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Degradation of conventional, biodegradable and oxo-degradable microplastics in a soil using a d13C technique
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Scientists tracked how four types of plastic — a conventional bag, two biodegradable plastics, and an oxo-degradable straw — break down in soil over 185 days using a carbon isotope technique. Even the most degradable plastic (a biodegradable bin liner) only mineralised 5.7% of its carbon, while conventional plastic bags barely degraded at 0.3%. Crucially, all plastics also triggered a 'priming effect' that accelerated the breakdown of existing soil organic carbon, meaning microplastics don't just persist — they alter the soil chemistry around them.
Context A significant amount of conventional plastics waste, especially in the form of microplastics (MPs), has accumulated in soils due to its limited degradation. Oxo-degradable and biodegradable plastics have also contributed to MP contamination in soils. Aims In this study, we examined the degradation of a conventional plastic [fruit and vegetable (F&V) bag], two biodegradable plastics (bin liner and mulch film) and an oxo-degradable plastic (drinking straw). Methods These plastics (5 mm) were mixed into a soil and incubated in the laboratory at 37 ± 1°C for 185 days. The CO2-carbon (C) mineralisation of the four plastics was determined using a d13C technique, because the difference in the d13C values of studied plastics and the experimental soil was =10‰. Key results Bin liner showed the greatest C mineralisation (5.7%), followed by mulch film (4.1%), straw (0.4%) and F&V bag (0.3%) at the end of the incubation period. All plastics, except the mulch film for 23–77 days of incubation, caused a positive priming effect on soil organic carbon (SOC). Fourier transform infra-red spectroscopy and scanning electron microscopy analyses were consistent with the C mineralisation data. Conclusions This study determines the degradation of various MPs in soil using a reliable and practical d13C method, which has been lacking in this field of study. The priming effect of various MPs on SOC is a significant finding. Implications The lack of consideration of priming effect on SOC may overestimate the mineralisation of plastics in soil.
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