Abstract:The distribution patterns of bacteria and fungi in different water\|stable aggregates (WSA), and the effects of tillage methods on bacterial and fungal biomass, were investigated in a long\|term field experiment. Tillage treatments examined in this study included Conventional Tillage (CT), Flooded Paddy Field (FPF), Combining Ridge and No\|tillage (NT). A wet sieving method was used to separate soil aggregates into six groups based on particle diameter: >4.76 mm, 2.0-4.76 mm, 1.0-2.0 mm, 0.25-1.0 mm, 0.053-0.25 mm and <0.053 mm. Fungal and bacterial biomass were estimated by measuring the amount of ergosterol and muramic acid, respectively, in the samples. Both bacterial and fungal biomass were lowest in the smallest soil particles (<0.053 mm) and highest in the 0.053-0.25 mm fraction. As the size of the soil micro\|aggregate increased, the associated bacterial or fungal biomass also rose, however, there were notable response differences between the two taxonomic groups. Of the three tillage methods examined, maximum bacterial and fungal biomass was found in the Combining Ridge and No\|tillage (NT) treatment; minimum biomass occurred under Conventional Tillage (CT). There were significant (P＜005) differences in fungal and bacterial biomass in the 025-10 mm and 2.0-4.76 mm particle\|size groups among the three tillage methods; there were no significant differences (P＞0.05) in the remaining size groups. The results of this study indicate that both fungal and bacterial biomass in the soil are related to the particle size of the soil aggregates as well as to the tillage method. The Combining Ridge and No\|tillage(NT) treatment yielded the highest fungal and bacterial biomass associated with WSA, which suggests soil health is maximized when soil disturbance is kept to a minimum.