Evaluation of the carbon sink potential of wooden foundation piles embedded in oxygen-depleted soils using density profile analysis

Abstract Wooden foundation piles are a renewable resource that contributes to mitigating climate change because of the minimal processing requirements and remarkably long service life. Previous studies have shown that while these piles can have a long service life, the buried wood is also susceptible to bacterial activity; however, the extent to which this affects the carbon stock in wooden piles remains uncertain. This study addressed this by estimating the decrease in the mass of a pile buried in soil using the density profile, because bacterially induced deterioration was observed to initiate on the pile surface. Piles extracted from various locations in Japan were prepared as thin specimen strips. Then, these specimens were exposed to soft X-rays, and the resulting images were acquired on film. The images were analyzed to calculate the mean wood density at 0.5 cm intervals from the pile surface. The results revealed that no significant decrease in density was observed near the surface of a pile, indicating that the bacterial effect on the carbon stock was negligible when a pile was installed in waterlogged conditions. By contrast, the possibility remains that piles exposed to oxygen-depleted conditions, such as fluctuating groundwater levels, might have experienced a reduction of approximately 10% of their initial mass after 60–84 years of exposure to soil conditions. This reduction in mass is significantly lower than the 50% reduction observed in lumber after 35 years and wooden panels after 25 years; therefore, wooden foundation piles have promising potential as carbon sinks.