Decomposition of Nelumbo nucifera Litters with Three Initial Biomasses Significantly Affected the Microbial Community and Assembly in Sediment.

The decomposition of aquatic macrophytes plays a critical role in releasing nutrients and driving biogeochemical cycles in freshwater ecosystems, processes largely mediated by microbial communities. However, the dynamics, assembly mechanisms, and ecological functions of microbial communities, particularly nitrogen-cycling microorganisms, in response to litter decomposition remain poorly understood. This study investigated the impact of Nelumbo nucifera litter decomposition on bacterial and nitrogen-cycling microbial communities, including ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA), and nirS-type denitrifying bacteria, in freshwater sediments using high-throughput sequencing. Decomposition experiments were conducted 240 days using three initial litter biomasses (15, 45, and 75 g). Results revealed that higher initial biomass and extended decomposition times increased mass loss and nutrient release, altering the physicochemical properties of both sediment and overlying water, such as pH, DO, and nutrient concentrations. These changes significantly altered microbial community dynamics, influenced assembly mechanisms and potential ecological functions. Dominant phyla, including Acidobacteriota, Bacteroidota, Chloroflexi, Myxococcota, and Firmicutes, exhibited higher relative abundances in litter-decomposed groups compared to the CK group. Stochastic processes dominated the assembly of AOB and AOA communities, while deterministic processes were more influential for nirS-type denitrifying bacteria. Functional predictions using KEGG pathways highlighted distinct microbial functional profiles associated with nitrogen cycling and carbon metabolism in response to litter decomposition. These findings emphasize the importance of considering both initial biomass and decomposition time when evaluating the effects of plant litter on microbial communities and their functional roles in nutrient cycling, providing valuable insights into the ecological impacts of macrophyte decomposition in freshwater ecosystems.