Elevated salinity amplifies polyethylene microplastic-induced soil nitrous oxide emissions

Microplastics (MPs) have been shown to enhance nitrous oxide (NO) emissions and soil salinization potentially amplifying this effect. This study investigated the individual and combined impacts of polyethylene (PE) MPs and salinity on NO emissions from paddy soils, while simultaneously analyzing related microbial parameters. MPs significantly increased cumulative NO emissions by 9.0-46.9, 150.3-422.9 and 1194.9-1410.7 μg N kg in soils cultivated for 3, 15 and 40 years, respectively. While increasing salinity amplified MP-induced NO emissions in soils cultivated for ≤ 15 years, this effect was negligible in 40-year cultivated soil. The observed NO increase was primarily attributed to MP-induced changes in nitrification, as evidenced by consistent increases in ammonia-oxidizing archaeal (AOA) amoA gene abundance across all soils. Although MPs showed limited effects on most denitrifying bacterial genera, they significantly increased the relative abundance of Azoarcus (a nitrate reducing bacterium) by 38.0-48.1 % in 3-year soil, and generally enhanced the nirS gene (encoding nitrite reduction) in soils cultivated for ≥ 15 years. Notably, NO emissions exhibited positive linear relationships with AOA amoA, Nitrosomonas and Thermodesulfovibrio, suggesting their potential roles in regulating MPs-induced NO emissions. These findings demonstrate that soil salinity enhances PE MP-driven NO emissions, with more pronounced effects in soils with shorter cultivation histories.