Effects of microplastics and excessive nitrogen pollution on oat growth and soil nitrogen cycling

Both excessive nitrogen (N) and microplastic (MPs) pollution pose global change challenges to ecosystems and human health. The effects of MPs coexisting with excessive N on plant growth and N cycling are still largely unknown. This study employed a pot experiment to assess how polypropylene (PP) MPs influence oat (Avena sativa L.) growth and soil N cycling under conditions of excessive N fertilization. In the treatments of excessive N treatment, 2 % PP MPs significantly increased underground biomass (54.5 %). ¹⁵N stable isotope results showed that 2 % PP MPs treatment significantly reduced the percentage of N from fertilizer in oat by 10.4 %. Compared to without N treatment, excessive N treatment reduces the Shannon index of the rhizosphere soil bacterial community. Only at the 0.5 % PP MPs level, excessive N treatment significantly reduced the diversity of bacterial communities in the bulk soil compared to the without N treatment. The Shannon index of fungal community was not affected by N treatment and MPs. Microbial biomass nitrogen (MBN) and pH were significant related to sensitive species in microbial communities. The coexistence of excess N and 2 % PP MPs significantly reduced the abundance of genes (norB, nosZ and nirB) related to denitrification process. Structural equation modeling showed that MPs could promote underground biomass, reduce soil pH, inorganic N content, and reduce N uptake efficiency, thus resisting growth damage caused by high N. In the future, the effects of different concentrations and types of MPs and different nitrogen forms on soil and crops should be further investigated.