Coastal landscapes are increasingly exposed to seawater due to sea level rise and extreme weather events. The biogeochemical responses of these vulnerable ecosystems are poorly understood, limiting our ability to predict how their role in local and global biogeochemical cycles will shift under future conditions. Here we evaluate how antecedent conditions influence the biogeochemical response of soil to seawater inundation events based on a 42-day laboratory incubation experiment with soils collected from a natural salinity gradient across a coastal floodplain. We quantified influences of seawater inundation on intact soil cores through high-frequency carbon dioxide (CO₂) and methane (CH₄) gas fluxes measurements as well as ultrahigh resolution characterization of organic matter chemistry and metabolites. Mean CO₂ and CH₄ fluxes were higher after inundation compared to control cores for soils that had low in situ electrical conductivity (EC). Soils with low in situ EC also exhibited significant shifts in organic matter profiles after inundation, with surficial soils (0–7.5 cm) becoming more enriched in phenolic compounds, compared to deeper soils (7.5–15 cm). The number of biochemical transformations inferred from mass spectrometry increased significantly after inundation for soils with low in situ EC. Our results suggest that seawater inundation of low-salinity terrestrial environments can lead to increased microbial activity and increasing likelihood of soil carbon release, with sites experiencing infrequent or new seawater exposure likely to be more sensitive to saltwater exposure relative to sites with more frequent exposure. We conclude that the biogeochemical impacts of future seawater exposure will be modulated by antecedent conditions associated with landscape position within coastal watersheds.

由于海平面上升和极端天气事件，沿海景观越来越暴露于海水中。人们对这些脆弱生态系统的生物地球化学反应知之甚少，这限制了我们预测它们在未来条件下在本地和全球生物地球化学循环中的作用将如何变化的能力。在这里，我们根据一项为期42天的实验室孵化实验，评估了从前沿沿海洪泛区的天然盐度梯度收集的土壤，这些条件如何影响土壤对海水淹没事件的生物地球化学响应。我们通过高频二氧化碳（CO ₂）和甲烷（CH ₄）量化了海水淹没对完整土壤核心的影响气体通量测量以及有机物化学和代谢产物的超高分辨率表征。对于原位电导率（EC）低的土壤，淹没后的平均CO ₂和CH ₄通量高于对照岩心。淹没后原位EC较低的土壤也表现出明显的有机质变化，与较深的土壤（7.5-15 cm）相比，表层土壤（0-7.5 cm）的酚类化合物含量更高。对于原位低的土壤，淹没后通过质谱法推断的生化转化次数显着增加EC。我们的结果表明，低盐度陆地环境中的海水泛滥会导致微生物活动增加和土壤碳释放的可能性增加，相对于频繁接触的地点，很少或新接触海水的地点对盐水的接触更敏感。我们得出结论，未来海水暴露的生物地球化学影响将由与沿海流域内景观位置相关的先决条件来调节。