Evaluation of respiration-based methods for measuring microbial biomass in metal-contaminated acidic mineral and organic soils

Abstract

To select the most suitable method for determining microbial biomass in metal-polluted acidic soils both the chloroform fumigation-incubation (FI) method and the substrate-induced response (SIR) method were used on four acidic (pH 2.9–4.4) mineral and four organic soils from sites situated 1–42 km from a copper smelter. The FI biomass was calculated with or without nonfumigated controls by using either the general 0.41 or individual soil k c values, and through the kinetic model of Chaussod and Nicolardot ( Revue d'Écologie et de Biologie du Sol 19, 501–512, 1982) which does not use a nonfumigatedcontrol (because it may have a microbial population different to that of the fumigated sample). As SIRbiomass calculated by the standard procedure was found not to correlate with the FI biomass in the soils, a new calibration was derived on the basis of the respiration rate during the first hour after glucose addition. Somewhat at variance with the few other reports on FI biomass in soils of pH below 4.5: (a) only one of the eight soils showed a negative “flush”; (b) the k c values for 14C-labelled fungal biomass were > 0.41 in six soils, and > 0.45 in five; (c) the k c values were not dependent solely on the soil metal content or pH; and (d) the FI biomass-C estimated by the usual methods was up to 2.6% of the total C in the mineral soils, and the biomass did not seem to be influenced by the concentrations of toxic metals in the organic soils. The FI biomass-C values determined by the kinetic model of Chaussod and Nicolardot were, on an average, 0.32 and 0.71% of the total C in the organic and mineral soils, respectively. The method also discerned the effect of a high level of Cu pollution. The single organic and mineral soils containing biocidal concentrations of Cu had biomass-C values, as percentages of total soil organic C, that were 44 and 36% less than the mean values for other soils within their respective groups. Also, the FI biomass values determined by the use of the model correlated significantly ( P > 0.01) with total C in both the organic and mineral soils groups.