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Background:Karst ecosystems are fragile and characterized by poor soil fertility. As emerging contaminants, the ecological risks of microplastics in such systems remain poorly understood.Objective: In this study, we conducted a pot experiment to systematically investigate the cascading effects of two types of microplastics-non-degradable polyethylene (PE) and degradable polylactic acid (PLA)-at different doses on a karst soil-plant system.Methods: A pot experiment was conducted to evaluate plant growth responses, soil physicochemical properties, enzyme activities, microbial community structure, and metabolic fluxes. A comprehensive toxicity index (CTI) based on the entropy weight method was used to compare ecological risks.Results: Microplastics generally inhibited plant growth, with PLA exerting a stronger inhibitory effect than PE. High-dose PLA significantly reduced fresh plant weight and root development, and induced a decoupling between soil available nutrients and plant growth. Microplastics significantly altered key soil enzyme activities and microbial community structure in a type- and dose-dependent manner. Specifically, PE primarily regulated the microenvironment through physical effects, with low doses positively stimulating acid phosphatase (ACP) activity, whereas PLA exerted chemical interference through its degradation products, affecting lignin peroxidase (LiP) and laccase activities. Fungi were more sensitive to microplastic stress than bacteria, and microplastics selected for specific functional bacterial taxa and tolerant fungi, directly driving enzyme activities and elemental cycling processes. Metabolic flux analysis revealed that microplastics disrupted the coupling among carbon, nitrogen, and sulfur cycles: low doses enhanced synergies, whereas high doses suppressed functional interactions. A CTI based on the entropy weight method indicated that PLA exhibited higher ecological toxicity than PE.Conclusion: This study reveals that microplastics in karst soils induce a cascade of effects, characterized by the decoupling of nutrients from plant growth and the inhibition of plant development, mediated by shifts in microbial communities and enzyme activities. These effects are type- and dose-dependent, with PLA posing a higher risk than PE. Our findings provide a theoretical basis for the early warning and management of microplastic pollution in ecologically vulnerable regions.