EFFECTIVENESS OF ANTIBIOTICS TO DISTINGUISH THE CONTRIBUTIONS OF FUNGI AND BACTERIA TO NET NITROGEN MINERALIZATION, NITRIFICATION AND RESPIRATION

Streptomycin and cycloheximide were applied singly or in combination t o an arable or a forest soil to determine their effects on N mineraliz ation, nitrification, CO2 evolution, and microbial biomass C and N. On ly slight differences were observed in concentrations of both inorgani c N forms, CO2 evolution and amounts of microbial biomass C and N betw een the streptomycin-treated arable soil and the respective control. C ycloheximide singly or in combination with streptomycin stimulated N m ineralization, depressed NO3- production as well as the increase in mi crobial biomass C and N observed in the untreated amble soil. The C:N ratio of soil microbial biomass fluctuated around 4 with cycloheximide singly or in combination with streptomycin during the 0-10 day incuba tion; the ratio was generally higher than 4.5 but never exceeded 5 in the streptomycin-treated arable soil and in the respective control. Th e inhibition of nitrification in the arable soil by the fungicide prom pted the question concerning the relative importance of autotrophic an d heterotrophic nitrification rates; the inhibition of the NO3- format ion by nitrapyrin and the absence of stimulation by peptone indicated that the potential for autotrophic nitrification in this soil is great er than that for heterotrophic nitrification. The addition of antibiot ics to the forest soil increased respiration and NH4+-N concentration with the highest effect with both antibiotics. Both biocides singly or in combination depressed the decrease of NO3--N concentration observe d in the control; they markedly depressed microbial biomass C and N du ring the early incubation period; then both microbial biomass values i ncreased probably because surviving microorganisms utilized organic co mpounds released from antibiotic-killed cells.