| Abstract | Microplastic contamination of agricultural soils is a recognized but poorly resolved threat to flooded paddy systems, where waterlogged, anaerobic conditions are expected to amplify the biogeochemical consequences of polymer accumulation. Most existing evidence, however, derives from short incubations at exposure levels (0.5–8 % w/w) that are one to two orders of magnitude above field-realistic loads, and tropical paddy systems remain markedly underrepresented in this literature. This study evaluated how graded concentrations of polyethylene (PE) microplastics (0, 0.05, 0.15, and 0.50 g kg⁻¹) affect rice productivity, soil and floodwater nitrogen pools, total organic carbon (TOC), and methane (CH₄) emissions under continuously flooded conditions representative of tropical paddy agriculture. At field-realistic PE concentrations (0.05–0.50 g kg⁻¹), this study identified three measurable responses: a +127 % rise in soil TOC at 0.50 g kg⁻¹ (R² = 0.62, p = 0.002), a +133 % rise in floodwater NH₃-N (R² = 0.65, p = 0.002), and a 28 % reduction in fresh grain yield (p = 0.041). Methane emissions were not significantly elevated in cumulative terms but showed a clear seasonal restructuring, with PE treatments sustaining a late-season flux roughly twice the control. These results indicate that the carbon- and nitrogen-related signals previously reported at PE doses of 0.5–8 % w/w remain detectable at concentrations an order of magnitude lower, with a parallel reduction in grain yield. The simultaneous rise in floodwater ammonium and decline in soil ammonium suggests that nitrogen redistribution toward the floodwater compartment may contribute to the yield response, although the present design does not partition this effect from possible direct phytotoxicity. Given this redistribution pattern, nitrogen-management practices that limit floodwater ammonium accumulation, such as deep urea placement, urease inhibitors, or controlled-release formulations which may warrant evaluation in PE-contaminated paddies. Confirming this would require direct measurement of NH₃ and N₂O fluxes, bioavailable carbon, and replication across additional polymers and rice cultivars. |
