Patterns and Drivers of Nitrogen Cycling Response to Micro/Nano-Plastics in Soil and Sediment

Micro/nano-plastics (M/NPs) as emerging particulate pollutants pose significant risks to ecosystem health. Nitrogen (N) cycling plays a crucial role in biogeochemistry and largely depends on microbe-driven N transformation. However, how and why N cycling responds to M/NP exposure in different environmental media such as soil and sediment remains largely unknown. Herein, through a meta-analysis of 116 publications, we found that M/NP exposure significantly reduced soil NO₃ ^- concentrations (24.9%) and enhanced N₂O emissions (32.6%), while increasing sediment NH₄ ⁺ concentrations (21.6%) and N₂O emissions (38.1%). The dynamics of N and N₂O emissions were jointly regulated by M/NPs exposure characteristics and the environmental medium. Particularly, soil N₂O emissions increased when the exposure dose exceeded 0.2% or the exposure duration was within 34.7 days. In sediments, N₂O emissions were enhanced under a wider range of conditions: when the exposure dose was either between 0.003% and 0.5% or above 1%, the particle size was less than 398.8 μm, or the exposure duration ranged from 5.5 to 353.1 days. This broader responsiveness indicates that sediment ecosystems are more sensitive to M/NPs-induced N₂O emissions than soil ecosystems. The mechanistic basis for the media-dependent effects of M/NPs on N cycling lies in their distinct regulation of key microbial functional gene abundance. Specifically, in soils, N₂O emissions were driven by an increased abundance of genes encoding nitrate reductase (nar) and nitrite reductase (nir), stimulating the denitrification pathway. Conversely, in sediments, the upregulation of nitric oxide reductase (nor) genes enhanced the conversion of NO to N₂O. Overall, by revealing how M/NP properties and environmental media interact to govern N cycling, this work provides a scientific foundation for predicting and mitigating N₂O emissions from terrestrial and aquatic ecosystems under increasing M/NP pollution.