Organic carbon (C) in soil is the single largest pool of C on Earth's surface [Watson et al., 2000] and controls much of its ecosystem services with respect to greenhouse gas emissions (of both C and N) and C sequestration, soil fertility and plant productivity, and filtration of water during its passage through soil [Stevenson and Cole, 1999]. Not only the total amounts of C are relevant for the biogeochemistry of C in soil, but also its forms. Black C has only recently been recognized as an important organic C pool in soils and sediments forming a continuum from partly charred organic materials to charcoal, coal, soot, and graphite [Schmidt and Noack, 2000]. Black C is found in any soil analyzed for black C contents [Bird et al., 1999; Schmidt et al., 1999; Skjemstad et al., 2002; Glaser and Amelung, 2003], but questions remain about its residence time, alterations, and accumulation in terrestrial and aquatic systems [Dickens et al., 2004; Schmidt, 2004]. Given that existing studies show both rapid [Shneour, 1966; Bird et al., 1999] and slow [Shindo, 1991] rates of decomposition, it appears that a large part of black C is mineralized over short timescales, whereas a small part remains in a very stable, highly altered form displaying greater 14C ages than the oldest soil organic matter fractions [Pessenda et al., 2001]. The pool size of black C was successfully used to parameterize the inert organic matter pool of the RothC model of C turnover in soil [Skjemstad et al., 2004], demonstrating that soil organic matter dynamics can be better described with an improved understanding of black C. While Skjemstad et al.  successfully treated black C as an inert pool with respect to soil C dynamics, strong evidence suggests that nutrient dynamics are significantly influenced by black C [Glaser et al., 2001; Lehmann et al., 2003a], as, for example, Sombroek et al.  found a greater cation exchange capacity in soils with larger proportions of black C. Important interactions exist between black C surfaces and inorganic as well as organic pollutants [Ghosh et al., 2000; Accardi-Dey and Gschwend, 2002] and dissolved organic matter [Pietikäinen et al., 2000] which are controlled by C forms on black C particles that influence, for example, hydrophobicity and surface charge. Abiotic oxidation and mineralization by microorganisms first occurs on surfaces of black C particles, and quantification of its extent will provide important information about the longevity of black C in the biogeosphere as well as the factors influencing its disappearance. Finally, the solubility of black C in water is highly dependent on the degree of surface oxidation [Decesari et al., 2002] with consequences for transport in soil and water.
 Previous studies of black C were restricted to analyses of the properties of bulk black C isolates after oxidation of non-black C using ultraviolet (UV), thermal, or chemical oxidation [Skjemstad et al., 1999; Gelinas et al., 2001; Masiello et al., 2002]. Surface properties of unaltered C forms, however, may be more relevant to the study of black C effects on soil biogeochemistry than bulk properties of isolates as pointed out above. Owing to the alterations during oxidative separation and the small size of black C particles [Skjemstad et al., 1996], surface properties of black C have to our knowledge not been investigated up to now with respect to C compounds. Methods exist to study the morphology of single particles and their surfaces such as high-resolution transmission electron microscopy [Palotas et al., 1996; Gustafsson et al., 2001], to study certain C compounds such as PAH on black C using microprobe laser desorption and laser ionization mass spectrometry in 50-μm intervals or to identify C-rich patches in sediments using infrared analyses [Ghosh et al., 2000]. Micro-raman spectroscopy and electron energy loss spectroscopy were used to characterize broad categories of C forms, but were restricted to single points of a particle [Schmidt et al., 2002]. 13C NMR was able to resolve nano-scale spatial associations between C forms but without the possibility to study spatial distributions in situ [Skjemstad et al., 1997]. In contrast, the development of a method to characterize forms of C with high spatial resolution in sub-micrometer scale would, for example, allow the test of hypotheses that surface functional groups of black C particles significantly differ from those in the center of the black C particles, and that C on black C surfaces is more oxidized than at the center.
 Scanning Transmission X-ray Microscopy (STXM) using synchrotron radiation provides this opportunity to investigate nano-scale variations of C forms in soil through recent advances in X-ray microfocusing techniques [Jacobsen et al., 2000; Scheinost et al., 2001]. Using a tunable monochromator, Near-Edge X-ray Absorption Fine Structure (NEXAFS) can be obtained by increasing the photon energy through the absorption edge which is specific for each element and increases with atomic weight (284 eV for C) [Stöhr, 1992]. First, the core electron will be excited to the lowest unoccupied molecular orbital, while higher energy levels beyond the ionization threshold of C (∼290 eV) will promote the core electron to continuum. The excited stages of the inner electron are characteristic for the geometric and electronic structure of the molecule and can be correlated to specific C forms [Schäfer et al., 2003]. These features make NEXAFS using STXM a very sensitive technique to distinguish between, for example, aromatic-C, phenol-C, carbonyl-C, and carboxyl-C with a resolution of only several tens of nanometers and are ideal for the determination of very fine heterogeneity of C forms in soil.
 The challenge is to obtain thin sections with a thickness of around 100 nm that allow enough energy to penetrate the sample for detection. Since the objective of such an analysis would be to investigate functional groups of C, traditional C-based embedding materials cannot be used to obtain the thin sections as they interfere with high-resolution C analyses. This constraint excludes all materials that have commonly been used for embedding soil or geological materials such as resins and epoxy. In addition to water [Ghosh et al., 2000], elemental sulfur (S) possesses unique properties that make it an appropriate material to embed samples that will be analyzed for C, since elemental S does not contain C, is liquid not too far above room temperature (melting point at 113°C), and is solid at room temperature. Matrajt et al.  embedded micrometeorites in melted elemental S encapsulated in epoxy but were not interested in quantifying surface functional groups. A temperature of 100°C has been shown to significantly change C forms of humic substances [Lu et al., 2001], and C organic chemistry on particle surfaces exposed to liquid elemental S with temperatures above 113°C may therefore change significantly, while C at the center of the particle is most likely not affected. Bradley et al.  and Flynn et al.  embedded interplanetary dust particles in elemental S by super-cooling small droplets of melted S that stay liquid for several seconds to hours below the melting point (L. Keller, personal communication, 2002). The droplets have to be very small to remain in this super-cooled liquid state, which requires the size of the embedded specimen to be in the micrometer range. Black C particles from biomass burning are usually several micrometers large and are therefore difficult to embed in droplets. Additionally, the crystalline nature of the sulfur droplets after solidification poses challenges for sectioning. These constraints required the development of a new embedding procedure to explore the opportunities provided by NEXAFS for soil organic matter studies.
 The objectives of this study were (1) to develop a new method to embed small black C particles for the investigation of their C forms using STXM and C(1s) NEXAFS spectroscopy and (2) to evaluate the suitability of NEXAFS data and cluster analyses to distinguish between surface properties and different areas within the particles. NEXAFS data were compared with data obtained from synchrotron-based Fourier Transform Infrared (FTIR) spectroscopy, and the potential of the method for examining black C within global biogeochemical cycles is discussed.