Estimation of fungal growth rates in soil using C-14-acetate incorporationinto ergosterol

A technique to estimate fungal growth rates in field samples was tested in soil. The technique is based on the addition of C-14-acetate to a soil slur ry and the subsequent uptake and incorporation of the labelled acetate into the fungus specific substance ergosterol by the fungi. The addition of fun gal inhibitors decreased acetate incorporation rates, while bacterial inhib itors did not. Fungus-free soil exhibited no incorporation of acetate into ergosterol, indicating that the method was specific for measuring fungal ac tivity. Incorporation rates were linear up to 18 h after the addition of ac etate indicating that changing the conditions (adding acetate as a solution to a soil slurry) did not affect the incorporation rate. Problems associat ed with saturation of the incorporation of the added acetate were encounter ed, which together with uncertain conversion factors made calculations of a bsolute growth rates difficult. However, for relative comparisons the techn ique worked well. This was exemplified by measuring the relationship betwee n temperature and growth rate of the soil fungal community, where an optimu m temperature between 25 and 30 degreesC and an apparent minimum temperatur e for fungal growth of -11 degreesC were found. The technique was also used to indicate which nutrients limited instantaneous fungal growth in soil by adding carbon, nitrogen and phosphorus in different combinations and measu ring the rate of acetate incorporation into ergosterol 2 days later. Carbon appeared to be the limiting nutrient for fungal growth in both an agricult ural soil and a forest humus soil. (C) 2001 Elsevier Science Ltd. All right s reserved.