In search of a binding agent: Nano-scale evidence of preferential carbon associations with poorly-crystalline mineral phases in physically-stable, clay-sized aggregates

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, high15N natural abundance, and more positive ∆14C 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.