Simultaneous measurement of S, macronutrients, and heavy metals in the soil microbial biomass with CHCl3 fumigation and NH4NO3 extraction

The study was carried out to investigate whether 1 M NH4NO3 extraction is a useful alternative to 10 mM CaCl2 extraction for estimating soil microbial biomass S and whether the data of CHCl3-labile NH4NO3-extractable macronutrients and heavy metals are useful and in agreement with the available data on element concentrations in soil microorganisms. Microbial biomass C was followed by microbial biomass S after CaCl2 extraction with an average C/S ratio of 82, and by microbial biomass S after NH4NO3 extraction with an average C/S ratio of 57. The mean contribution of CHCl3-labile metals in relation to the NH4NO3-extractable fraction from non-fumigated soils ranged from 0.1 to 112% in the order potassium < magnesium < cadmium < sodium < zinc + nickel < manganese < copper. The mean contribution of CHCl3-labile metals in relation to the microbial biomass C ranged from 0.03 to 22‰ in the order cadmium < nickel < zinc < manganese < magnesium < copper < sodium < potassium. These relative contributions varied within the different metals from a 4-fold (Na+) to a more than 200-fold range (Cu2+). Significant positive correlations with microbial biomass C were observed for CHCl3-labile zinc, sodium and especially potassium. The concentration of all elements except copper in relation to microbial biomass C were in the range known from the limited literature on fungi grown on heavy metal contaminated soils.