USE OF STAINING AND INHIBITORS TO SEPARATE FUNGAL AND BACTERIAL-ACTIVITY IN SOIL

The relative contribution of two major soil microbial groups, fungi and bacteria, to soil respiration was investigated because of their importance in decomposition and nutrient cycling in agroecosystems. The bactericide streptomycin and the fungicide cycloheximide were applied singly and in combination in two glucose-amended (5 g kg-1 soil) and texturally differing agricultural soils, a Valentine sand (Typic Ustipsamment) and a Sharpsburg silty clay loam (Typic Argiudoll). Soils receiving only glucose served as controls. Total and metabolically-active fungal and bacterial microscopic counts were made at 1, 3 and 10 days after amendment. CO2 evolution was measured for 24 h before each sampling for the purpose of relating soil respiration to specific microbial metabolic activity. Intracellular reduction of iodonitrotetrazolium to formazan and accumulation of fluorescein from fluorescein diacetate were used as indicators of metabolically-active bacteria and fungi, respectively. Production of new biomass in controls and fluctuations in CO2 evolution were primarily associated with bacterial activity on the first day, indicating an advantage of bacteria over fungi in initial glucose metabolism. Fungal activity was associated with CO2 production at later sampling times. Reduced respiration, as a result of biocide treatment, occurred only on the first day whereas nontarget or indirect effects of both inhibitors were apparent at later times. Fungal activity rose two to five times above the level of the glucose-amended control on day 3 in streptomycin-treated soils, and bacterial activity was stimulated in the cycloheximide treatment of the Sharpsburg soil on day 10. The effectiveness of streptomycin inhibition on total and active bacterial densities was greater in the Sharpsburg silty clay loam than in the Valentine sand. There was no reduction of total hyphal length due to biocide treatment, and fungal activity with cycloheximide did not differ significantly from that of the control. For our experimental conditions the use of biocides alone did not effectively differentiate between bacterial and fungal activity. Selective staining of metabolically-active microorganisms, however, may offer promise in achieving this goal and appears useful in understanding the importance of bacteria and fungi to nutrient cycling in soil ecosystems. © 1990.