Objective This study aimed to investigate the effects of polyethylene microplastics (PE-MPs) and iron addition—applied individually or in combination—on carbon and nitrogen fractions and cadmium speciation in cadmium-contaminated paddy soils. The objective was to elucidate the regulatory effects of these co-occurring pollutants on soil environmental behavior from the perspective of their combined interactions.

Methods A typical paddy soil from Guangdong Province, China, was selected as the study material. Four treatments were established: control (CK), polyethylene microplastics (PE), iron (Fe), and a combined treatment of iron and polyethylene microplastics (Fe-PE). Soil carbon and nitrogen fractions as well as cadmium speciation were measured. Principal component analysis and correlation analysis were employed to explore the effects of Fe addition under co-contamination conditions on changes in carbon and nitrogen fractions, and cadmium speciation.

Results (1) Compared with CK, all treatments significantly increased soil electrical conductivity (by 135.8% to 204.4%). However, only the Fe-PE treatment significantly decreased soil pH (P＜0.05). (2) PE treatment alone significantly increased the content of dissolved organic carbon (DOC) by 37.60%, while Fe treatment alone significantly increased the dissolved organic nitrogen (DON) content by 162.31% (P＜0.05). In the Fe-PE treatment, easily-oxidized organic carbon (EOC), microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and DON contents increased significantly by 18.10%, 17.29%, 60.97%, and 158.60% (P＜0.05). The carbon pool management index (CPMI) followed the order: Fe-PE＞PE＞CK＞Fe. (3) Compared with CK, Fe-containing treatments (Fe and Fe-PE) significantly reduced total cadmium content (by 40.98%–41.18%) and acid-extractable cadmium fractions (by 70.67%–77.18%). In contrast, the PE treatment alone significantly increased the reducible cadmium fraction by 16.88%. Principal component analysis revealed clear separations among CK, PE, Fe, and Fe-PE treatments. Correlation analysis indicated that soil pH was significantly positively correlated with both total cadmium and acid-extractable cadmium contents, while EOC was positively correlated with MBC and MBN.

Conclusion Both PE-MPs and iron exert notable effects on soil carbon and nitrogen fractions. The combined application of PE-MPs and iron effectively enhanced the labile carbon components (DOC and EOC) and improved the CPMI, thereby contributing to greater carbon pool stability. Moreover, this combination significantly reduced the bioavailability of cadmium by promoting its adsorption and co-precipitation with iron oxides. This study facilitates a precise assessment of the composite risk from microplastics and cadmium in iron-rich soils, providing a scientific basis for improved understanding of their co-pollution ecology.