Degradation of conventional, biodegradable and oxo-degradable microplastics in a soil using a d13C technique

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 MPs contamination in soils. Degradation of various types of MPs in soil is uncertain because of the lack of a reliable and practical method for its measurement. In this study, we examined the degradation of a conventional (fruit and vegetable (F&V) bag), two biodegradable plastics (bin liner and mulch film), and an oxo-degradable (drinking straw) plastic. These plastics (5 mm) were mixed into a soil and incubated in the laboratory at 37±1 °C for 185 days. The CO2-C mineralization of the four plastics was determined using a δ13C technique, as the difference in the δ13C values of studied plastics and the experimental soil was ≥ 10‰. Bin liner showed greatest C mineralization (5.7%), followed by mulch film (4.1%), straw (0.4%) and F&V bag (0.3%) at the end of incubation period. All plastics, except the mulch film between 23 to 77 days of incubation, caused a positive priming effect on soil organic carbon (SOC). Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) analyses provided results consistent with the C mineralization data. This study demonstrates that δ13C is a reliable and practical method to determine the degradation of MPs in soil, which has been lacking in this field of study. The priming effect of various MPs on SOC is a significant finding. The lack of consideration of priming effect on SOC may overestimate the mineralization of plastics in soil.