A β-sitosterol assay for fine-root mass in soils

The structural complexity and spatial heterogeneity of soils make the direct measurement of biomass a tedious task. These difficulties have led to the development of indirect assays based on biochemical indicators. We have investigated the potential of β-sitosterol, a component of plant membranes, as an indicator of phytomass in soils. β-sitosterol was extracted from soil, root and litter samples with hot methanol, following the procedure used for fungal ergosterol, and quantified by HPLC. Concentrations in fine roots (<2 mm dia) of 16 herbaceous and woody plants ranged from 0.2 to 2.0 μg mg−1 organic mass (mean = 1.0, SD = 0.5). Concentrations in 2 cm dia × 15 cm deep soil cores collected under old field, deciduous hardwood and pine canopies ranged from 2.4 to 26.5 μg g−1 soil dry mass, equivalent to fine-root standing crops of 5–60 Mg ha−1. Microcosm studies indicated that in the early stages of decomposition β-sitosterol degrades at about the same rate as bulk litter. The β-sitosterol assay may facilitate the analysis of mycorrhizal associations and the quantification of total fine-root mass.