Effects of plastic film residues on the photosynthetic characteristics and biomass accumulation of soybean (Glycine max)

Agricultural plastic film mulching technology has greatly promoted the development of agricultural production and social economics, especially in the arid and semi-arid areas of China. However, due to high fragmentation, low recovery, and long-term degradation, the accumulation of plastic residues in the soil has increased annually, which threatens crop growth, soil health, and the sustainable development of agriculture. Although many studies have focused on the effects of agricultural film residues on soil quality, the effects of plastic type (degradable or non-degradable) and the cumulative abundance of plastic on crop photosynthetic characteristics have rarely been reported. In this study, soybean (Glycine max) was investigated for its light and carbon dioxide (CO₂) response characteristics under different plastic residue addition (polyethylenePE and biodegradable plasticBP mulch film; plastic size:0.5-2 cm; addition levels:0, 0.1%, 0.5%, and 1.0%) at the flowering and early pod stages. Plant biomass and soil samples were collected at the flowering and harvesting stages to examine the effects of the different plastic residues on plant growth and soil quality. The results showed that the light compensation point (LCP) of soybean leaves decreased by 23.96% at the flowering stage and increased by 51.38% at the beginning of the pod stage in the PE treatment groups, suggesting that the weak light utilization ability of soybean leaves increased at the flowering stage and decreased at the beginning pod stage. LCP decreased by 54.82%, and the light saturation point increased by 58.12% in the BP treatment groups at the beginning of the pod stage, which improved the ability of strong light adaptation and increased the range of light energy utilization. The PE and BP residues increased the dark respiration rate (R_d) by 30.56% and 22.28%, respectively, increasing dry substance consumption. With increasing amounts of plastics, the maximum photosynthetic capacity decreased by 36.49% and 23.56% in the PE and BP treatments, respectively, indicating that the CO₂ utilization capacity of soybean was inhibited. Furthermore, the CO₂ compensation point (CCP) decreased by 67.96% and 38.91% in the PE and BP treatments, respectively, which indicated the improved CO₂ utilization capacity of the leaves at low CO₂ levels. The photorespiration rate (R_p) also decreased, reducing dry substance consumption. At the flowering stage in the BP treatment with 0.1% and 0.5% plastic addition, the underground biomass decreased significantly with increased plastic residue (P < 0.05), but there were no significant differences in the aboveground and underground biomass among the other treatments. Pearson correlation analysis was used to analyze the fitting parameters of the light response and CO₂ response curves with biomass. At the harvesting stage in the PE treatments, the aboveground biomass was negatively correlated with LCP, whereas R_p, CCP, and the initial carboxylation efficiency were strongly correlated with biomass accumulation (aboveground + underground). Further research is required to identify the mechanisms by which plastic residues affect crop growth, especially for the photosynthetic properties. Such work will enable a better understanding of the ecological risk of microplastics.