Associations of salinity and microplastics with microbial community assembly and functional potential in saline-alkali soils.

Soil salinization and microplastic (MP) pollution are important pressures on saline-alkali ecosystems, yet their associations with soil microbial communities along natural salinity gradients remain unclear. We investigated low (LS), medium (MS), and high-salinity (HS) soils in Qinghai Province using MP characterization and high-throughput sequencing. MP abundance was highest at MS sites (12660 items/kg), compared with LS (7395 items/kg) and HS (4660 items/kg), indicating a non-monotonic pattern. Bacterial community composition differed significantly among salinity groups (ANOSIM, R = 0.7193, P = 0.006), whereas fungal communities did not (R = 0.0288, P = 0.3910). Bacterial assembly shifted from predominantly deterministic under LS conditions to stochastic under MS conditions, with deterministic processes increasing again under HS conditions; fungal communities remained predominantly deterministic. Total soil salinity and MP abundance were among the variables most strongly associated with microbial community variation. Predicted functional profiles were broadly stable, although several functions showed contrasting bacterial and fungal trends. These findings highlight distinct bacterial and fungal responses and support targeted MP monitoring and source control in saline-alkali farmland.