ACTIVITY AND DEGRADATION OF STREPTOMYCIN AND CYCLOHEXIMIDE IN SOIL

Streptomycin and cycloheximide were added (3 and 2 mg g-1 dry soil, re spectively) singly and in combination to a forest soil to follow their possible degradation and their effects on soil mineralization-immobil ization processes. After 0, 1, 2, 4, 7, and 10 days of incubation at 2 5-degrees-C and 60% water-holding capacity, measurements were taken of microbial biomass C and N, the evolution of CO2, exchangeable NH4+, 0 .5 M K2SO4-extractable organic C, and total N in both unfumigated and CHCl3-fumigated soil. The results indicated that during the first 2 da ys of incubation, soil microorganisms were killed by the antibiotics a nd/or by CHCl3 and used subsequently as a substrate by the survivors. Thereafter, surviving microorganisms probably also started to use bioc idal molecules as an energy and nutrient source. The ratios of biomass C to biomass N and of CO2 evolved to net NH4+ produced indicated that both biocides had non-target effects for most of the incubation. Thus , streptomycin and cycloheximide are not suitable in determining the r elative contribution from fungi and bacteria to mineralization-immobil ization processes in soils.