Trace metal are essential for microbially-mediated biogeochemical processes occurring in anoxic wetland soils and stream bed sediments, but low availability of these elements may inhibit anaerobic element cycling and transformations. Solid-phase speciation is likely a critical control on trace metal availability but has seen limited study in anoxic systems having concentrations similar to geological background levels, where metal limitations may be most prevalent. We have investigated trace metal concentrations and solid-phase speciation in three freshwater subsurface aquatic systems: marsh wetland soils, riparian wetland soils, and the sediments of a streambed. These systems displayed low solid-phase trace metal concentrations, generally at or below geological background levels, which generally followed the trend Zn > Cu ≈ Ni > Co and showed no correlation with major element compositions. All soils and sediments were dominated by quartz but varied in clay mineralogy as well as the organic matter, total sulfur, and total iron contents. X-ray absorption near-edge structure (XANES) spectroscopy shows that sulfur speciation in both wetlands is dominated by organic sulfur. Elemental sulfur and iron sulfides together made up <25% of the sulfur in the wetland soils, but the distribution between inorganic and organic forms was reversed in the stream sediments. Ferrous and ferric iron in clay minerals were common species identified by both XANES and extended X-ray absorption fine structure (EXAFS) spectroscopies at all sites. Iron(III) oxides were substantial components in all but the marsh wetland soils. Quantitative analysis of copper, nickel, and zinc XANES spectra revealed similar metal speciation across all sites. Copper speciation was dominated by sulfides, adsorbed species, and minor amounts of copper bound to organic matter; no metallic copper was detected. Nickel speciation also varied little and was dominated by nickel in clay mineral octahedral sheets and nickel sulfide, with adsorbed species also present. Zinc speciation was slightly more varied, with the marsh wetland soils and stream bed sediments containing adsorbed species, zinc associated with clay mineral structures, and zinc bound to reduced sulfur groups on organic matter, whereas the riparian wetland soils lacked clay-associated zinc but contained zinc sulfide. Trace metals bound to reduced sulfur occurred at every site, with a greater sulfur-bound fraction for copper. The fractional abundance of sulfur-bound species showed no relationship with soil or sediment total sulfur content, which varied by two orders of magnitude. More broadly, the observations in this study suggest that trace metal speciation in freshwater wetland soils and stream sediments is consistently dominated by a small set of recurring components which are distinct for each metal. This may represent a general geochemical phenomenon in anoxic soils and sediments containing trace metals at background concentrations (as low as 3 µg g) that was not predicted from systems that are contaminated with or naturally-enriched in copper, nickel, or zinc.

中文翻译：

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对湿地土壤和溪流沉积物中痕量金属微量营养素形态的一致控制

微量金属对于缺氧湿地土壤和河床沉积物中发生的微生物介导的生物地球化学过程至关重要，但这些元素的低可用性可能会抑制厌氧元素的循环和转化。固相形态可能是对痕量金属可用性的关键控制，但在浓度与地质背景水平相似的缺氧系统中的研究有限，其中金属限制可能是最普遍的。我们研究了三种淡水地下水生系统中的痕量金属浓度和固相形态：沼泽湿地土壤、河岸湿地土壤和河床沉积物。这些系统显示出较低的固相痕量金属浓度，通常等于或低于地质背景水平，通常遵循 Zn > Cu ≈ Ni > Co 的趋势，并且与主要元素组成没有相关性。所有土壤和沉积物均以石英为主，但粘土矿物学以及有机质、总硫和总铁含量各不相同。 X 射线吸收近边结构 (XANES) 光谱表明，两个湿地中的硫形态均以有机硫为主。元素硫和硫化铁合计占湿地土壤中硫的<25%，但在河流沉积物中无机和有机形式的分布却相反。粘土矿物中的二价铁和三价铁是所有地点的 XANES 和扩展 X 射线吸收精细结构 (EXAFS) 光谱鉴定的常见物种。除沼泽湿地土壤外，氧化铁（III）是所有土壤中的重要成分。对铜、镍和锌 XANES 光谱的定量分析显示，所有位点的金属形态相似。铜的形态主要由硫化物、吸附物质和少量与有机物结合的铜组成；没有检测到金属铜。镍形态也变化不大，主要是粘土矿物八面体片中的镍和硫化镍，也存在吸附物质。锌的形态略有变化，沼泽湿地土壤和河床沉积物含有吸附物质、与粘土矿物结构相关的锌以及与有机质上还原的硫基团结合的锌，而河岸湿地土壤缺乏与粘土相关的锌，但含有硫化锌。与还原硫结合的痕量金属在每个地点都有出现，其中铜与硫结合的比例更大。硫结合物种的丰度分数与土壤或沉积物总硫含量没有关系，总硫含量相差两个数量级。更广泛地说，本研究的观察结果表明，淡水湿地土壤和溪流沉积物中的痕量金属形态始终由一小部分重复成分主导，这些成分对于每种金属都是不同的。这可能代表了缺氧土壤和沉积物中含有背景浓度（低至 3 µg g）痕量金属的普遍地球化学现象，而受到铜、镍或锌污染或天然富集的系统并未预测到这一现象。