Abstract

Changes in the soil bacterial community of a coniferous forest were analyzed to assess microbial responses to wildfire. Soil samples were collected from three different depths in lightly and severely burned areas, as well as a nearby unburned control area. Direct bacterial counts ranged from 3.3-22.6 x10^8 cells/(g . soil). In surface soil, direct bacterial counts of unburned soil exhibited a great degree of fluctuation. Those in lightly burned soil changed less, but no significant variation was observed in the severely burned soil. The fluctuations of direct bacterial count were less in the middle and deep soil layers. The structure of the bacterial community was analyzed via the fluorescent in situ hybridization method. The number of bacteria detected with the eubacteria-targeted probe out of the direct bacterial count varied from 30.3 to 84.7%, and these ratios were generally higher in the burned soils than in the unburned control soils. In the surface unburned soil, the ratios of [alpha]-, [beta]- and [gamma]-proteobacteria, Cytophaga-Flavobacterium group, and other eubacteria groups to total eubacteria were 9.9, 10.6, 15.5, 9.0, and 55.0%, respectively, and these ratios were relatively stable. The ratios of [alpha]-, [beta]- and [gamma]-proteobacteria, and Cytophaga-Flavobacterium group to total eubacteria increased immediately after the wildfire, and the other eubacterial proportions decreased in the surface and middle layer soils. By way of contrast, the composition of the 5 groups of eubacteria in the subsurface soil exhibited no significant fluctuations during the entire period. The total bacterial population and bacterial community structure disturbed by wildfire soon began to recover, and original levels seemed to be restored 3 months after the wildfire.

• Keywords: wildfire; bacterial community structure; fluorescent in situ hybridization (FISH)