Sources of error in direct microscopic methods for estimation of fungal biomass in soil

Sources of error in direct microscopic measurement of fungal hyphae in soil were examined and fungal biomass estimates and associated variability obtained by the direct counting method and the ergosterol technique were compared. Nested random effects ANOVA indicated that the major source of variance in the direct microscopic counting method were the people examining the prepared microscope slides, accounting for 83% of the total variance. Sampling variability accounted for approximately 14% of the total variance. Fungal biomass values calculated from soil ergosterol concentrations were close to the range of values derived from hyphal length estimates but coefficients of variation were much lower for soil ergosterol determinations (6–13%) than for hyphal length estimates (16–32%). For one soil sample, we compared total hyphal length and fungal biomass estimates from our lab to those of another lab. Values obtained by the other laboratory were outside the range of values and 95% confidence intervals reached in our lab. Comparison of fungal hyphal length estimates from undisturbed prairie soil and an adjacent cultivated soil indicated that the undisturbed soil contained more than twice as much fungal biomass. Results of our study indicate: (1) extreme caution must be used when comparing hyphal length and fungal biomass estimates made by different laboratories using the direct counting method; and (2) soil ergosterol determinations can provide information on fungal biomass that may be useful in comparing direct count estimates by different labs.