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Soil Syst. 2018, 2(2), 32; https://doi.org/10.3390/soilsystems2020032

In Search of a Binding Agent: Nano-Scale Evidence of Preferential Carbon Associations with Poorly-Crystalline Mineral Phases in Physically-Stable, Clay-Sized Aggregates

1
Faculty of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan
2
Institute for Agro-Environmental Science, National Agriculture and Food Research Organization, 3-1-3 Tsukuba, Ibaraki 305-8604, Japan
3
Institute of Materials Structure Science, High-Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
4
Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, 1-3-2 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
5
Natural Science Center for Basic Research and Development, Hiroshima University, 1-3-2 Kagamiyama, Higashi-Hiroshima, Hiroshima 739–8526, Japan
6
Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Bunkyo-Ku, Tokyo 113-0033, Japan
These authors contributed equally to this work.
*
Authors to whom correspondence should be addressed.
Received: 2 December 2017 / Revised: 16 May 2018 / Accepted: 17 May 2018 / Published: 29 May 2018
(This article belongs to the Special Issue Soil Organic Matter Dynamics)
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Abstract

Mechanisms of protecting soil carbon (C) are still poorly understood despite growing needs to predict and manage the changes in soil C or organic matter (OM) under anticipated climate change. A fundamental question is how the submicron-scale interaction between OM and soil minerals, especially poorly-crystalline phases, affects soil physical aggregation and C stabilization. Nano-sized composites rich in OM and poorly-crystalline mineral phases were presumed to account for high aggregate stability in the Andisol we previously studied. Here we searched for these nanocomposites within a sonication-resistant aggregate using scanning transmission X-ray microscopy (STXM) and near-edge X-ray absorption fine structure (NEXAFS) as well as electron microscopy (SEM, TEM). Specifically, we hypothesized that nanometer-scale spatial distribution of OM is controlled by poorly-crystalline minerals as both co-exist as physically-stable nanocomposites. After maximum dispersion of the cultivated Andisol A-horizon sample in water, one aggregate (a few µm in diameter) was isolated from 0.2–2 µm size fraction which accounted for 44–47% of total C and N and 50% of poorly-crystalline minerals in bulk soil. This fraction as well as <0.2 µm fraction had much higher extractable Al and Fe contents and showed greater increase in specific surface area (N2-BET) upon OM oxidation compared to bulk and >2 µm size fractions, implying high abundance of the nanocomposites in the smaller fractions. The isolated aggregate showed a mosaic of two distinctive regions. Smooth surface regions showed low adsorption intensity of carbon K-edge photon energy (284–290 eV) with well-crystalline mineralogy, whereas rough surface regions had features indicative of the nanocomposites: aggregated nanostructure, high C intensity, X-ray amorphous mineral phase, and the dominance of Si, O, Al, and Fe based on SEM/EDX and TEM/EDX. Carbon functional group chemistry assessed by NEXAFS showed the dominance of amide and carboxyl C over aromatic and aliphatic C with some variation among the four rough surface regions. Together with C and N isotopic patterns among the size fractions (relatively low C:N ratio, high 15N natural abundance, and more positive Δ14C of the <2 μm fractions), our results provided the direct evidence of preferential binding of microbially-altered, potentially-labile C with poorly-crystalline mineral phases at submicron scale. The role of the nanocomposite inferred from this study may help to bridge the knowledge gap between physical aggregation process and biogeochemical reactions taking place within the soil physical structure. View Full-Text
Keywords: carbon sequestration; volcanic ash soil; Andisols; scanning transmission X-ray microscopy; aggregate hierarchy; specific surface area; allophane; particle size fractionation carbon sequestration; volcanic ash soil; Andisols; scanning transmission X-ray microscopy; aggregate hierarchy; specific surface area; allophane; particle size fractionation
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Asano, M.; Wagai, R.; Yamaguchi, N.; Takeichi, Y.; Maeda, M.; Suga, H.; Takahashi, Y. In Search of a Binding Agent: Nano-Scale Evidence of Preferential Carbon Associations with Poorly-Crystalline Mineral Phases in Physically-Stable, Clay-Sized Aggregates. Soil Syst. 2018, 2, 32.

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