%0 Journal Article
%T In Search of a Binding Agent: Nano-Scale Evidence of Preferential Carbon Associations with Poorly-Crystalline Mineral Phases in Physically-Stable, Clay-Sized Aggregates
%A Hiroki Suga
%A Maki Asano
%A Makoto Maeda
%A Noriko Yamaguchi
%A Rota Wagai
%A Yasuo Takeichi
%A Yoshio Takahashi
%J -
%D 2018
%R https://doi.org/10.3390/soilsystems2020032
%X 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 ¦Ìm in diameter) was isolated from 0.2¨C2 ¦Ìm size fraction which accounted for 44¨C47% of total C and N and 50% of poorly-crystalline minerals in bulk soil. This fraction as well as <0.2 ¦Ìm fraction had much higher extractable Al and Fe contents and showed greater increase in specific surface area (N 2-BET) upon OM oxidation compared to bulk and >2 ¦Ìm 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¨C290 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 ¦¤ 14C of the <2 ¦Ìm 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
%K carbon sequestration
%K volcanic ash soil
%K Andisols
%K scanning transmission X-ray microscopy
%K aggregate hierarchy
%K specific surface area
%K allophane
%K particle size fractionation
%U https://www.mdpi.com/2571-8789/2/2/32