Metagenomic Insights into the Microbial Regulation of Soil Organic Carbon Dynamics Under Microplastic Exposure in Saline-Alkali Soils

Microplastics (MPs) can influence soil microbial communities, this may further affect the transformation of soil organic carbon (SOC) in saline-alkali soils. However, the combined effects of MPs and varying salinity on this process remain unclear. We combined incubation experiments with metagenomic sequencing to investigate the impacts of polyethylene (PE) and biodegradable polylactic acid (PLA) MPs on SOC dynamics and microbial metabolic functions. The study was conducted across a salinity gradient (EC: 4.75, 20, and 40 mS/cm) with varying MPs addition (0%, 0.5%, 1%, and 14% w/w). Our results showed MPs addition increased microbial richness. Compared to PE, biodegradable PLA significantly increased SOC and soil microbial biomass carbon (MBC) by 0.96%–118.71% and 9.18%–362.46%, respectively. Metagenomic analysis revealed that PLA enhanced carbon fixation primarily through the Calvin–Benson–Bassham cycle ( Prk ;3.31%–98.43%) and the Wood–Ljungdahl pathway ( acsABCDE ;0.035%–0.44%), whereas PE stimulated the 3-Hydroxypropionate cycle ( mcr :17.3%–163.06%). In addition, PE addition increased the metabolic activity of the TCA cycle (0.8%–26.67%). Relative to the control, both PE and PLA addition promoted the relative abundance of ethanol fermentation related genes ( adh , mdh , ald ). The relative abundance of some carbon fixation genes was higher under low-to-medium EC conditions than under high EC conditions, whereas the enhanced fermentation pathways under high EC, suggests a more complex carbon cycle. KEGG metabolic pathway prediction showed that PLA treatments exhibited greater changes in functional abundance in response to EC than PE treatments. PE addition increased abundance of metabolic pathways but decreased abundance of translation and replication and repair pathways. Meanwhile, PLA has a stronger ability to promote cellular processes than PE. This study provides new insights into assessing the impact of MPs on carbon functions in saline-alkali soils.