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https://doi.org/10.5194/soil-2020-32
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/soil-2020-32
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: original research article 11 Jun 2020

Submitted as: original research article | 11 Jun 2020

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Iron and aluminum association with microbially processed organic matter via meso-density aggregate formation across soils: organo-metallic glue hypothesis

Rota Wagai1, Masako Kajiura1, and Maki Asano2 Rota Wagai et al.
  • 1National Agriculture and Food Research Organization, Institute for Agro-Environmental Sciences. 3-1-3 Kan-nondai, Tsukuba, Ibaraki, 305-8604, Japan
  • 2Faculty of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan

Abstract. 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 density fraction locations of major metal phases and OM using 23 soil samples from 5 climate zones and 5 soil orders (Andisols, Spodosols, Inceptisols, Mollisols, Ultisols), including several subsurface horizons and both natural and managed soils. Each soil was separated to 4 to 7 density fractions using sodium polytungstate with mechanical shaking, followed by the sequential extraction of each fraction with pyrophosphate (PP), acid oxalate (OX), and finally with dithionite-citrate (DC) to estimate pedogenic metal phases of different solubility and crystallinity. The extractable Fe and Al concentrations (per fraction) generally showed unique unimodal distribution along particle density gradient for each soil and each extractable metal phase. Across the studied soils, the maximum metal concentrations were always at meso-density range (1.8–2.4 g cm−3) 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, consisted largely of microaggregates 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 appeared microbially processed from the original plant material. The amounts of PP- and OX-extractable metals correlated positively with co-dissolved C among 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 – the formation of OM-rich, mixed metal phases having relatively fixed OM : metal stoichiometry and subsequent development of meso-density microaggregates via gluing properties of these organo-metallic phases by incorporating other organic and mineral particles such as phyllosilicate clays. Given that stable OM is mainly located in meso-density fractions, soil's capacity to protect OM may be controlled by the balance of following 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 scales interaction (e.g., OM adsorption on mineral surface, coprecipitation) and larger-scale processes such as aggregation, C accrual, and pedogenesis.

Rota Wagai et al.

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Rota Wagai et al.

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Short summary
Global significance of metals (extractable Fe and Al phases) to control organic matter (OM) in recognized. Next key questions include the identification of their localization and mechanisms behind OM-metal relationships. Across 23 soils of contrasting mineralogy, Fe and Al phases were mainly associated with microbially-processed OM as meso-density microaggregates. OM- and metal-rich nanocomposites having narrow OM : metal ratio likely acted as binding agents. A new conceptual model was proposed.
Global significance of metals (extractable Fe and Al phases) to control organic matter (OM) in...
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