Hydrological regime modulates nitrogen retention-removal shifts in a glacier-oasis alpine river by restructuring multi-trophic interactions and microbial assembly.

Nitrogen (N) cycling in glacier-oasis alpine rivers exhibits distinct spatiotemporal patterns driven by multi-trophic community interactions. This study integrated 16S/18S rRNA sequencing with metagenomic analysis to investigate N-transformation dynamics across trophic levels and their response to varying hydrological regimes. The α-diversity of multi-trophic communities exhibited trophic-level-specific longitudinal patterns, with bacteria and algae generally showing higher diversity in the oasis reach (OR), whereas protozoans and metazoans were more diverse in the glacial reach (GR). In the OR, the species turnover of microeukaryotes exceeded 40%, and the pooled OR-irrigation channel reach (ICR) group exhibited 9.1-22.6-fold greater network complexity than the GR. The abundances of functional genes associated with nitrification and denitrification were 12.3-13.8 and 4.7-9.6 times higher in the OR than in the GR, respectively; N fixation potential was 2.4-14.1 times greater and bacterial α-diversity was 0.94-1.42 times higher in the OR than in the GR. Notably, only during the dry season did the GR exhibit 63-84% higher nitrate assimilation than the OR. Path analysis revealed that algae-protozoan symbiosis promoted N assimilation and retention (β = 0.87), whereas bacterial communities enhanced dissimilatory nitrate reduction and denitrification, facilitating N removal (β > 0.66). In contrast, metazoan predation (β = -0.78) and dissolved oxygen (β = -0.24) suppressed denitrification. The differentiation of N-cycling functions was governed by high α- and β-diversity within microbial communities. Heterogeneous selection and dispersal limitation during community assembly, acting through cross-trophic cascading effects, collectively balanced N retention against removal and ultimately determined the fate of N in the river ecosystem. Climate change may redistribute N-cycling hotspots along alpine rivers by altering hydrological regimes and riverine gradients, potentially increasing eutrophication risk by reducing N removal and enhancing N retention.