Bacterial communities and nitrogen transformation genes in streambank legacy sediments and implications for biogeochemical processing

Streambank legacy sediments may be important sources of sediment and nutrients from Mid-Atlantic watersheds. However, little is known about the nutrient processing roles of microorganisms that inhabit legacy sediments, let alone their composition, diversity, and distributions. In this study, we sampled 15 streambanks at multiple depths throughout four watersheds in the Mid-Atlantic Region of the USA. High throughput sequencing of 16S ribosomal RNA genes indicated that streambank microbial community composition varied within site depth and across contemporary land uses. Collectively, the most abundant microbial taxa in legacy sediments included Acidobacteria (25-45%), Proteobacteria (15-40%), Nitrospirae (2-10%), Chloroflexi (1-5%), and Actinobacteria (1-10%). Bacterial community composition was distinct between agriculture and urban sites as well as suburban and urban sites. There was significant variation in community composition between the top (1-25%), upper-middle (26-50%), and bottom layers (76-100%) of sediments, while the relative abundances differed between layers for only Acidobacteria and Proteobacteria. Several streambank chemistry variables (metals, %TC, and %TN) had weak positive correlations with community composition. Compared to ammonia-oxidizing bacteria, nitrifying archaea were more predominant. This study provides the first insights into detailed microbial composition of legacy sediments and identifies environmental drivers for community structure and nitrogen processing. Future studies should consider exploring the role of this unique microbial environment for nutrient processing and leaching from legacy sediments and its implications for watershed water quality.