Polyhydroxybutyrate (PHB), one of the most widely used biodegradable plastics, has gained attention as an environmentally friendly material synthesized and decomposed by microorganisms. However, its biodegradation behavior under landfill conditions remains insufficiently understood. In Japan, approximately 70% of incineration ash is disposed of in landfills, where soils are often exposed to high pH environments. Despite this, limited research has been conducted on PHB degradation under such alkaline landfill conditions. This study aims to evaluate the biodegradability of PHB in operational landfill soil and to elucidate the influence of high pH and the associated microbial community on its decomposition.

Soil samples were collected from the Nishi-Iburi Regional Union Final Disposal Site in Muroran, Hokkaido. As a control, uncontaminated campus soil (pH 7.0) from the Muroran Institute of Technology was used. PHB films were buried in each soil, and weight loss was measured over 28 days. On day 28, PHB in the control soil showed a 26.8% weight loss, compared to only 13.2% in the landfill soil. Microbial community structures were analyzed, and PHB-degrading bacteria were isolated using a mineral salt agar medium with PHB as the sole carbon source. A total of 16 strains were isolated from the campus soil, while only 12 were obtained from the landfill soil. These findings indicate that the strongly alkaline conditions in landfill soil suppress both the abundance and activity of PHB-degrading bacteria. Additionally, although incineration ash is stabilized before disposal, the potential presence of residual heavy metals may further affect biodegradation processes.

This study offers new insights into the fate of biodegradable plastics after disposal and contributes to the development of improved evaluation and management strategies for landfill environments.