Temporal–Spatial Differences of Nitrogen Source–Sink in Sediments of Wetland–River Connected System and Response Mechanism of Microbial Community Function

The spatiotemporal succession of microbial community structure influences sediment nitrogen (N) release. To compare the N release and microbial response between a large-scale wetland and its connecting rivers, sediment samples were collected across three seasons (October 2024, March 2025, and July 2025) and analyzed using sorption isotherms and sequencing to elucidate source–sink dynamics and microbial mechanisms. The results showed that the maximum sorption capacity (Qmax, 9.931 mg/g) exhibited significant seasonal variation (March > July > October) and a vertical decreasing pattern (surface > middle > bottom). The Qmax of wetland sediments (SS) was generally higher than that of river sediments (SH). The N source–sink analysis indicated that SS consistently served as a stable N sink, while SH primarily served as a N source. Among them, the internal N release pressure in the rivers was highest in July, and a relatively high diffusion flux was still maintained in October. Microbial diversity was significantly higher in the warm seasons (July and October) than in spring, and spatially, diversity was higher in SS than in SH. Proteobacteria were the dominant phylum, with a relative abundance ranging from 8.11% to 35.59%. Gammaproteobacteria was the dominant class, with a maximum relative abundance of 28.36%. Anaerolineae in SH were significantly enriched in summer and autumn. The driving factors shifted from the physical particle size (D50) in spring to the organic load and nutrients (total nitrogen or total phosphorus) in summer, and then to the synergistic effect of pH and physical structure in autumn. Functional prediction indicated that the microbial functions in river channels evolved from reserve-type heterotrophic metabolism to high-activity energy metabolism, with the highest predicted potential observed in July. In contrast, the wetland consistently maintained steady-state regulatory functions centered on signal transduction and membrane transport.