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 <i>Rhodoferax</i> 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 (<i>Rhodoferax </i>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.

孔隙水（porewater）占据土壤颗粒间的空隙，为河漫滩土壤中的微生物活动提供了“热点区域”与“河流状网络”。然而迄今为止，人们对栖息于孔隙水中的微生物及其对环境变化的响应机制仍知之甚少。本研究旨在阐明河岸带孔隙水中的微生物分布与群落组装模式，以及它们如何响应与水文过程相关的水化学特征与氧化还原梯度。我们以美国宾夕法尼亚州克里斯蒂娜河流域-关键带观测站（Critical Zone Observatory）的白粘土溪（White Clay Creek）东西两岸为研究样地，分析了孔隙水微生物群落的年度变化。微生物丰度通过落射荧光显微镜法进行定量，群落结构则通过高通量测序（high throughput sequencing）进行解析。同时监测并记录了水化学特征与氧化还原梯度，并通过相关性分析与多变量分析探究了其与孔隙水微生物组的相互作用。微生物细胞丰度在夏季与晚秋时节升高。湿地孔隙水微生物组主要包含酸杆菌门（Acidobacteria）、拟杆菌门（Bacteroidetes）、硝化螺旋菌门（Nitrospirae）与变形菌门（Proteobacteria），其群落结构与下伏潜流砾石层的微生物组存在显著差异。东西两岸湿地的细菌群落结构季节动态存在明显差异，响应于河漫滩地形与相关的水文/地球化学过程。铁循环细菌（主要为嘉利翁氏菌科（Gallionellaceae）与红育菌属（*Rhodoferax*）物种）主导了孔隙水微生物组，且其相对丰度在东岸湿地显著高于西岸。此外，在东岸湿地，铁氧化细菌（嘉利翁氏菌科）与铁还原细菌（*Rhodoferax* 属）呈显著负相关。孔隙水中的微生物丰度（细胞密度）在两岸呈现相似的季节模式，但微生物群落结构则不然。孔隙水微生物组的组装与主要受局地水文过程调控的水化学特征及氧化还原梯度显著相关。鉴于其在陆水界面碳矿化与铁还原过程中的重要作用，河岸带孔隙水微生物组及其相关微生物活动在碳与矿物动态中发挥着关键作用。本研究结果可为未来探究全球气候变化影响下水文动态对淡水生态系统的影响提供理论参考。