Particle size fractionation of fungal and bacterial biomass in subalpine grassland and forest soils

Characterization of soil aggregates according to particle size fractions is a useful tool in process-oriented research into soil organic matter and biological properties. Substrate-induced respiration (SIR) inhibition was used to quantify microbial, fungal and bacterial biomass in particle size fractions of soils ranging from forest to grassland in a subalpine region of central Taiwan. In addition, ergosterol content was determined in the same samples to verify fungal biomass measured by SIR inhibition technique. Surface soil (0-10 cm) was fractionated into four particle size fractions: coarse sand (250-2000 mu m), fine sand (53-250 mu m), silt (2-53 mu m) and clay (0.2-2 mu m). The larger sized fractions (>250 mu m and 53-250 mu m) contained higher levels of fungal ergosterol than the smaller sized ones (2-53 mu m and 0.2-2 mu m). The largest particle size fraction (250-2000 mu m) from all studied habitats showed the highest level of microbial biomass, with no clear trend in microbial biomass level among the other size fractions. SIR-calculated fungal biomass level and ergosterol converted fungal biomass content were positively correlated (r=0.71, p<0.05), and such correlation decreased as biomass levels were high. Ratios of fungi to bacteria ranged between 0.6 and 1.3 in fractions obtained in this study. This study indicates a high variability of microbial (fungal and bacterial) biomass level among particle size fractions in soil, and that the large-sized fractions tend to contain a high level of microbial biomass in a given ecosystem. (C)2005 Elsevier B.V. All rights reserved.