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PLA vs PE microplastics with cadmium: Time-dependent divergent and microbial disruption of soil carbon and nitrogen cycling in medicinal plant soils
Summary
Scientists studying soil contaminated with both microplastics and cadmium (a toxic heavy metal) found that "biodegradable" plastic, when combined with cadmium, actually disrupted soil health more than regular plastic did—damaging the bacteria that plants like medicinal herbs rely on to get nutrients from soil. This matters because it challenges the common belief that biodegradable plastics are automatically better for the environment, suggesting that in polluted farmland, they could harm soil quality and potentially the safety or nutrient content of crops grown there.
Microplastics (MPs) and cadmium (Cd) co-contamination is an emerging concern in agricultural soils, but its dynamic effects on carbon (C) and nitrogen (N) cycling in medicinal plant systems remain unclear. Here, we conducted a full-growth-cycle pot experiment using Epimedium as a model plant, covering seedling (S1), vegetative (S2), and maturity (S3) stages. Polyethylene (PE) and polylactic acid (PLA) were applied at 0.01-0.15% (w/w) combined with Cd at 2 mg/kg. Using 16S rRNA sequencing, PICRUSt2, and structural equation modeling, we assessed soil C/N pools, enzyme activities, bacterial communities, and functional genes. Pollution effects exhibited clear growth-stage-dependent thresholds. The strongest disturbance to C/N pools occurred at S2, with partial recovery at S3. PLA-Cd induced significantly stronger disturbances than PE-Cd, driven by fundamentally different pathways: PE-Cd effects are primarily associated with physicochemical pathways (direct enzyme inhibition), whereas PLA-Cd effects are strongly correlated with microbial community restructuring. Under PLA-Cd, keystone taxa shifted from functional genera (Sphingomonas, Flavisolibacter) to stress-tolerant Acidobacterium, and bacterial co-occurrence network modularity collapsed from 0.362 to 0.227. Predicted abundances of C-fixation, N-fixation, and nitrification genes decreased by 44.9-64.0%, forming a metabolic pattern of suppressed N input and weakened C retention. These findings propose the "growth stage dependent response pattern" and a "differentiated mechanism of synergistic toxicity", elucidating how degradable vs. non-degradable MPs exert divergent toxic effects. This challenges the common assumption that biodegradable plastics are environmentally friendly under heavy metal co-contamination.