Degradation of conventional, biodegradable and oxo-degradable microplastics in a soil using a δ13C technique

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 degrees C for 185 days. The CO2-carbon (C) mineralisation of the four plastics was determined using a delta C-13 technique, because the difference in the delta C-13 values of studied plastics and the experimental soil was >= 10 parts per thousand.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.ConclusionsThis study determines the degradation of various MPs in soil using a reliable and practical delta C-13 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.