dominating factors related to microbial activity and biomass. The variation in microbial parameters related to organic matter quality could be removed, and then a significant 10-20% decrease in soil respiration rate and microbial biomass was observed in mor with a copper + zinc concentration of 300-1000 microg g-1 dry soil compared to control mor. In Skutskaer, the variation of soil respiration rate, heavy metals and organic matter quality was investigated. The factors that most highly influenced respiration rate in Skutskaer were K, Mn and Ca contents, and pH. The heavy metal contents were correlated to organic matter quality in such a way that the correlation between respiration rate and heavy metals could not be attributed to a direct effect of the heavy metals on soil respiration. The correations between several microbial parameters were also studied. Soil respiration rate and microbial biomass measured by the substrate induced respiration method were always highly correlated. Furthermore, they were both negatively correlated to lag time after substrate addition, and correlated to differences in the composition of microbial phospholipid fatty acids.