Elevated CO2 synergizes with nanoplastics to intensify arsenic mobilization, biotransformation, and plant uptake in soil-spinach systems.

The co-occurrence of elevated atmospheric CO2 and micro-nano plastics (MNPs) in agroecosystems presents a compounding threat to food safety, particularly regarding the fate of toxic metalloids like arsenic (As). This study provides the first integrated assessment of how elevated CO2 (e[CO2]) interacts with polyethylene microplastics (mPE) or nanoplastics (nPE) to influence As transformation and bioavailability in a soil-spinach (Spinacia oleracea L.) system. Our results demonstrated that e[CO2] not only promoted spinach biomass but also significantly increased total As accumulation in plant tissues. This was driven by a shift in soil arsenic speciation towards the more mobile and toxic arsenite (As(III)), resulting from the enhanced microbial reduction of arsenate (As(V)). The presence of MNPs, especially nPE, dramatically amplified this effect. Soil physicochemical analyses revealed that the combined treatment of e[CO2] and nPE lowered soil pH and redox potential (Eh), increased dissolved organic carbon (DOC) content, and consequently expanded the pool of bioavailable As. High-throughput qPCR and 16S rRNA gene sequencing indicated that e[CO2] coupled with nPE induced a substantial shift in the soil bacterial community structure, enriching As-transforming taxa (e.g., Acidobacteria and Firmicutes) and upregulating the expression of key functional genes involved in As redox cycling (arsC, arrA, aioA) and methylation (arsM). Structural equation modeling suggested that labile DOC and these specific microbial functional groups were closely correlated with As mobilization and plant uptake, representing hypothetical associations rather than definitive causal drivers. Our findings reveal a significant statistical synergy between e[CO2] and nanoplastics in altering arsenic dynamics, suggesting an underestimated risk pathway that warrants further mechanistic validation. This work emphasizes the necessity of integrating plastic particle size and climate change factors into environmental risk assessment frameworks, and provides critical insights for developing targeted mitigation strategies to safeguard food safety and agroecosystem sustainability under global change.