Purpose <p>The application of biodegradable plastics is increasing gradually, but how the generated biodegradable microplastics (MPs) affect soil health is still far from well understood, especially aged MPs. This study compared the effects of pristine and UV-aged biodegradable MPs on soil microbial community and functioning associated with nitrogen (N) metabolism.</p> Methods <p>An agricultural soil was treated with 0.5% (w/w) poly(butylene adipate-co-terephthalate) (PBAT) MPs, either pristine or photo-aged. Soil chemical and biochemical properties were characterized, and soil bacterial community and abundance of ammonia oxidizer were analyzed using high throughput sequencing and quantitative polymer chain reaction (qPCR), respectively.</p> Results <p>PBAT MPs positively affected soil microbial biomass N, total N, and urease/glutamate synthase/nitrate reductase activity, which was supported by an enrichment of microbes capable of N fixing and denitrification such as <i>Bradyrhizobium</i> and <i>Ralstonia</i> compared to the control. On the contrary, available N (NH₄⁺ and NO₃⁻) and potential nitrification rate were substantially negatively affected (by up to 68%), possibly due to an inhibiting effect of its degradation products on ammonia-oxidizing archaea and <i>Nitrospira.</i> UV-aging in air or water greatly altered plastic properties (e.g., color, surface roughness and crosslinking), resulting in a weakened effect of PBAT MPs on N metabolism parameters and slightly interfering their interaction with microbial taxa on day 20. After 90 days, the effects of pristine and aged MPs became largely the same.</p> Conclusion <p>The results indicated that biodegradable MPs input could significantly influence N transformation processes, resulting in reduced N availability and altered soil N pool. UV-aging may temporarily weaken the effects of PBAT MPs. The findings enhance our understanding of the influence of aged biodegradable MPs in soil ecosystems.</p>