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 Am), fine sand (53–
250 Am), silt (2–53 Am) and clay (0.2–2 Am). The larger sized fractions (N250 Am and 53–250 Am) contained higher levels of
fungal ergosterol than the smaller sized ones (2–53 Am and 0.2–2 Am). The largest particle size fraction (250–2000 Am) 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, pb0.05), 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.

基于粒径组分（particle size fractions）的土壤团聚体（soil aggregates）表征方法，是开展土壤有机质与生物学特性过程导向研究的实用工具。本研究采用底物诱导呼吸抑制（Substrate-induced respiration inhibition, SIR）法，对台湾中部亚高山区域内覆盖森林至草地不同植被类型的土壤各粒径组分中的微生物、真菌及细菌生物量进行定量分析；此外，本研究对同一样品中的麦角固醇（ergosterol）含量进行测定，以验证通过SIR抑制法测得的真菌生物量。本研究将表层土壤（0~10 cm）划分为4种粒径组分：粗砂（250~2000 μm）、细砂（53~250 μm）、粉粒（2~53 μm）及黏粒（0.2~2 μm）。粒径较大的组分（>250 μm及53~250 μm）相较于粒径较小的组分（2~53 μm及0.2~2 μm），其真菌麦角固醇含量更高。所有研究生境中粒径最大的组分（250~2000 μm）均表现出最高的微生物生物量水平，其余粒径组分间的微生物生物量未呈现明确的变化趋势。通过SIR计算得到的真菌生物量与经麦角固醇换算得到的真菌生物量呈显著正相关（r=0.71，p<0.05），且该相关系数随生物量水平升高而降低。本研究所得各粒径组分的真菌细菌比介于0.6~1.3之间。本研究表明，土壤各粒径组分间的微生物（真菌与细菌）生物量水平存在显著差异，且在特定生态系统中，大粒径组分往往含有更高水平的微生物生物量。