<p>To reveal the effects of different types of microplastics (particle size &lt; 5&#xa0;mm) on crop nitrogen absorption and metabolism, biodegradable microplastics made of polylactic acid (PLA) and polybutylene terephthalate (PBAT) andnonbiodegradable microplastics made ofpolyethylene (PE) were selected for plant pot experiments atthree mass fractions(0.5%, 1% and 3%, respectively). The responses of inorganic nitrogen content, crop growth, nitrogen absorption and nitrogen-metabolizing enzyme activity to soil microplastic pollution in the soil–Chenopodium quinoa system were studied. The results revealed that microplastics significantly limited the decomposition of soil organic carbon (SOC), reduced the accumulation of quinoa nitrate nitrogen (NO₃^- -N), and increased the soil C/N ratio. The total nitrogen (TN) content under the PE treatment was lower than that under the PLA and PBAT treatments by 0.03–0.60&#xa0;g·kg⁻¹ and 0.27–0.31&#xa0;g·kg⁻¹, respectively. The dry weight and cumulative nitrogen absorption of quinoa treated with PBAT were significantly lower than those of quinoa treated with PLA or PE (P &lt; 0.05). The activities of antioxidant enzymes and nitrate reductase (NR) were the lowestin the 1% microplastic treatment, and the content of malondialdehyde (MDA) was the highest; the activities of NR and the MDA content were 40.3–54.3% and 2.54–2.61 times greater in the 1% treatment than in the control, respectively. In terms of mechanism, nitrogen absorption inquinoa was significantly negatively correlated with soil SOC content (P &lt; 0.01). In conclusion, the input of microplastics to the soil can reduce the activity of nitrogen-metabolizing enzymes in quinoa, and biodegradable PBAT significantly reducednitrogen absorption inquinoa. Oxidative damage in quinoa wasgreater when the degree of pollution wasmoderate.</p>