DYNAMICS OF RESIDUAL ENZYME-ACTIVITIES, DENITRIFICATION POTENTIAL, AND PHYSICOCHEMICAL PROPERTIES IN A GAMMA-STERILIZED SOIL

The use of sterilized reinoculated soils appears to be valuable in the study of relationships between numbers and activity of microorganisms, especially in the case of denitrifiers. In this context, residual enzymatic activities and physico-chemical changes after gamma-irradiation have been studied. The dynamics of disappearance of the residual activities of denitrifying enzymes, CO2 release and beta-glucosidase activity were followed for 8 weeks. Evolution of some physico-chemical properties after gamma-irradiation were also tested: pH, N and C status, concentrations of exchangeable cations, soil porosity. The soil treatments were wet or dry gamma-sterilized and wet or dry stored after irradiation. Complete sterilization was achieved by a 25 kGy dose in all treatments. No important changes were noticed concerning the pore volume distribution. Very small variations in pH and exchangeable cation concentrations were considered negligible. On the contrary, an increase in soluble organic C, especially in the wet treatment, was attributed both to the lysis of killed cells and to the release of organic acids, particularly through the action of peroxides in wet conditions. The measurement of the enzymatic activities showed the resistance of enzymes to irradiation, leading for example to CO2 production, to lysis of beta-glucosidic bonds or to variations in the N status through deamination of nitrogenous organic compounds or progressive proteolysis of microbial cells. Nevertheless, the location of enzymes was considered as a factor controlling resistance to gamma-rays and to its subsequent activity. We suggested that denitrifying enzymes were remaining active in non-proliferating cells and that the loss of potential denitrifying activity, observed for the 8 weeks period, resulted from cell lysis. It was concluded that gamma-irradiation of dry soil is less damaging than that of wet soil. Nevertheless, the soil must stabilize before use and a control (uninoculated irradiated soil) must be compared in order not to attribute residual activities of the irradiated soil to the introduced microbial populations.