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

Evolution of CO2 was measured during a 24 h CHCl3 fumigation at 25°C in eight different soils (CO2F) The cumulative production of such CO2 was always lower than that evolved under the same conditions by unfumigated soils (CO2NF). However, for five of the eight soils, CO2F accounted for more than 50% of CO2NF. Neither CO2F nor CO2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 CO2 from soil. Also other considerations suggest that abiological evolution of CO2 was likely to be negligible; thus, both CO2F and CO2NF were mostly of biological origin and, probably, the biomass surviving chloroform exposure, or partially lysed, was substantial. The ratio CO2F to CO2NF was taken as an indirect physiological assessment of the efficiency of CHCl3 in lysing microbial cells as, in principle, the lower CO2F compared to CO2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 CO2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 CHCl3 (EC: 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 CHCl3 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 KC and KEC generate quite dissimilar values.