To clarify the direct quantitative responses of microorganisms to fine roots of different diameter size, we examined the relationship between specific microbial respiration normalized at 20 °C and mass loss, chemical composition (C, N), and surface area of fine roots of two tree species (Quercus serrata, Ilex pedunculosa) across two diameter classes (<0.5, 0.5-2 mm) at Ryukoku Forest in Shiga, Japan. The roots were enclosed in litterbags, buried, and harvested at 1 or 3 month-intervals for 18 months.

For both species, the mean values of microbial respiration and root properties for each sampling interval varied with decay time. The <0.5 mm roots were higher respiration rate than those of the 0.5-2 mm roots. They showed slower mass loss, constant C/N dynamics, and higher decline of root surface area. The diameter size, which reflects chemical and morphological properties, is an important indicator for microbial respiration.

When pooling all the data, the respiration rate was significantly correlated negatively with mass remaining, C/N, and root surface area across two diameters and species. Thus microbial respiration was determined by resource quality of dead fine root beyond the diameter classes and species. Quantitative relationship between microbial decomposition respiration and resource quality could improve our understanding of the mechanisms in fine root decomposition.