Influence of thermo-oxidatively aged mulch debris on the seedling emergence of field pea (Pisum sativum var. arvense L.) seeds

The implementation of appropriate flame-spraying treatment on used polyethylene (PE) mulch film is a potential method for the rapid abatement of PE contamination in dryland fields. This study utilized self-made flame-spraying equipment to prepare thermo-oxidatively aged PE microplastics (aPE-MPs), with their structural properties characterized through analytical techniques. The effects of varying concentrations of aPE-MP suspensions (corresponding to residual film levels equivalent to 5-300 years of continuous mulching) on seed germination and seedling growth of field pea (Pisum sativum var. arvense L.) were also investigated. The surface of aPE-MPs exhibited wrinkles and cracks, forming abundant oxygen-containing functional groups and double bonds, while showing a decrease in pH, but with no significant alterations in crystalline morphology. Exposure to aPE-MP suspensions for 7 days did not inhibit seed germination rate or vigor index but evidently enhanced root elongation. Following 24 days of prolonged aPE-MP exposure, the root and shoot biomass, chlorophyll content, osmoregulatory substance, and antioxidant enzyme activities in seedlings were significantly higher than those of the control group within the medium-high suspension concentration range, with no apparent malondialdehyde accumulation except at the highest concentration. Elemental profiling revealed enhanced root acquisition of macro- and micro-nutrients (Ca, Mg, Fe, Mn, Cu, and Zn). Conclusively, the thermo-oxidative treatment markedly increased hydrophilicity and decreased pH of the used PE residues, and aPE-MP suspensions even promoted the seedling development of field pea. This method shows potential for application in the disposal of used mulch film in dryland farming, though its long-term ecological effects require further assessment.