Near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy for mapping nano‐scale distribution of organic carbon forms in soil: Application to black carbon particles

Abstract

[1] Small-scale heterogeneity of organic carbon (C) forms in soils is poorly quantified since appropriate analytical techniques were not available up to now. Specifically, tools for the identification of functional groups on the surface of micrometer-sized black C particles were not available up to now. Scanning Transmission X-ray Microscopy (STXM) using synchrotron radiation was used in conjunction with Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy to investigate nano-scale distribution (50-nm resolution) of C forms in black C particles and compared to synchrotron-based FTIR spectroscopy. A new embedding technique was developed that did not build on a C-based embedding medium and did not pose the risk of heat damage to the sample. Elemental sulfur (S) was melted to 220°C until it polymerized and quenched with liquid N 2 to obtain a very viscous plastic S in which the black C could be embedded until it hardened to a noncrystalline state and was ultrasectioned. Principal component and cluster analysis followed by singular value decomposition was able to resolve distinct areas in a black carbon particle. The core of the studied biomass-derived black C particles was highly aromatic even after thousands of years of exposure in soil and resembled the spectral characteristics of fresh charcoal. Surrounding this core and on the surface of the black C particle, however, much larger proportions of carboxylic and phenolic C forms were identified that were spatially and structurally distinct from the core of the particle. Cluster analysis provided evidence for both oxidation of the black C particle itself as well as adsorption of non-black C. NEXAFS spectroscopy has great potential to allow new insight into black C properties with important implications for biogeochemical cycles such as mineralization of black C in soils and sediments, and adsorption of C, nutrients, and pollutants as well as transport in the geosphere, hydrosphere, and atmosphere.

Long term change in chemical properties of preindustrial charcoal particles aged in forest and agricultural temperate soil

“…Through weathering, we evaluated from our data that charcoal gains on average 1.36 H atoms for one O. This may result from hydroxylation and carboxylation at edges of the graphene sheets in charcoal (Lehmann et al, 2005), but also from contamination by organic acids from soil solution (Pignatello et al, 2006) or microbial films in the porosity of charcoal (Lehmann et al, 2011). Nguyen et al (2008) estimated from XPS analysis an organic O/ C ratio of ca.…”

“…4). Ageing of charcoal in soil results mainly from oxidation starting from the surface and propagating to the core of the particles (Lehmann et al, 2005). Oxidation creates phenol, carbonyl and carboxyl functionalities at the edge of aromatic rings, responsible for the increase in cation exchange capacity of charcoal over time (Cheng et al, 2008a).…”

“…The drivers of oxidation can be biotic (Hamer et al, 2004;Wengel et al, 2006) but are mainly abiotic, at least shortly after introduction to soil (Cheng et al, 2006). The adsorption of dissolved organic compounds can contribute to the increase in O content at the surface of BC (Lehmann et al, 2005). Climatic factors such as water regime (Nguyen and Lehmann, 2009) and temperature were shown to influence the kinetics of alteration and decomposition of BC (Cheng et al, 2008b;Nguyen et al, 2010).…”

“…Ageing consists mainly of oxidation of exposed C rings with a high density of p electrons and free radicals , which creates a high density of O-rich functional groups at the BC surface. Oxidation starts at the surface and propagates to the core of particles over time (Lehmann et al, 2005), promoting further physical, chemical and microbial degradation (Hammes and Schmidt, 2009). The drivers of oxidation can be biotic (Hamer et al, 2004;Wengel et al, 2006) but are mainly abiotic, at least shortly after introduction to soil (Cheng et al, 2006).…”

Synchrotron‐Based Near‐Edge X‐Ray Spectroscopy of Natural Organic Matter in Soils and Sediments

“…Epoxy embedding media traditionally used in soil or sediment investigations (Tippkotter and Ritz, 1996) SyNCHROTRON-BASED NEAR-EDgE X-RAy SPECTROSCOPy compounds and are therefore not suitable for organic matter studies. Lehmann et al (2005) introduced an embedding approach for black carbon particles using elemental sulfur based on prior research that was done on interplanetary dust (Bradley et al, 1993;Flynn et al, 2004). The spatial integrity of entire aggregates is better preserved, however, when they are misted, frozen, and cryosectioned either with or without (Lehmann et al, 2007) additional embedding in elemental sulfur.…”