Does soil ergosterol concentration provide a reliable estimate of soil fungal biomass?

Our aim was to determine if soil ergosterol concentration provides a quantitative estimate of the soil fungal biomass concentration, as is usually assumed. This was done by comparing soil ergosterol measurements with soil fungal biomass (fungal biomass C) concentrations estimated by microscopic measurements and by the selective inhibition technique linked to substrate-induced respiration (SIR). The measurements were compared in a silty-clay loam soil given a range of previous treatments designed to increase or decrease the soil fungal biomass and so also to change the soil ergosterol concentration. The treatments used were ryegrass amendment, to increase the total and fungal biomass, and CHCl3-fumigation and the addition of the biocides, captan, bronopol and dinoseb, to decrease both ergosterol and fungal biomass C concentrations. The mineralization of ergosterol following addition to sand innoculated with soil extract, and to a sandy loam soil, was also determined. The added ergosterol was little, if at all, degraded following addition to either sand or the unfumigated or fumigated soil during a 10 d aerobic incubation. Similarly, pesticide addition did not significantly change soil ergosterol concentrations yet the soil fungal biomass C concentration decreased significantly. Thus, the ratio: (soil ergosterol concentration/soil fungal biomass C concentration) was much higher in the pesticide-treated soils than the control soil. Following ryegrass amendment, soil ergosterol concentration increased from about 6-12 microgram(-1) soil within 5 d and then decreased gradually to about 7 microgram g(-1) soil by 20 d incubation. Changes in fungal biomass C (measured by direct microscopy) closely mirrored changes in soil ergosterol over this period. However, when the amended soil was fumigated and then incubated for a further 5 d, the initial ergosterol concentration declined from 7 to 5 microgram g(-1) soil by 20 d incubation (a decline of about 0.4). The comparable decline in fungal biomass C was about eight-fold. Thus the ratio of ergosterol to fungal biomass C increased from 0.005 to about 0.01. There was a significant correlation (r>0.84, P<0.001) between soil ergosterol concentration and fungal biomass measured by either SIR or microscopy. However, three data points played a vital role in the correlation. When these points were excluded the relationship was very poor (r<0.4). Our results therefore suggest that substantial amounts of ergosterol may exist, other than in living cells, for considerable periods, with little, if any mineralization. Thus, these results indicate that ergosterol and fungal biomass C concentrations are not always closely correlated, due to the slow metabolism of ergosterol in recently dead fugal biomass and/or the existence of exocellular ergosterol in soil.