From surface variability to interaction- and soil-mediated regulation: depth-dependent microbial organization for nitrogen cycling in the dry season in lakeside wetlands

Lakeside wetlands play a critical role in mitigating non-point source nitrogen (N) pollution, yet the mechanisms underlying vertical microbial stratification across the water-sediment interface remain unclear. Based on a single dry-season sampling campaign conducted in February 2023, microbial communities and N-cycling functional genera exhibited a clear depth-dependent stratification under hydrological fluctuation, but their responses were not vertically synchronized. The sediment profile functioned as a three-tier system, shifting from disturbance-buffered surface processes to subsurface-dominated regulation: weak soil-microbe coupling in surface layers (0-10 cm), coupling between soil and microbial indicators strengthened at intermediate depths (10-40 cm), and soil-mediated control became more evident in the lower-profile layer (40-50 cm). Environmental effects emerged at 20-30 cm, whereas microbial network connectivity peaked at 30-40 cm, suggesting a delayed and partially dissociated relationship. Denitrifying taxa showed a weak V-shaped depth distribution, with the highest relative abundance in the top 5 cm (32.5% ± 17.0%) and a minimum at 10-20 cm (23.8% ± 14.1%). Soil parameters explained most of the community variation, while vegetation and hydrological factors contributed less. These findings identified the intermediate layer (10-40 cm) as a critical functional zone for N transformation, while the lower-profile layer (40-50 cm) may retain residual denitrification-related potential under resource-limited conditions. These findings suggested that subsurface soil-microbe interactions should be considered when evaluating dry-season N-cycling potential in the lakeside wetlands, although their temporal stability and actual functional activity require further verification.