Ergosterol and microbial biomass in the rhizosphere of grassland soils

Fungal and microbial biomass were determined by ergosterol and fumigation–extraction, respectively, in bulk grassland soil (<2 mm), rhizosphere and root material. The aim was to quantify the contribution of these three microbial fractions to the total soil microbial biomass and to soil organic matter. In the bulk soil, the average concentration was 3.37 μg g−1 for ergosterol, 860 μg g−1 for microbial biomass C, and 30.4 mg g−1 for organic C. In the rhizosphere material, the corresponding concentrations exceeded those of the bulk soil by 80, 80 and 50%, respectively. The large average ergosterol concentration of 74.2 mg g−1 root revealed a strong fungal colonisation of the root material. About 75% of the total ergosterol was found in the bulk soil fraction, 11% in the rhizosphere and 14% in the root material. In one soil nearly half of total ergosterol was found in the root material. However, the average ergosterol-to-biomass ratio in the root material was less than a third of the bulk soil or the rhizosphere soil, indicating that approximately two-thirds of CHCl3-labile C are presumably root-derived.