APPLICATION OF THE SELECTIVE-INHIBITION METHOD TO DETERMINE BACTERIAL- FUNGAL RATIOS IN 3 BEECHWOOD SOILS RICH IN CARBON - OPTIMIZATION OFINHIBITOR CONCENTRATIONS

Bacterial and fungal contributions to microbial respiration in three b eechwood soils rich in C (two basalt soils and one limestone soil) wer e investigated by using streptomycin and cycloheximide to inhibit subs trate-induced respiration after glucose (8000 mu g g(-1)), N, and P ad dition to soil samples. The inhibitors were added as solutions (2000, 8000, and 16000 mu g g(-1)) and the reduction in substrate-induced res piration after separate and combined inhibitor addition was measured i n an automated electrolytic microrespirometer. Bacterial and fungal co ntributions to microbial respiration were calculated using the interva l 6-10h after inhibitor application. The microbial biomas was smaller in the two basalt soils (Oberhang and Mittelhang) than in the limeston e soil (Unterhang). In the presence of both inhibitors, microbial resp iration was inhibited by a maximum of 45, 45, and 25% in the two basal t soils and the limestone soil, respectively. Inhibition of microbial respiration was at a maximum at streptomycin and cycloheximide concent rations of 16000 mu g g(-1). The inhibitor additivity ratio approached 1.0 even at high inhibitor concentrations, indicating high inhibitor selectivity. Calculated prokaryote: eukaryote ratios indicated lower b acterial contributions to the microbial. biomass in the Mittelhang (0. 74) and Unterhang (0.73) than in the Oberhang (0.88) soil.