Wood deterioration assessment and microbial community analysis of foundation piles experimentally installed for 8 and 10 years

Buried timber piles play a critical role in ground improvement, yet their long-term deterioration under waterlogged and anaerobic conditions remains insufficiently understood. This study investigated microbial communities and wood degradation in Japanese cedar ( Cryptomeria japonica ) and Japanese larch ( Larix kaempferi ) logs used for ground improvement, a geotechnical method that stabilizes soft or liquefiable soils. The logs were buried for 8–10 years at two sites with differing soil compositions and groundwater conditions. Bacterial communities were analyzed using next-generation sequencing (NGS). In cedar, bacterial infiltration extended from the outermost sapwood to the inner sapwood and into the heartwood, likely due to its lower density and higher water permeability. In contrast, bacterial colonization in larch was confined primarily to the outermost sapwood, likely reflecting its higher density and lower permeability. Microscopic analysis revealed erosion of parenchyma cell walls in cedar and slight deterioration of tracheid cell walls in larch, but no apparent reduction in wood density was observed. Dominant bacterial taxa included the anaerobic classes Clostridia and Limnochordia in cedar, and Clostridia and Negativicutes in larch. Facultative anaerobes dominated the surrounding soil, particularly members of the order Chitinophagales . These findings suggest that initial bacterial colonization of the logs was influenced by the soil microbiota and subsequently shaped by their internal environmental conditions. Although fungal communities were identified by ITS amplicon analysis and microscopic observation of cell walls, no apparent signs of fungal degradation were observed. Overall, the results underscore the importance of wood density and permeability in microbial resistance, with larch exhibiting greater durability under anaerobic soil conditions. This study contributes to a deeper understanding of the long-term stability of buried wood in geotechnical applications. • Subterranean decay of cedar and larch piles evaluated for ground improvement durability. • Bacterial colonization depth reflects wood density and water permeability. • Density and microscopy show minimal wood cell wall degradation. • NGS revealed cellulose-degrading bacteria dominated pile communities.