Terrestrial microplastic (MP) pollution is an escalating threat to soil productivity. This study investigates the impact of polyethylene (PE) microplastics (50–150 μm) on the model earthworm species Eisenia fetida and the subsequent effects on soil nutrient cycling. Over a 28-day experimental period, we observed dose-dependent reductions in earthworm biomass (15.2%) and cocoon production (64.8%) at high concentrations (1000 mg/kg). Concurrently, the study quantifies a 25% decline in available Nitrogen and a 17.6% decrease in available Phosphorus. These findings demonstrate that MPs disrupt the biological and chemical synergy of soil, leading to a measurable decline in soil richness.