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Degradation of commercial biodegradable plastics and temporal dynamics of associated bacterial communities in soils: A microcosm study
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"Biodegradable" plastic bags and mulch films didn't fully break down in soil even after a full year, they lost less than half their weight, with the easy-to-digest starch component vanishing quickly while the plastic-based ingredients lingered much longer. This matters because it shows these products may not disappear from farmland as fast as marketing suggests, meaning plastic residues (and the soil bacteria disrupted by them) could stick around and potentially affect soil health and the food grown in it longer than expected.
Biodegradable plastics (BDPs) have been introduced to replace conventional fossil-based non-biodegradable plastics in agricultural production to reduce the accumulation of plastic debris in soils. However, the degradation performance of commercially available BDP products in real soils and the response of soil microbial communities to biodegradation remain unclear. Here, we explored the degradation characteristics of a commercial BDP product (made from starch, polylactic acid (PLA) and polybutylene adipate terephthalate (PBAT)) in different soils in a microcosm system over a period of 360 days. Temporal dynamics of associated bacterial communities in different soil niches (control soil, plastic surface soil and bulk soil (soil without close contact with plastics)) were profiled. Weight loss reached 42.0±1.2% to 48.0±2.2% in different soils after 360 days. The degradation of BDP followed the same pattern in different soils characterized by two distinct stages. In the first stage (day 0-30), BDPs experienced major weight loss (35.8-41.9%) which coincided with a drastic increase in the soil dissolve organic carbon (1.53-2.25 times the control soil) and the forming of distinct bacterial communities in the plastic surface soil. Thermalgravimetric analysis (TGA) and fourier transform infrared (FTIR) analysis confirmed the fast depletion of starch in this stage. In addition, observations with naked eyes and scanning electron microscope confirmed intensive microbial colonization on BDP surfaces. In the second stage (day 30-360), the degradation of remaining PLA and PBAT continued at a relatively slow rate. Meanwhile bacterial communities in the plastic surface soil started to gradually recover from the disturbance caused by fast biodegradation in the first stage in a soil-dependent manner. Our findings indicate that the degradation performance of BDPs was limited by the degradation rate of relatively recalcitrant components and the temporal dynamics of associated soil bacterial communities synchronized with the degradation of BDPs.
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