DEGRADATION OF POLYLACTIC ACID (PLA) MICROPLASTICS AT DIFFERENT TEMPERATURES IN A CAMBISOL UNDER DISTINCT AGRICULTURAL PRACTICES

The widespread contamination of ecosystems by plastic debris has emerged as a defining environmental challenge of the Anthropocene. When exposed to environmental conditions, plastics can fragment into microplastics (MPs; 0.001–5 mm), which may accumulate in the environment and persist for extended periods.  Among terrestrial systems, agricultural soils are increasingly recognized as major sinks for MPs, raising concerns about their potential impacts on soil health, microbial communities, and ecosystem functioning. In response to this growing challenge, biodegradable polymers such as polylactic acid (PLA) have been introduced as promising alternatives to conventional petroleum-based plastics. However, the breakdown of PLA in natural environments depends heavily on specific environmental conditions, such as the composition and activity of local microbial communities and temperature. Agricultural practices, such as the use of cover crops and organic mulching, can increase soil biological activity and alter microbial community dynamics, potentially influencing the fate of bioplastics in soil and organic matter stability. Evaluating the fate and behaviour of these materials in soil environments under distinct agricultural practices is essential to determine whether they represent a sustainable solution or a new source of pollution.

This study investigated the biodegradation of commercial PLA MPs under different agricultural practices in soils incubated for 112 days under three temperatures. Soil samples (Cambisol) were collected from a 12-year field experiment conducted by the Soil and Water Management and Crop Nutrition Laboratory of the FAO/IAEA Joint Centre of Nuclear Techniques in Food and Agriculture at the Austrian Agency for Health and Food Safety (AGES), in Grabenegg, Austria. The treatments represent three distinct agricultural practices: summer vetch cultivation with organic mulch, summer vetch without mulch, and bare soil. The soil incubation experiment was conducted with PLA added at 1% w/w, under 60% water holding capacity (WHC) and incubated at 5°C, 20°C, and 35°C. For each soil condition, three replicates were prepared with PLA, alongside a control treatment (soil without PLA) with four replicates. Additionally, three blank vessels (without soil or MPs) were included as controls for background CO₂ measurements, pH was measured before the beginning of the experiment. The concentration and carbon isotopic composition (δ¹³C) of the CO₂ emitted during the incubation were monitored using a cavity ring-down (CRDS) Laser Analyser. Soil samples were analyzed for their elemental and isotopic composition of carbon and nitrogen (%C, %N, δ¹³C, δ¹⁵N) using an Elemental Analyzer coupled to an Isotope Ratio Mass Spectrometer (EA-IRMS).

Temperature significantly affected soil CO₂ emissions, with cumulative emissions lowest at 5 °C and highest at 20 °C, particularly in the summer vetch with mulch treatment. While the addition of PLA increased overall CO₂‑C emissions during the incubation, with exception for the summer vetch treatment under 5°C and 20°C, δ¹³C analysis revealed that this increase did not originate from PLA mineralization. Instead, the isotopic signature of the CO₂ matched that of native soil organic matter, indicating a positive priming effect. PLA stimulated microbial decomposition of existing soil carbon without undergoing substantial biodegradation of the polymer itself under the tested conditions.