Hydrological Fragmentation Driving Microbial Carbon Necromass Reduction in Columnar Sediments: Evidence from CAZyme Genomic Signatures in Cascade Reservoirs

Microbial necromass carbon (MNC), a key component of soil organic carbon, plays a vital role in aquatic carbon sequestration. Its accumulation and transformation are highly sensitive to environmental changes, particularly in reservoir sediments-critical zones for organic matter storage and biogeochemical cycling. This study investigated the vertical distribution and regulatory mechanisms of MNC in cascade reservoir systems through sediment analysis and metagenomic sequencing. Our findings reveal that MNC constitutes ​​15 ~ 35% of total sediment organic carbon (SeOC)​​, with fungal-derived necromass consistently dominating over bacterial contributions. Metagenomic data highlight distinct functional potentials in carbon cycling, showing that bacterial necromass exhibits higher lability than fungal necromass, as evidenced by shifts in carbohydrate-active enzyme (CAZyme) gene abundances-particularly those involved in glucan and peptidoglycan degradation. Notably, cascade damming introduced ​​spatial heterogeneity in MNC distribution​​, with downstream reservoirs experiencing ​​reduced MNC accumulation​​ due to altered hydrological connectivity and nutrient regimes. These results underscore the ​​pivotal role of MNC in aquatic carbon storage​​ while highlighting the complex interplay between ​​environmental factors, microbial metabolic traits, and anthropogenic disturbances​​ in regulated river systems. Therefore, our findings demonstrate that fungal necromass is a dominant and relatively stable component of sediment carbon, and its dynamics must be integrated to accurately assess and predict carbon sequestration in dammed rivers.