<b>Supplementary Files: Seagrass-mediated rhizosphere redox gradients are linked with ammonium accumulation driven by diazotrophs</b>

Seagrasses can enhance nutrient mobilization in their rhizosphere via complex interactions with sediment redox conditions and microbial populations. Yet, limited knowledge exists on how seagrass-derived rhizosphere dynamics affect nitrogen cycling. Using optode and gel27 sampler based chemical imaging, we show that radial O2 loss (ROL) from rhizomes and roots leads to formation of redox gradients around below-ground tissues of seagrass (Zostera marina), which are co-localised with regions of high ammonium concentrations in the rhizosphere. Combining such chemical imaging with fine-scale sampling for microbial community and gene expression analyses indicated that multiple biogeochemical pathways and microbial players can lead to high ammonium concentration within the oxidized regions of the seagrass rhizosphere. Symbiotic N2-fixing bacteria (Bradyrhizobium) were particularly abundant and expressed the diazotroph functional marker gene nifH in Z. marina rhizosphere areas with high ammonium concentrations. Such association between Z. marina and Bradyrhizobium can facilitate ammonium mobilization, the preferred nitrogen source for seagrasses, enhancing seagrass productivity within nitrogen-limited environments. ROL also caused strong gradients of sulphide at anoxic/oxic interfaces in rhizosphere areas where we found enhanced nifH transcription by sulphate-reducing bacteria. Furthermore, we found a high abundance of methylotrophic and sulphide-oxidizing bacteria in rhizosphere areas, where O2 was released from seagrass rhizomes and roots, and these bacteria could play a beneficial role for the plants in terms of their methane and sulphide oxidation, as well as their formation of growth factors and phytohormones. ROL from below-ground tissues of seagrass thus seems crucial for ammonium production in the rhizosphere via stimulation of multiple diazotrophic associations.海草（Seagrasses）可通过与沉积物氧化还原环境及微生物种群的复杂相互作用，提升其根际（rhizosphere）的养分活化能力。然而，目前对于海草介导的根际动态如何影响氮循环（nitrogen cycling），仍缺乏足够的认知。本研究借助基于光极（optode）与gel27采样器（gel27 sampler）的化学成像（chemical imaging）技术，证实了海草根茎与根系的径向氧气流失（radial O2 loss, ROL）会在鳗草（Zostera marina）的地下组织周围形成氧化还原梯度，该梯度与根际内高铵盐浓度区域共定位。将此类化学成像技术与微生物群落（microbial community）、基因表达分析（gene expression analyses）的精细尺度采样相结合后，研究发现多种生物地球化学途径（biogeochemical pathways）与微生物类群可在海草根际的氧化区域内形成高铵盐浓度。共生固氮细菌（symbiotic N2-fixing bacteria）慢生根瘤菌属（Bradyrhizobium）在高铵盐浓度的鳗草根际区域中丰度尤其显著，且表达了固氮菌（diazotroph）功能标记基因nifH。鳗草与慢生根瘤菌的此类共生关系可促进铵盐活化——这是海草偏好利用的氮源，进而在氮限制环境（nitrogen-limited environments）中提升海草的生产力。径向氧气流失还会在根际区域的缺氧/有氧界面（anoxic/oxic interfaces）形成强烈的硫化物（sulphide）梯度，本研究在此处观察到硫酸盐还原菌（sulphate-reducing bacteria）的nifH转录水平显著上调。此外，在海草根茎与根系释放氧气的根际区域中，研究人员发现甲基营养菌（methylotrophic bacteria）与硫化物氧化菌（sulphide-oxidizing bacteria）具有较高丰度；这些细菌可通过氧化甲烷与硫化物，以及合成生长因子（growth factors）与植物激素（phytohormones），为宿主植物提供有益作用。由此可见，海草地下组织的径向氧气流失，通过激活多种固氮菌共生体系（diazotrophic associations），对根际的铵盐生成至关重要。