Microplastics, as emerging contaminants, have drawn widespread attention; however, the impact of different types of microplastics on wetland plants remains poorly understood. Radial oxygen loss (ROL) is a unique trait of wetland plants that reflects their growth status. This study investigated the effects of nondegradable (polyethylene terephthalate, PET) and biodegradable (polylactic acid, PLA) microplastics on the ROL of Canna indica, as well as on height, chlorophyll, and root traits, and additionally examined the correlations between soil microorganisms and ROL. After 60 days of incubation, microplastics migrated toward plant roots. The results exhibited that both low and medium concentrations of PET and PLA promoted ROL (103.02–170.44%, in comparison with the control group), reaching up to 74.28–96.36 mmol/(μm·h). However, as the microplastic concentration increased, the ROL decreased. The high concentration of 3 g/kg PLA appeared to weakly inhibit the secretion of ROL to 31.84 mmol/(μm·h). Additionally, microplastic addition can promote growth indicators, manifesting the improvement of physiological indicators such as plant height, chlorophyll, and porosity. Moreover, adding different concentrations of PET and PLA led to changes in the bacterial communities. Norank_f_Roseiflexaceae, the genus with the greatest variation, increased in abundance with increasing microplastic concentrations, whereas the opposite was true for Sphingomonas. The change in the PET colony was relatively small, whereas the PLA group exhibited a significant difference, suggesting that the bacterial community was more sensitive to PLA. Research indicates that microplastics can affect ROL and reshape soil microbial structure, especially in the PLA group.
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