Polyhydroxybutyrate (PHB) is a biodegradable polymer synthesized and degraded by
microorganisms and is widely regarded as an environmentally friendly alternative to conventional
plastics. However, its fate under landfill conditions, especially in alkaline soils, remains poorly
understood. In Japan, incineration has been the predominant municipal waste treatment method since
the 1970s, and the resulting incineration ash accounts for approximately 70% of landfill waste.
Therefore, landfill soils are often highly alkaline, yet studies on PHB degradation under such
conditions are limited. This study aimed to evaluate the biodegradability of PHB in operational
landfill soil and to elucidate the effects of elevated pH and associated microbial communities with its
degradation. Soil samples were collected from the Nishi-Iburi Regional Union Final Disposal Site in
Muroran, Hokkaido, and uncontaminated campus soil (pH 7.0) from the Muroran Institute of
Technology was used as a control. PHB films were buried in both soils, and their weight loss was
measured periodically. As a result, delayed PHB biodegradation was observed in the landfill soil. In
the university soil, fragmentation of the films progressed by day 28, and substantial degradation was
evident by day 49. In contrast, in the landfill soil, although fragmentation was observed in some
films, little degradation occurred overall. Specifically, on day 49, the residual PHB content had
decreased to 32.5% in the control soil, whereas it remained at 63.8% in the landfill soil.
Microbial counts also differed substantially: bacterial colonies were 4.0×10⁸ CFU/g in the control
versus 1.0×10⁷ CFU/g in landfill soil, while fungal counts decreased from 2.0×10⁶ CFU/g to 7.0×10³
CFU/g, corresponding to 40-fold and 300-fold reductions, respectively. Microbial community
analysis and PHB-specific isolation revealed actinomycetes (Actinomadura, Kitasatospora,
Streptomyces), and spore-forming bacteria (Priestia). These taxa are known for their resilience in
harsh environments, particularly through spore formation, suggesting adaptation to alkaline stress.
These results suggest that although landfill soils harbor microorganisms adapted to the highly
alkaline conditions caused by incineration ash, such alkalinity may suppress both the viable cell
numbers and the degradation activity of PHB-degrading bacteria. Furthermore, according to the
literature, incineration ash may contain heavy metals, and their effects, such as inducing oxidative
stress, should also be considered.
Our findings provide new insights into the post-disposal fate of biodegradable plastics and contribute
to improving evaluation and management strategies for landfill environments. Understanding the
interactions between environmental conditions, microbial communities, and PHB degradation is
essential for promoting the effective use of biodegradable plastics in sustainable waste management.