Global significance of iron (Fe) and aluminum (Al) for the storage of organic matter (OM) in soils and surface sediments is increasingly recognized. Yet specific metal phases involved or the mechanism behind metal–OM correlations frequently shown across soils remain unclear. We identified the allocation of major metal phases and OM to density fractions using 23 soil samples from five climate zones and five soil orders (Andisols, Spodosols, Inceptisols, Mollisols, Ultisols) from Asia and North America, including several subsurface horizons and both natural and managed soils. Each soil was separated into four to seven density fractions using sodium polytungstate with mechanical shaking, followed by the sequential extraction of each fraction with pyrophosphate (PP), acid oxalate (OX), and finally dithionite–citrate (DC) to estimate pedogenic metal phases of different solubility and crystallinity. The concentrations of Fe and Al (per fraction) extracted by each of the three reagents were generally higher in meso-density fractions (1.8–2.4 g cm⁻³) than in the lower- or higher-density fractions, showing a unique unimodal pattern along the particle density gradient for each soil. Across the studied soils, the maximum metal concentrations were always at the meso-density range within which PP-extractable metals peaked at 0.3–0.4 g cm⁻³ lower-density range relative to OX- and DC-extractable metals. Meso-density fractions, consisting largely of aggregated clusters based on SEM observation, accounted for on average 56 %–70 % of total extractable metals and OM present in these soils. The OM in meso-density fractions showed a 2–23 unit lower C : N ratio than the lowest-density fraction of the respective soil and thus appeared microbially processed relative to the original plant material. The amounts of PP- and OX-extractable metals correlated positively with co-dissolved C across the soils and, to some extent, across the density fractions within each soil. These results led to a hypothesis which involves two distinct levels of organo-metal interaction: (1) the formation of OM-rich, mixed metal phases with fixed OM : metal stoichiometry followed by (2) the development of meso-density microaggregates via “gluing” action of these organo-metallic phases by entraining other organic and mineral particles such as phyllosilicate clays. Given that OM is mainly located in meso-density fractions, a soil's capacity to protect OM may be controlled by the balance of three processes: (i) microbial processing of plant-derived OM, (ii) dissolution of metals, and (iii) the synthesis of organo-metallic phases and their association with clays to form meso-density microaggregates. The current hypothesis may help to fill the gap between well-studied molecular-scale interaction (e.g., OM adsorption on mineral surface, coprecipitation) and larger-scale processes such as aggregation, C accrual, and pedogenesis.

中文翻译：

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铁和铝通过土壤中观密度聚集体的形成与微生物处理的有机物缔合：有机金属胶假设

铁（Fe）和铝（Al）对于在土壤和表层沉积物中存储有机物（OM）的全球重要性日益得到认可。然而，尚不清楚具体涉及的金属相或在土壤中经常显示出的金属-OM相关性背后的机理。我们使用来自亚洲和北美的5个气候区和5个土壤阶（Andisols，Spodosol，Inceptisols，Mollisols，Ultisols）的23种土壤样品，确定了主要金属相和OM对密度分数的分配，包括一些地下视野以及天然和受控土壤。使用多钨酸钠在机械震荡下将每种土壤分成4至7个密度馏分，然后依次用焦磷酸盐（PP），草酸酸（OX），最后是连二亚硫酸柠檬酸盐（DC）来估计溶解度和结晶度不同的成岩金属相。三种试剂中每种试剂提取的铁和铝的浓度（每部分）的中密度部分（1.8-2.4 g cm^-3），而不是较低或较高的密度分数，沿每种土壤的颗粒密度梯度显示唯一的单峰模式。在整个研究的土壤中，最大金属浓度始终在中密度范围内，在该范围内，PP可萃取金属的峰值为0.3–0.4 g cm ^-3。相对于OX和DC可萃取金属，密度范围较低。中等密度分数主要由基于SEM观察的聚集团簇组成，平均占这些土壤中总可提取金属和OM的56％–70％。中密度分数的OM比各自土壤的最低密度分数显示的C：N比低2–23个单位，因此相对于原始植物材料，经过微生物处理。PP和OX萃取金属的含量与土壤中共同溶解的C呈正相关，在某种程度上，每种土壤中的密度分数也呈正相关。这些结果导致了一个假设，该假设涉及两个不同级别的有机金属相互作用：（1）形成具有固定OM的富含OM的混合金属相：金属化学计量，然后（2）通过夹带其他有机和矿物颗粒（例如页硅酸盐粘土），通过这些有机金属相的“胶合”作用，形成中密度微聚集体。鉴于OM主要位于中密度部分中，土壤的保护OM的能力可通过以下三个过程的平衡来控制：（i）植物来源的OM的微生物处理，（ii）金属的溶解和（iii）有机金属相的合成及其与粘土的缔合以形成中密度微聚集体。当前的假设可能有助于填补经过充分研究的分子尺度相互作用（例如，矿物表面上的OM吸附，共沉淀）与大规模过程（如聚集，C累积和成岩作用）之间的空白。