Polyethylene microplastic impairs soil gross nitrification and reduces fertilizer-derived nitrogen uptake in rice

Microplastics (MPs) pollution has been recognized as a limiting factor for plant growth, yet its effects on nitrogen (N) uptake efficiency and gross nitrification rates remain poorly understood. Here, we conducted a pot experiment using the N labeling method to evaluate the impact of polyethylene MP on soil gross nitrification and rice N uptake from fertilizer. Polyethylene MP exposure significantly inhibited rice growth, chlorophyll synthesis, soil nitrification processes, and N uptake while inducing oxidative stress, with dose-dependent effects. Even low MP concentrations (≥ 0.01 %) reduced (p < 0.05) rice chlorophyll a/b content and ammonia-oxidizing archaea (AOA) amoA gene abundance, while ≥ 0.1 % MPs significantly decreased gross nitrification rates and potential nitrification but increased superoxide dismutase (SOD) and peroxidase (POD) activities. At 1.0 % MPs, rice biomass, N uptake, and ammonia-oxidizing bacteria (AOB) amoA abundance were markedly suppressed. Fertilizer-derived N uptake was particularly sensitive to MPs, declining from 25.4 % in MP-free soil to 12.9-19.8 % at ≥ 0.01 % MPs. Notably, MP-induced oxidative stress in rice, rather than soil nitrification inhibition, was identified as the primary driver of reduced fertilizer N assimilation. Our findings highlight the need to mitigate MP contamination through strategies such as reducing MP sources to improve N management in rice agroecosystems.