Table S1. Detailed microbial taxa and their relative abundance found in porewaters from east and west wetland and gravels at the White Clay Creek. 0.000% indicates the relative abundance is less than 0.001%

Interstitial water or porewater occupies the space between soil particles and provides “hotspots” and “fluvial networks” for microbial activities in floodplain soil. However, to date, we know very little about the microorganisms living in porewater and how they respond to environmental changes. This study aimed to understand microbial distribution and assemblage in riparian porewaters and how they respond to water chemistry and redox gradients associated with hydrological processes. We analyzed the annual changes of porewater microbial communities from the east and west banks of the White Clay Creek, a site at the Christina River Basin - Critical Zone Observatory, Pennsylvania, U.S.A.. Microbial abundances were quantified by epifluorescence microscopy and detailed community structures were characterized by high throughput sequencing. Water chemistry and redox gradients were also monitored and recorded, and their interactions with porewater microbiomes were analyzed using correlations and multivariate analyses. Abundance of microbial cells increased during summer and late autumn. Wetland porewater microbiomes mainly contained Acidobacteria, Bacteroidetes, Nitrospirae, and Proteobacteria, and microbiome structures were easily distinguishable from those in the underlying hyporheic gravel layer. Seasonal dynamics of bacterial community structure in the east and west wetlands were distinct, responding to floodplain topography and associated hydrological/geochemical processes. Iron (Fe)-cycling bacteria (mainly Gallionellaceae and Rhodoferax spp.) dominated the porewater microbiome, and their relative abundance was significantly higher in the east than the west wetland. Furthermore, Fe-oxidizing bacteria (Gallionellaceae) were negatively correlated with Fe-reducing bacteria (Rhodoferax spp.) at the east wetland. Microbial abundances (cell density) in porewaters showed similar seasonal patterns across streambanks, but microbial community structure did not. Microbiome assembly in porewater is correlated with water chemistry and redox gradients primarily associated with local hydrological processes. Due to their significance for C mineralization and Fe reduction at terrestrial-aquatic interfaces, microbiomes in riparian porewaters and associated microbial activity play an essential role in C and mineral dynamics. These findings will inform future studies of the response of freshwater ecosystems to hydrological dynamics influenced by global climate change.