Abstract Tidal salt marsh ecosystems store copious amounts of carbon (C) within sediments. In order to predict how these C stores may be affected by environmental change, it is critical to assess current CO2 and CH4 production and efflux from these ecosystems. Production and efflux of these greenhouse gases (GHGs) are governed by coupled geochemical, hydrological, physical and biological processes in sediments that are sensitive to local conditions, which can result in large spatial and temporal heterogeneity of GHGs dynamics within the ecosystem. To understand how the drivers of GHGs dynamics vary across salt marsh ecosystems, we coupled solid-phase geochemistry to measurements of porewater chemistry (to ∼1 m), CO2 and CH4 production in sediments and efflux to the atmosphere in a temperate tidal salt marsh for over one year to capture seasonal patterns within two vegetation zones of the marsh landscape that have distinct biogeochemical and hydrologic conditions: Tall Spartina (TS) and Short Spartina (SS). The SS vegetation zone experienced nearly constant inundation, low redox values (−200 to 200 mV), porewater pH 6–7 that did not vary with depth or time, an enrichment of pyrite and goethite with depth and up to 3 mM porewater sulfide. In contrast, the TS vegetation zone on the natural levee proximal to a tidal channel experienced large water level oscillations due to spring-neap tides that resulted in variable but higher redox values (0–700 mV), porewater pH 6–7 at depth but surface (0–3 cm) as low as 4 in the spring, an enrichment of ferrihydrite and a depletion of pyrite at ∼30 cm, and up to 0.8 mM ferrous Fe in porewater. At 50–56 cm, solid phase analyses (STXM-NEXAFS) revealed differential C speciation between the two vegetation zones, with stronger C-Fe spatial association at TS and stronger C-Ca co-association at SS despite both having similar soil pH of 3–4. These results suggest that soil pH may not be strongly predictive of C-mineral control in flooded marsh sediments. Both vegetation zones showed consistent CO2 and CH4 emissions from sediments to the atmosphere throughout the study period with TS having ∼60% higher median CO2 and SS having ∼55% higher median CH4 efflux. Using depth profiling, unexpectedly high concentrations of CO2 (>200 μM) and CH4 (>200 μM) were observed at depths 50–75 cm at both zones that were higher for SS in these sulfate-rich (up to 17 mM) sediments, which suggests methylotrophic methanogenesis occurs deep within the profile of salt marsh sediments away from the tidal channel. Moreover, if we extrapolate our median depth values of CH4 and CO2 to the 5.3 Mha of global salt marshes, this could account for a conservative estimate of ∼70 Gg of unaccounted C stored in gaseous form (i.e., CH4 and CO2) in marsh sediments, which should be considered when attempting to understand the current patterns and future responses of carbon dynamics from these ecosystems.

中文翻译：

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潮汐盐沼中碳循环地球化学控制的时空异质性

摘要 潮汐盐沼生态系统在沉积物中储存了大量的碳 (C)。为了预测这些碳储存如何受到环境变化的影响，评估这些生态系统当前的 CO2 和 CH4 产生和流出至关重要。这些温室气体 (GHG) 的产生和流出受沉积物中地球化学、水文、物理和生物耦合过程的控制，这些过程对当地条件很敏感，这可能导致生态系统内的 GHG 动态在空间和时间上存在很大的异质性。为了了解盐沼生态系统中温室气体动力学的驱动因素如何变化，我们将固相地球化学与孔隙水化学测量（至 ∼1 m）相结合，沉积物中 CO2 和 CH4 的产生以及在温带潮汐盐沼中排放到大气中超过一年的时间，以捕捉具有不同生物地球化学和水文条件的沼泽景观的两个植被带内的季节性模式：Tall Spartina (TS) 和 Short Spartina ( SS）。SS 植被区经历了几乎恒定的淹没、低氧化还原值（-200 至 200 mV）、孔隙水 pH 值 6-7 不随深度或时间变化、黄铁矿和针铁矿随深度的富集以及高达 3 mM 的孔隙水硫化物。相比之下，靠近潮汐通道的天然堤坝上的 TS 植被区由于春季小潮而经历了大的水位振荡，导致可变但更高的氧化还原值（0-700 mV），深层孔隙水 pH 值 6-7，但表面（0-3 厘米）低至 4 的春天，水铁矿的富集和黄铁矿的耗尽在~30 cm 处，以及孔隙水中高达 0.8 mM 的亚铁。在 50-56 cm 处，固相分析 (STXM-NEXAFS) 显示两个植被区之间存在差异 C 形态，TS 处的 C-Fe 空间关联更强，SS 处的 C-Ca 共关联更强，尽管两者具有相似的土壤 pH 值3-4。这些结果表明土壤 pH 值可能不能强烈预测淹没沼泽沉积物中的 C 矿物控制。在整个研究期间，两个植被区都显示出从沉积物到大气的一致 CO2 和 CH4 排放，TS 的 CO2 中值高约 60%，SS 的 CH4 流出中值高约 55%。使用深度剖析，意外高浓度的 CO2 (>200 μM) 和 CH4 (> 在这些富含硫酸盐（高达 17 mM）沉积物中 SS 的两个区域的 50-75 厘米深度处观察到 200 μM），这表明甲基营养甲烷生成发生在远离潮汐通道的盐沼沉积物剖面深处. 此外，如果我们将 CH4 和 CO2 的中值深度值外推至 5.3 Mha 的全球盐沼，这可以解释在沼泽沉积物中以气态形式（即 CH4 和 CO2）储存的 70 Gg 未计算碳的保守估计，在试图了解这些生态系统中碳动态的当前模式和未来响应时应该考虑这一点。