Effects of Temperature and Enzyme Addition on the Biodegradation of Polylactic Acid and Soil Quality

Abstract

Biodegradable plastics have attracted attention as eco-friendly alternatives to conventional non-degradable plastics; however, their degradation behavior in natural soil environments remains insufficiently understood. In this study, the biodegradation of polylactic acid (PLA) and enzyme-incorporated PLA (PLA+C and PLA+B) was compared under ambient (25℃) and composting (58℃) conditions using field-collected agricultural soils, and the accompanying changes in soil quality were analyzed. Cryogenically pulverized polyethylene terephthalate (PET), PLA, PLA+C, and PLA+B were each mixed with soil at 1% (w/w) and incubated for 224 days, during which CO2 evolution was measured periodically to determine biodegradation. After incubation, soil quality was assessed by analyzing pH, electrical conductivity (EC), exchangeable cations, total Kjeldahl nitrogen (TKN), available phosphorus, and soil enzyme activities such as fluorescein diacetate hydrolase (FDAse) and urease. Under the ambient condition, CO2 evolution from PLA and enzyme-incorporated PLA treatments was comparable to that of the control, corresponding to a biodegradation within 2%. In contrast, under the composting condition, the PLA, PLA+C, and PLA+B treatments reached 31.30%, 30.81%, and 29.85% biodegradation, respectively, confirming that elevated temperature markedly promoted PLA degradation. However, under both temperature conditions, enzyme incorporation did not significantly enhance degradation relative to PLA alone, and none of the treatments met the EN 13432 industrial composting criterion of ≥ 90% biodegradation within 180 days in natural soil. Soil quality analysis revealed changes in EC and exchangeable cations in some treatments under the composting condition, but the overall changes in soil quality were limited. These results indicate that temperature, rather than enzyme incorporation, is the dominant factor governing PLA biodegradation, and that the degradation promoting effect of the enzyme-incorporated PLA used in this study is limited. The findings further suggest that practical environmental application of biodegradable plastics will require optimization of enzyme content and manufacturing processes, together with further research on long term degradation behavior under diverse soil conditions.