Microplastics Modulate Carbon Sequestration in Paddy Fields by Regulating Rhizosphere Silicon Mobility

Although microplastics (MPs)-induced alterations in microbial carbon (C) and nitrogen (N) cycling have been increasingly documented, their integrated effects on silicon (Si)-mediated C sequestration in paddy ecosystems remain poorly understood. Using a rice (Oryza sativa L.) growth-cycle microcosm experiment, this study investigated how biodegradable (poly(lactic acid), PLA) and nondegradable (polyethylene, PE) MPs alter rhizosphere Si dynamics, microbial C/N metabolism, and soil C storage. PLA treatment increased C accumulation in grains (+33%) and shoots (+10%) relative to the control, whereas PE reduced both by 26–40%, coinciding with divergent Si uptake patterns. Transient stimulation of Si translocation (up to 2-fold under PLA) was associated with short-term mitigation of microbial-driven C losses. However, both MP types progressively reduced rhizosphere Si bioavailability and disrupted aggregate stability, indicating long-term depletion of labile Si pools. Moreover, PLA enhanced N mineralization via enriching Chloroflexi and Actinobacteriota, elevated labile organic C, and downregulated key genes involved in C fixation (e.g., korA/B), thereby undermining persistent C storage. These findings reveal a MPs-induced dual role of short-term elevated C accumulation via rhizosphere Si uptake by plants versus long-term disruption of C–Si coupled biogeochemical cycle in paddy fields.