Sediment pore-network architecture may shape spatial patterns of denitrification in freshwater littoral zones

Freshwater littoral zones are increasingly recognized as highly active biogeochemical interfaces for permanent nitrogen removal via denitrification; however, the physical mechanisms sustaining this activity remain poorly understood. Here, we combined membrane inlet mass spectrometry, X-ray μCT-based multifractal analysis, and functional gene quantification across geomorphic gradients (littoral slopes to bottom sediments) in eight rivers and ponds of the Yangtze alluvial plain. We show that littoral sediments sustained 1.8-3.0 times higher denitrification rates than bottom sediments across all four seasons. PLS-SEM indicated that macro-scale sediment physical state was associated with denitrification rates mainly through indirect links with substrate availability and denitrification-related functional gene abundance (R² = 0.82). Representative high-resolution μCT imaging showed pore-network features directionally consistent with greater connectivity and transport potential in the examined littoral-position cores than in the examined bottom-position cores, whereas bottom sediments showed fragmented pore space with high tortuosity. These results indicate that bulk substrate concentrations alone do not fully predict spatial patterns of denitrification-dominated N₂ production; pore-network architecture may provide a plausible structural context for effective substrate accessibility.