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

A technique to estimate fungal growth rates in field samples was tested in soil. The technique is based on the addition of 14C-acetate to a soil slurry and the subsequent uptake and incorporation of the labelled acetate into the fungus specific substance ergosterol by the fungi. The addition of fungal inhibitors decreased acetate incorporation rates, while bacterial inhibitors did not. Fungus-free soil exhibited no incorporation of acetate into ergosterol, indicating that the method was specific for measuring fungal activity. Incorporation rates were linear up to 18 h after the addition of acetate indicating that changing the conditions (adding acetate as a solution to a soil slurry) did not affect the incorporation rate. Problems associated with saturation of the incorporation of the added acetate were encountered, which together with uncertain conversion factors made calculations of absolute growth rates difficult. However, for relative comparisons the technique worked well. This was exemplified by measuring the relationship between temperature and growth rate of the soil fungal community, where an optimum temperature between 25 and 30°C and an apparent minimum temperature for fungal growth of −11°C 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 measuring the rate of acetate incorporation into ergosterol 2 days later. Carbon appeared to be the limiting nutrient for fungal growth in both an agricultural soil and a forest humus soil.