Vegetation and microtopography drive microbial necromass carbon sequestration in wetland soils

Floodplain wetlands are important carbon sinks, yet drought-induced water level declines threaten this function by triggering mudflat-to-meadow transitions that alter soil organic carbon (SOC) stocks and stability. Microtopography shapes wetland hydrology and vegetation productivity; however, its interactive effects with vegetation on microbial necromass carbon (MNC)—the main component of stable SOC derived from microbial death—remain unknown. Combining amino sugar biomarkers, amplicon and metagenomic sequencing, we investigated MNC distribution and drivers across vegetation covers (meadow and mudflat) and microtopographic units (dish-shaped depressions, delta slopes, and riparian slopes) up to 30 cm depth in Poyang Lake floodplains. In the top 10 cm, MNC pool shifted from bacterial (BNC) to fungal necromass carbon (FNC) dominance from mudflats to meadows, with FNC/BNC ratio increasing from 0.5 to 1.7. This shift was driven by drainage that stimulated plant growth and C input belowground as well as oxygenation, thereby enriching fungal saprotrophic and symbiotrophic guilds, cellulose-hydrolyzing enzymes, and genes responsible for aerobic lignin-degradation. Conversely, lower meadow pH suppressed bacterial richness and functions critical for carbon, nitrogen, and sulfur cycling. Microtopography further mediated MNC/SOC ratio following vegetation effects. In the top 10 cm, delta meadow soil had higher FNC/SOC than dish-shaped and riparian meadows, driven by recalcitrant dissolved organic matter that enriched saprotrophic fungi. Aerated riparian mudflat had higher BNC/SOC than other mudflats due to efficient nitrogen turnover and reduced CO₂ emissions. Below 10 cm, BNC exceeded FNC owing to oxygen limitation for fungi. Delta meadow and riparian mudflat also maintained higher BNC/SOC than other microtopography units, primarily driven by clay-silt mineral protection. Overall, drought-induced meadow expansion restructured topsoil microbial communities, shifting microbial carbon sequestration pathway from bacterial toward fungal dominance. Slope wetlands mitigate climate change more effectively than depressions through greater SOC stability, mediated by depth-dependent drivers of microbial necromass—substrate availability in the top 10 cm and mineral protection below. These findings reveal that the impact of microbial life-and-death processes on long-term carbon sequestration and stability is regulated by the hotspot-specific conditions created by vegetation, microtopography, and soil depth, highlighting the need for hotspot-differentiated wetland management strategies.