Mineralization and microbial utilization of poly(lactic acid) microplastic in soil

Area of Science:

• Environmental Science
• Microbiology
• Soil Science

Background:

• Standard CO2 evolution methods for biodegradable microplastics (MPs) overestimate or underestimate degradation due to soil organic matter priming effects.
• Accurate assessment of biodegradable MP fate in diverse agricultural soils is crucial for environmental risk evaluation.

Purpose of the Study:

• To develop and apply a 13C natural abundance method for precise poly(lactic acid) (PLA) microplastic mineralization quantification.
• To investigate the microbial utilization pathways of PLA microplastics in various soil types.
• To compare the accuracy of the 13C method against the traditional CO2 evolution method.

Main Methods:

• Utilized the 13C natural abundance technique to trace PLA microplastic mineralization in different agricultural soils.
• Analyzed PLA-derived carbon dioxide (CO2) emissions and incorporation into microbial biomass.
• Compared results with the conventional CO2 evolution method across varying soil C/N ratios and pH.

Main Results:

• PLA microplastic mineralization ranged from 3-33% in alkaline soils and 1-5% in acidic soils.
• The 13C method provided more accurate mineralization estimates than the CO2 method, which overestimated PLA degradation by 1.3- to 3.3-fold in acidic soils.
• Gram-negative bacteria preferentially assimilated PLA hydrolysates, while Gram-positive bacteria contributed significantly to CO2 release at low PLA concentrations.

Conclusions:

• The 13C natural abundance method is a reliable tool for tracking biodegradable microplastic mineralization and microbial assimilation in soils.
• PLA microplastic degradation is influenced by soil properties like C/N ratio and pH.
• Bacterial groups play a key role in the assimilation and decomposition of microplastic-derived carbon.