Microbial biomass in soils with calcium accumulation associated with the application of composted lime-treated sewage sludge

We investigated the factors affecting the microbial biomass in upland soils where calcium (Ca) had accumulated because of the application of composted lime-treated sewage sludge (LSS). Soil samples (0-10 cm) were taken from field plots that had received LSS compost at the rates of 0, 20, 40 and 60 Mg ha[superscript [-]1] year[superscript [-]1] for 4 years. The pH values and the amount of exchangeable Ca[superscript 2[+]] increased with increasing amounts of compost. Total concentrations of cadmium, copper and zinc increased with the application of LSS compost, and at a rate of 60 Mg ha[superscript [-]1] year[superscript [-]1], they were 0.7, 41 and 144 Mg kg[superscript [-]1] soil, respectively. Electrical conductivity (EC) increased in the soils that had received LSS compost, and this was ascribed to an increase in the amount of water-soluble Ca[superscript 2[+]]. The application of LSS compost led to a decrease in the ratio of microbial biomass C to total soil C and an increase in biomass specific respiration (qCO₂), indicating adverse effects of LSS compost application on soil microbial biomass, as observed in sewage sludge-amended soils. Analysis of the ergosterol content did not reveal any increase in the proportion of fungal biomass in the total microbial biomass in contrast to earlier findings on sewage sludge-amended soils. The concentrations of heavy metals in the soils used were below the levels known to exert adverse effects on soil microbial biomass. In contrast, the ratio of microbial biomass C to total soil C decreased significantly and the qCO₂ increased significantly with the increase in EC and in the amount of water-soluble Ca[superscript 2[+]]. Therefore, it was suggested that the electrolyte concentration associated with the amount of water-soluble Ca[superscript 2[+]] was the major factor affecting microbial biomass in soils amended with LSS compost.