Heavy metal risk and sediment environment jointly shape hyporheic microbial communities in a typical loess watershed

Hyporheic zones in erosion-prone loess watersheds are key interfaces for solute and contaminant transport and biogeochemical transformation. Heavy metals can alter sediment microbial communities, while microorganisms mediate nutrient and organic-matter cycling. However, the relative roles of metal gradients and sediment environmental conditions in structuring bacterial and fungal communities in these systems remain poorly resolved. This study investigated 16 sites across the Jinghe River Basin on the Loess Plateau during the hydrologically stable dry season, combining sediment metal and environmental measurements with high-throughput sequencing. The results showed that 88% of the sites had low pollution loads, whereas moderate pollution was confined to a confluence site and the lower downstream reach. Arsenic exhibited the highest potential ecological risk, with a risk factor about 15 times that of the second-ranked metal, V. Bacterial richness in downstream was 20% higher than upstream, fungal species richness was higher upstream than downstream. Fungal genera exhibited associations with more metal species, while significant correlations between bacteria and metals were predominantly limited to Pb and Zn. PLS-SEM showed that total metal concentrations were strongly associated with ecological risk but only weakly with microbial attributes, while sediment conditions represented the main pathway associated with microbial distribution, accounting for nearly half of bacterial and fungal community variation. These findings support ecological assessment and targeted management that integrate metal and nutrient source control with protection of hyporheic habitats in loess watersheds.