Aging reduces the negative effect of low-density polyethylene microplastics on soil-plant-microbial systems

Microplastics (MPs) are pervasive pollutants in terrestrial ecosystems, where they inevitably undergo aging driven by light, heat, and biological activity. Aging alters MP properties and may reshapes their interactions with plants, soil, and microbial communities. This study evaluated the effects of original and UV-aged low-density polyethylene MPs (LDPE-MPs) at different particle sizes (5 and 500 μm) and concentrations (0.1 % and 1 %, w/w) on pak choi (Brassica rapa subsp. chinensis) growth, soil properties, and microbial communities via a controlled pot experiment. The addition of 1 % (w/w) 500 μm original LDPE-MPs significantly inhibited plant height, chlorophyll content (SPAD), and biomass by up to 75.9 %, whereas aged LDPE-MPs caused minimal growth inhibition. Notably, aged LDPE-MPs enhanced nitrogen and phosphorus uptake by 82.3 % and 14.2 %, respectively. Both MP types elevated oxidative stress indicators, including malondialdehyde (MDA), peroxidase (POD) and catalase (CAT) activities, but reduced superoxide dismutase (SOD) activity. MP treatments also improved soil organic matter (SOM) and cation exchange capacity (CEC), with the 0.1 % (w/w) 5 μm LDPE-MPs treatment showing the strongest effects. Larger MPs raised soil nitrate-nitrogen (NO₃^--N) content, while smaller MPs enhanced available phosphorus (AP). Microbial analysis revealed that aged LDPE-MPs inhibited bacterial groups involved in carbon mineralization and denitrification but promoted saprophytic fungi. Structural equation modeling indicated that UV-aging reduced the toxicity of LDPE-MPs by mitigating negative impacts on plant growth and bacterial communities, enhancing beneficial fungal populations, and stabilizing soil carbon and nitrogen cycling. These findings provide mechanistic insights into the long-term ecological risks of aged MPs in agroecosystems.