SSSAJ: Volume 73: Number 6 • November–December 2009 1817 Soil Sci. Soc. Am. J. 73:1817-1830 doi:10.2136/sssaj2008.0228 Received 7 July 2008. *Corresponding author (DS278@cornell.edu). © Soil Science Society of America 677 S. Segoe Rd. Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. N atural organic matter is a heterogeneous mixture of organic molecules representing both compounds released from living plant and microbial cells (e.g., extracellular enzymes, sur- face-active proteins, chelating compounds, etc.) to plant, animal, microbial, and charred carbonaceous residues ranging in size and complexity from simple monomers to mixtures of complex biopolymers (Ladd et al., 1993; Solomon et al., 2007a). Natural organic matter is a highly active component of soils, water, and sediments and plays an important role in ecosystem processes. Present ubiquitously in the environment, NOM has deied mo- lecular-level structural and functional characterization for nearly a century, owing primarily to its existence mostly as highly func- tionalized polyelectrolytes, which as such do not lend themselves to analytical techniques for molecular characterization (Sleighter and Hatcher, 2007). Variations along the decomposition and size continuum, as well as its ability to form strong associations with minerals create further analytical problems that have made Dawit Solomon* Johannes Lehmann James Kinyangi Biqing Liang Karen Heymann Lena Dathe Kelly Hanley Cornell University College of Agriculture and Life Sciences Ithaca, NY 14853 Sue Wirick Chris Jacobsen State University of New York at Stony Brook Department of Physics and Astronomy Stony Brook, NY 11794 SOIL CHEMISTRY Carbon (1s) NEXAFS Spectroscopy of Biogeochemically Relevant Reference Organic Compounds Natural organic matter (NOM) is a highly active component of soils and sediments, and plays an important role in global C cycling. However, NOM has deied molecular-level structural characterization, owing to variations along the decomposition continuum and its existence as highly functionalized polyelectrolytes. We conducted a comprehensive systematic overview of spectral signatures and peak positions of major organic molecules that occur as part of NOM using near-edge x-ray absorption ine structure (NEXAFS) spectroscopy. he spectra of carbohydrates and amino sugars show resonances between 289.10 and 289.59 eV, attributed to 1s-3p/σ* transitions of O-alkyl (C-OH) moieties. hey also exhibited distinct peaks between 288.42 and 288.74 eV, representing C 1s–π* C = O transition from COOH functionalities. Amino acids produced a strong signal around 288.70 eV, which can be identiied as a C 1s–π* C=O transition of carboxyl/carbonyl (COOH/ COO-) structures. Spectral features near 285.29 eV were ascribed to C 1s–π* C=C transition of ring structure of aromatic amino acids, while spectra between 287.14 and 287.86 eV were attributed to C 1s–π* C-H and C 1s–σ* C-H/ 3p Rydberg-like excitations from CH and CH 2 groups. Phenols and benzoquinone produced strong resonances between 285.08 and 285.37 eV, attributed to the π* orbital of C (C 1s–π* C=C ) atoms connected to either C or H (C–H) in the aromatic ring. he next higher excitation common to both phenols and quinone appeared between 286.05 and 286.35 eV, and could be associated with C 1s–π* C=C transitions of aromatic C bonded to O atom in phenols, and to C 1s–π* C=O transitions from aromatic C connected to O atom (C-OH) in phenols or to a C=O in p-benzoquinone and some phenols with carbonyl structures, respectively. Nucleobases exhibited complex spectral features with pronounced resonances between 286.02 and 286.84 eV and between 288.01 and 288.70 eV. Molecular markers for black C (benzenecarboxylic acid and biphenyl- 4,4’-dicarboxylic acid) exhibit sharp absorption bands between 285.01 and at 285.43 eV, possibly from C 1s–π* C=C transition characteristic of C-H sites or unsaturated C (C=C) on aromatic ring structures. hese aromatic carboxylic acids also exhibit broad peaks between 288.35 and 288.48 eV, relecting C 1s–π* C=O transition of carboxyl functional groups bonded to unsaturated C. his investigation provides a more comprehensive NEXAFS spectral library of biogeochemically relevant organic C compounds. he spectra of these reference organic compounds reveal distinct spectral features and peak positions at the C K-edge that are characteristic of the molecular orbitals bonding C atoms. Detailed structural information can be derived from these distinctive spectral features that could be used to build robust peak assignment criteria to exploit the chemical sensitivity of NEXAFS spectroscopy for in situ molecular-level spatial investigation and ingerprinting of complex organic C compounds in environmental samples. Abbreviations: EDX, energy dispersive x-ray; NEXAFS, near-edge x-ray absorption ine structure spectroscopy; NOM, natural organic matter; SEM, scanning electron microscopy; STXM, scanning transmission x-ray microscopy; TEM transmission electron microscope; XPS, x-ray photoelectron spectroscopy.