Earthworms Edge Out Composting in Breaking Down PLA/PBAT Bioplastic Bags, Study Finds

Compostable plastic bags have long carried a quiet promise: toss them into the organic waste stream and they will simply vanish, transformed into harmless soil. A new laboratory study from Silpakorn University in Thailand puts that promise under the microscope, tracking what actually happens to PLA/PBAT bags over 120 days in two very different biological treatment systems. The results, published in Environmental Challenges, are both encouraging and sobering: the bioplastic lost more than half of its mass in both composting and vermicomposting, but it never fully disappeared, and the reason lies in a single number on the thermometer.

The research team, led by Phishet Sungtong and Daoroong Sungthong, compared conventional composting with vermicomposting, a mesophilic process in which the African nightcrawler earthworm Eudrilus eugeniae works alongside microbes to decompose organic matter. Each reactor contained a carefully balanced feedstock of cow manure and discarded cabbage at a ratio of 89:11 on a dry-weight basis, with some reactors receiving 7 grams of cut PLA/PBAT bag pieces, equivalent to 1 percent of the initial dry mass. The bags, collected from a campus café, were cut into 5 by 5 centimeter squares following the specimen preparation procedure of ISO 20200, the international standard for laboratory-scale disintegration testing. Twenty-eight independent reactors were prepared, with destructive sampling at days 15, 30, 60, 90, and 120.

The temperature story turned out to be central. Conventional composting initially peaked at 27 to 28 degrees Celsius before stabilizing near ambient levels, while vermicomposting held a strictly mesophilic profile of 24 to 26 degrees throughout, a range chosen to keep the earthworms alive and reproducing. Neither system reached the thermophilic phase of 58 degrees Celsius that defines industrial composting, and that matters enormously for PLA. The glass transition temperature of PLA sits around 56 to 63 degrees Celsius; below it, the polymer remains glassy, with stiff chains that resist water penetration and hydrolytic chain scission. PBAT, by contrast, with a glass transition near minus 35 degrees, stays rubbery at any composting temperature, yet its aromatic terephthalate segments remain stubbornly resistant to microbial enzymes regardless.

By day 120, the recovered plastic fragments told a nuanced tale. Vermicomposting achieved a mean disintegration of 59.91 percent, compared with 55.96 percent for conventional composting, a numerically higher result of nearly four percentage points, though the ranges overlapped and the difference was not statistically distinguishable. Notably, the vermicomposting range extended to 72.97 percent, higher than any single composting reactor achieved. The researchers attribute this edge to the combined action of earthworms and microbes: worms ingest and transport plastic fragments into the drilosphere, the biologically rich zone around their burrows, and their gut passage and castings expose the polymer to dense microbial communities in oxygen-limited microsites.

Spectroscopic evidence confirmed that the mass loss was not merely physical fragmentation. Attenuated total reflectance Fourier transform infrared spectroscopy revealed a pronounced decline in the absorbance of the ester carbonyl band near 1712 wavenumbers, dropping from 0.3304 at day zero to 0.1892 in composted and 0.3083 in vermicomposted samples, alongside a sharp decrease in the carbon-oxygen-carbon stretching band. These changes signal cleavage of the ester bonds that link the polymer chains, generating terminal hydroxyl and carboxylic acid groups that microbes can then assimilate through the tricarboxylic acid cycle into carbon dioxide, water, and biomass. Scanning electron microscopy showed the once bumpy, sponge-like surface progressively cracking, pitting, and peeling into layers, with rod-shaped cell-like structures colonizing the fracture grooves.

An intriguing metabolic twist emerged when the researchers compared the two systems chemically. The vermicomposting reactors received fresh frozen cabbage every 30 days to feed the worms, providing microbes with an abundant, easily digestible carbon source. Under carbon catabolite repression, microorganisms preferentially consume such favored substrates and suppress the synthesis of the extracellular depolymerases needed to attack recalcitrant plastic. The composting reactors, starved of new food after day 30, may have been driven by metabolic stress to upregulate polymer-degrading enzymes, using PLA/PBAT as an alternative carbon source. This may explain why the vermicomposted plastic showed a less pronounced reduction in ester absorbance despite its slightly greater mass loss.

Beyond the plastic itself, the study asked whether the resulting compost and vermicompost were safe and effective fertilizers. Both systems matured convincingly: the carbon-to-nitrogen ratio fell from 27.1 to between 16 and 20, meeting Thai agricultural standards, while cation exchange capacity nearly doubled, reflecting the formation of humic substances. Vermicompost proved the richer fertilizer, accumulating roughly twice the calcium of conventional compost, reaching 12.7 percent, thanks to the calciferous glands of earthworms excreting calcium carbonate in their castings, along with elevated potassium and sodium. Germination tests with mung bean seeds showed that phytotoxicity dropped rapidly, with the germination index exceeding 100 percent by day 15 and reaching 136 percent in vermicompost at day 120, compared with 101 percent for compost.

Heavy metal analysis added a cautionary note. Cadmium and lead remained below detection limits in all systems, and chromium traced back to the cabbage feedstock rather than the plastic. However, nickel appeared exclusively in the bioplastic-amended reactors, peaking at 37.78 milligrams per kilogram in vermicompost, likely derived from nickel oxide additives used in PLA manufacturing to improve tensile strength and confer antimicrobial properties. The researchers warn that lactic acid released during PLA degradation could locally acidify the matrix and mobilize nickel into plant-available forms, a dynamic that warrants environmental monitoring even though all measured values fell below Thai regulatory limits.

Bacterial community sequencing of the day-120 samples revealed sharply different microbial worlds. Compost samples hosted greater diversity, with Shannon indices above 9 and over a thousand observed amplicon sequence variants, dominated by Pseudomonadota, Bacillota, and Chloroflexota, with genera such as Lysinibacillus and Litorilinea prominent. Vermicompost samples were less diverse but enriched in Bacillota and Bacteroidota, including anaerobe-associated taxa such as Clostridium and Romboutsia, plausibly reflecting oxygen-limited microsites in earthworm guts and moist organic particles. Both Lysinibacillus and Clostridium are recognized plastisphere colonizers capable of secreting esterases and lipases that cleave polyester bonds, making them candidates for further investigation, though the authors are careful to note that their presence at day 120 does not prove direct involvement in plastic degradation.

The study’s practical message is clear: compostable does not mean gone. Under mesophilic conditions, PLA/PBAT bags lost more than half their mass in 120 days but remained incompletely disintegrated, and the bioplastic-amended compost actually failed the Thai agricultural standard precisely because macroscopic plastic fragments persisted. The authors suggest extending treatment duration, maintaining continuous thermophilic conditions, introducing specific polymer-degrading microorganisms, or employing advanced in-vessel reactors with real-time temperature and moisture monitoring to push degradation to completion. They also acknowledge the limits of their design, including single reactors for most sampling times and endpoint-only microbial analysis, which constrain statistical inference. Until such improvements arrive, the study offers consumers and waste managers a grounded benchmark: earthworms give compostable plastics a modest but measurable boost, yet the thermophilic heat of industrial composting remains the gold standard for making these materials truly vanish.

Subject of Research: Disintegration of PLA/PBAT bioplastic blends during composting and vermicomposting and the resulting fertilizer quality and bacterial communities

Article Title: Lab-scale comparison of PLA/PBAT disintegration in composting and vermicomposting: fertilizer quality, phytotoxicity, and day-120 bacterial community profiles

Article References: Sungtong, P., & Sungthong, D. (2026). Lab-scale comparison of PLA/PBAT disintegration in composting and vermicomposting: fertilizer quality, phytotoxicity, and day-120 bacterial community profiles. Environmental Challenges, 25, Article 101664. https://doi.org/10.1016/j.envc.2026.101664

Image Credits: AI Generated

DOI: 10.1016/j.envc.2026.101664

Keywords: PLA/PBAT, biodegradable plastics, composting, vermicomposting, earthworms, Eudrilus eugeniae, disintegration, FTIR, scanning electron microscopy, bacterial community, compost quality, heavy metals