Variations of bacterial communities induced by Ceratophyllum demersum litter decomposition at the litter sediment interface and in bottom sediments.

Submerged macrophytes are keystone components in freshwater ecosystems. Upon senescence, most leaves and stems settle on surface sediments to form a dynamic litter-sediment interface (LSI), which serves as a key biogeochemical boundary regulating nutrient fluxes and driving essential ecosystem functions. In this study, a 65-day decomposition experiment was conducted using Ceratophyllum demersum litter to investigate changes in bacterial community structure, diversity, assembly processes, and ecological functions between the LSI and bottom sediments across three initial biomass gradients at two decomposition durations (28 and 65 days). Results revealed that nutrient release and mass loss increased with biomass doses and time, with the high-biomass group losing 23.27 g dry mass (releasing 9603.10 mg C, 436.05 mg N, and 48.24 mg P) by day 65. The LSI showed higher pH, organic matter, and nutrient contents than bottom sediments, confirming its sensitivity to litter input. Observed species (Sobs), Chao and Shannon index at the LSI were significantly lower than in bottom sediments. Litter decomposition parameters significantly shaped sedimentary microbial community structure. Co-occurrence network complexity and deterministic community assembly processes intensified with decomposition duration. Additionally, the LSI enriched several redox-sensitive phyla, and nitrogen-cycling functional gene abundances (e.g., nifK/H/D) correlated positively with C/N/P release. The LSI acts as a dynamic hotspot where microbial community composition and functions are tightly coupled to decomposition progress. This study systematically elucidates the effects of plant litter decomposition on sedimentary bacterial communities.