Effects of microplastics on carbon and nitrogen cycling in coastal wetlands: A critical meta-analysis

Coastal wetlands play a pivotal role in carbon and nitrogen cycling and are recognized as reservoirs for microplastics (MPs). While the impacts of MPs on these processes have gained wide attention, their general patterns and mechanisms remain poorly understood. Here, we conducted a meta-analysis of 1051 data points from 34 studies. Overall, the results indicated that MPs significantly influenced certain carbon and nitrogen cycling indicators, including increases in sediment total organic carbon (partly attributable to the direct carbon contribution from MPs themselves), microbial biomass carbon, and nitrification gene abundance, while inhibiting β-1,4-glucosidase activity. However, nearly all studies were conducted under laboratory conditions using intertidal sediments. Subgroup analyses indicated that, in tidal simulation experiments better representing natural intertidal environments, MPs had no significant effects on carbon and nitrogen cycling, suggesting that laboratory designs lacking tidal dynamics may overestimate the ecological impacts. Moreover, ecological effects did not differ significantly between sediments from vegetated wetlands and unvegetated bare flats, which may be attributed to the frequent omission of vegetation-related factors, such as litter inputs and rhizosphere effects, in laboratory experiments. Therefore, caution is warranted when extrapolating laboratory findings to natural coastal wetlands. Additionally, meta-regression analyses showed MP size and concentration influenced certain carbon and nitrogen indicators, whereas exposure time affected only nitrogen cycling. This study provides a theoretical basis for a more comprehensive understanding of MP impacts on carbon and nitrogen cycling in coastal wetlands.