Spatial and temporal heterogeneity of geochemical controls on carbon cycling in a tidal salt marsh
Spatial and temporal heterogeneity of geochemical controls on carbon cycling in a tidal salt marsh
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DOI：
10.1016/j.gca.2020.05.013
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发表时间：
2020-08
影响因子：
5
通讯作者：
A. Seyfferth;Frances Bothfeld;R. Vargas;J. Stuckey;Jian Wang;Kelli A Kearns;H. Michael;J. Guimond;Xuan Yu;D. Sparks
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文献类型：
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作者：
A. Seyfferth;Frances Bothfeld;R. Vargas;J. Stuckey;Jian Wang;Kelli A Kearns;H. Michael;J. Guimond;Xuan Yu;D. Sparks
潮汐盐沼生态系统在沉积物中储存了大量的碳（C）。为了预测这些碳库可能如何受到环境变化的影响，评估当前二氧化碳和甲烷的产生以及这些生态系统的流出至关重要。这些温室气体 (GHG) 的产生和流出受到沉积物中对当地条件敏感的耦合地球化学、水文、物理和生物过程的控制，这可能导致生态系统内温室气体动态存在较大的空间和时间异质性。为了了解盐沼生态系统中温室气体动态的驱动因素如何变化，我们将固相地球化学与温带潮汐盐沼中孔隙水化学（至~1m）、沉积物中二氧化碳和甲烷的产生以及温带潮汐盐沼排放到大气中的二氧化碳和甲烷排放量的测量相结合，以捕获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模式：高米草（TS）和短米草（SS）。 SS植被区经历了几乎持续的淹没、低氧化还原值（-200至200mV）、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富集以及高达3mM的孔隙水硫化物。相比之下，靠近潮汐河道的天然堤坝上的 TS 植被区由于春季小潮而经历了较大的水位波动，导致氧化还原值变化但较高（0-700mV），春季深层孔隙水 pH 为 6-7，但表面（0-3cm）低至 4，水铁矿富集，约 30cm 处黄铁矿贫化，并且亚铁 Fe 含量高达 0.8mM孔隙水。在 50-56cm 处，固相分析 (STXM-NEXAFS) 显示两个植被带之间的 C 形态存在差异，TS 处具有更强的 C-Fe 空间关联，SS 处具有更强的 C-Ca 共关联，尽管两者的土壤 pH 值相似，均为 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碳矿物质的控制。在整个研究期间，两个植被区都表现出从沉积物到大气的一致的 CO2 和 CH4 排放，其中 TS 的 CO2 中值高约 60%，SS 的 CH4 流出中值高约 55%。使用深度剖面，在两个区域的 50-75cm 深度处观察到出人意料的高浓度 CO2（>200μM）和 CH4（>200μM），这些富含硫酸盐的沉积物中的 SS 较高（高达 17mM），这表明甲基营养型产甲烷作用发生在远离潮汐河道的盐沼沉积物剖面深处。此外，如果我们将 CH4 和 CO2 的中位深度值外推至全球盐沼的 5.3 Mha，这可以解释保守估计约 70 Gg 未计算的碳以气态形式（即 CH4 和 CO2）储存在沼泽沉积物中，在试图了解这些生态系统碳动态的当前模式和未来响应时应考虑到这一点。
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 CO2and CH4production 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), CO2and CH4production 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 ShortSpartina(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 CO2and CH4emissions from sediments to the atmosphere throughout the study period with TS having ∼60% higher median CO2and SS having ∼55% higher median CH4efflux. 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 CH4and CO2to 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., CH4and CO2) in marsh sediments, which should be considered when attempting to understand the current patterns and future responses of carbon dynamics from these ecosystems.