DO PHYSICAL-PROPERTIES OF SOIL AFFECT CHLOROFORM EFFICIENCY IN LYSINGMICROBIAL BIOMASS

Evolution of CO2 was measured during a 24 h CHCl3 fumigation at 25 deg rees C in eight different soils (CO2-F) The cumulative production of s uch CO2 was always lower than that evolved under the same conditions b y unfumigated soils (CO2-NF). However, for five of the eight soils, CO 2-F accounted for more than 50% of CO2-NF. Neither CO2-F nor CO2-NF we re singly or multiply correlated to soil pH, water holding capacity, H g porosity and carbonate content, i.e. chemico-physical variables that could influence the release of CO2 from soil. Also other consideratio ns suggest that abiological evolution of CO2 was likely to be negligib le; thus, both CO2-F and CO2-NF were mostly of biological origin and, probably, the biomass surviving chloroform exposure, or partially lyse d, was substantial. The ratio CO2-F to CO2-NF was taken as an indirect physiological assessment of the efficiency of CHCl3 in lysing microbi al cells as, in principle, the lower CO2-F compared to CO2-NF the more efficient the chloroform fumigation. This ratio was significantly mul tiply correlated with several combinations of independent variables, i ncluding 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 corre lations were functionally confirmed because the dynamics of the CO2-F evolution during fumigation was related to the relative structural sta bility of the soils. Other correlations seemed to indicate that the po sitive relationships reported earlier between clay content and organic C made extractable by CHCl3 (E-C: from which biomass C is calculated in the FE method for estimating soil microbial biomass) could be large ly dependent on the relative ability of chloroform to permeate pores i n different soils. It is highly probable that the efficiency of CHCl3 in lysing microbial cells is strongly influenced by the soil structura l properties. This could be one of the reasons why the calibration pro cedures for calculating K-C and K-EC generate quite dissimilar values. (C) 1997 Elsevier Science Ltd.