Do physical properties of soil affect chloroform efficiency in lysing microbial biomass?

Abstract

Evolution of CO 2 was measured during a 24 h CHCl 3 fumigation at 25°C in eight different soils ( CO 2F ) The cumulative production of such CO 2 was always lower than that evolved under the same conditions by unfumigated soils ( CO 2NF ). However, for five of the eight soils, CO 2F accounted for more than 50% of CO 2NF . Neither CO 2F nor CO 2NF were singly or multiply correlated to soil pH, water holding capacity, Hg porosity and carbonate content, i.e. chemico-physical variables that could influence the release of CO 2 from soil. Also other considerations suggest that abiological evolution of CO 2 was likely to be negligible; thus, both CO 2F and CO 2NF were mostly of biological origin and, probably, the biomass surviving chloroform exposure, or partially lysed, was substantial. The ratio CO 2F to CO 2NF was taken as an indirect physiological assessment of the efficiency of CHCl 3 in lysing microbial cells as, in principle, the lower CO 2F compared to CO 2NF the more efficient the chloroform fumigation. This ratio was significantly multiply correlated with several combinations of independent variables, including a wide range of soil physico-chemical properties (Hg porosity, storage porosity, water holding capacity, sand, silt, organic C and carbonate contents) related to soil structure. These significant correlations were functionally confirmed because the dynamics of the CO 2F evolution during fumigation was related to the relative structural stability of the soils. Other correlations seemed to indicate that the positive relationships reported earlier between clay content and organic C made extractable by CHCl 3 ( E C: from which biomass C is calculated in the FE method for estimating soil microbial biomass) could be largely dependent on the relative ability of chloroform to permeate pores in different soils. It is highly probable that the efficiency of CHCl 3 in lysing microbial cells is strongly influenced by the soil structural properties. This could be one of the reasons why the calibration procedures for calculating K C and K EC generate quite dissimilar values.