COMPARISON BETWEEN FLUORESCEIN DIACETATE AND DIFFERENTIAL FLUORESCENTSTAINING PROCEDURES FOR DETERMINING FUNGAL BIOMASS IN SOILS

Soil microbial biomass is often used as an indicator of the state of p rocesses and fertility in soil, yet most currently available technique s determine the total hyphal length (active live + inactive live + dea d), rather than viable (active + dormant) or metabolically active port ions of the hyphae. Inability to determine which pool is being measure d can result in large errors in developing models of fungal community dynamics. The development of a quantitative relationship between activ e fungal biomass and viable hyphae needs to be developed in order to p roduce accurate measures of below-ground processes. Fluorescein diacet ate (FDA) determines the active portion of the fungal community in soi l while differential fluorescent stain (DFS) is a putative vital stain . We compared the amount of fungal biomass determined using FDA and us ing DFS, in three soils from southern California to determine the degr ee the to which the pools determined by the two stains is correlated. FDA and DFS measured different amounts of hyphae in each soil type sam pled, yet the two techniques measured the fungal biomass in the same o rder of magnitude at all sites, indicating that DFS is most probably a measure of the active fungal biomass in soil. While there were severa l possible explanations for the differences in the amount of hyphae de tected by the two stains, the most probable is that there are differen ces in the physical location of the stains within the hyphae, as shown by staining pure cultures of fungi. The large effects of storage on t he fungal biomass detected in this study indicate that soil samples mu st be sampled rapidly after removal from the field. These results sugg est that DFs is an activity stain that is more suitable for large samp le numbers, in that prepared slides can be stored and analyzed microsc opically at a later date. (C) 1997 Elsevier Science B.V.