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 to an arable or a forest soil to determine their effects on N mineralization, nitrification, CO2 evolution, and microbial biomass C and N. Only slight differences were observed in concentrations of both inorganic N forms, CO2 evolution and amounts of microbial biomass C and N between the streptomycin-treated arable soil and the respective control. Cycloheximide singly or in combination with streptomycin stimulated N mineralization, depressed NO3- production as well as the increase in microbial biomass C and N observed in the untreated arable 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 incubation; the ratio was generally higher than 4.5 but never exceeded 5 in the streptomycin-treated arable soil and in the respective control. The inhibition of nitrification in the arable soil by the fungicide prompted the question concerning the relative importance of autotrophic and heterotrophic nitrification rates; the inhibition of the NO3- formation by nitrapyrin and the absence of stimulation by peptone indicated that the potential for autotrophic nitrification in this soil is greater than that for heterotrophic nitrification. The addition of antibiotics 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 observed in the control; they markedly depressed microbial biomass C and N during the early incubation period; then both microbial biomass values increased probably because surviving microorganisms utilized organic compounds released from antibiotic-killed cells.