Elsevier

Organic Geochemistry

Volume 37, Issue 4, April 2006, Pages 501-510
Organic Geochemistry

Direct molecular evidence for the degradation and mobility of black carbon in soils from ultrahigh-resolution mass spectral analysis of dissolved organic matter from a fire-impacted forest soil

https://doi.org/10.1016/j.orggeochem.2005.11.003Get rights and content

Abstract

The molecular composition of water-soluble products generated by the natural degradation of charcoal particles over a period of 100 years in a temperate forest soil has been investigated by ultrahigh resolution mass spectrometry with electrospray ionization. The detectable products are condensed aromatic ring structures extensively substituted with oxygen-containing functional groups, indicating that oxidation and dissolution of charcoal black carbon occurs on a centennial timescale. Many of the same species are also detected within the dissolved organic matter (DOM) of the forest’s soil pore waters. We introduce the calculation of carbon normalized double bond equivalents (DBE/C) as a structural determinant for the empirical formulas obtained by mass spectral analysis. A threshold DBE/C value of 0.7 serves as a criterion for identifying species with condensed aromatic ring structures (CARS). A comparison with ultrahigh resolution mass spectra from previous studies shows that many of the CARS extracted directly from soil BC have the same mass (within 1 ppm) and empirical formulas as CARS detected in volcanic ash soil humic acid (HA) from Japan, and Amazonian Rio Negro DOM. The similarity of water-soluble condensed aromatics present within, and exported from fire-impacted soils of geographically and climatically disparate ecosystems indicates that the CARS reported herein are the molecular fingerprint of black carbon degradation in soils. Understanding the production mechanisms, reactivity, and fate of these molecular species should provide new insight to BC degradation and cycling. The soil charcoal particles at this site are infiltrated by filamentous microorganisms, suggesting that saprophytic fungi may be important to soil BC degradation processes.

Introduction

The ubiquity and abundance of black carbon (BC) in the environment have become increasingly urgent matters in geochemistry. Rapidly accumulating evidence suggests that BC, the refractory organic products of incomplete combustion, account for a substantial portion of molecularly uncharacterized natural organic matter (NOM) (Hedges et al., 2000). The contribution of BC as a percentage of NOM-C is 15–45% in soils (Schmidt et al., 1999) and 3–15% in ocean sediments (Gustafsson and Gschwend, 1998, Masiello and Druffel, 1998). Therefore, understanding the biogeochemical cycling of this carbon pool is essential to our efforts to model global-scale carbon cycling. A number of studies show that BC is an inherently refractory component of geomedia (Skjemstad et al., 1996, Glaser et al., 1998). Still other workers conclude that BC losses from soil and sediment over time are due to chemical degradation (Czimczik et al., 2003), and suggest that factors such as regional climatic conditions and soil properties mediate the residence time of BC (Schneour, 1966, Bird et al., 1999, Glaser and Amelung, 2003).

Despite the relative recalcitrance of BC in soils, it is clear that BC is not inert. For instance, charcoal plays important roles in soil formation (Glaser and Amelung, 2003), fertility (Glaser et al., 2000), humification (Shindo et al., 1986), carbon sequestration (Kuhlbusch and Crutzen, 1996, Glaser and Amelung, 2003), and pollutant availability (Gustafsson et al., 1997). Spectroscopic evidence that natural weathering or oxidative depolymerization of soil charcoal results in the formation of humic substances has been observed for several decades (e.g., Kumada, 1983, Schnitzer and Calderoni, 1985, Shindo et al., 1986, Kramer et al., 2004). The dominant structural features generated from pyrogen humification are condensed aromatics with carboxylic acid functionality. In controlled laboratory experiments, the nitric-acid-oxidation of furnace blacks and charcoals generated similar products: condensed aromatic structures substituted with carboxylic, phenolic, and carbonyl functional groups, with molecular weights in the range ca. 400–1200 Da (Haumaier and Zech, 1995, Kamegawa et al., 2002, Trompowsky et al., 2005). The high cation exchange capacity of these types of structures is thought to be responsible for correlations between soil fertility and BC content observed in tropical terra pretta soils and native grassland soils of North America (Glaser et al., 2000, Glaser and Amelung, 2003).

As discussed by Masiello (2004), natural degradation and mineralization of soil BC is certainly occurring, despite the paucity of evidence from environmental measurements explaining degradation pathways. For instance, Czimczik et al. (2003) showed that physical removal processes such as erosion and translocation through the soil profile can not account for the loss of BC from a boreal forest soil occurring in the 250 years following a biomass burning event. In situ chemical degradation/oxidation is also invoked as an explanation for BC depletion in sedimentary deposits that have become oxic after a period of anoxia (Middleburg et al., 1999). However, degradation mechanisms and byproducts remain marginally characterized at the molecular level and their biogeochemical fate remains unclear.

As the contribution of BC to the alkali-soluble fraction (HA) of soil organic matter is being recognized, Masiello and Druffel (1998) proposed that BC cycles as dissolved organic matter (DOM), and used the radioisotope signature with sedimentation rates to estimate that dissolved BC may comprise as much as 4–22% of marine DOM. Estimates from the thermal isolation of BC in ultrafiltered DOM from the Chesapeake Bay, suggest that BC contributes roughly 4–7% of the DOC in the coastal Atlantic Ocean (Mannino and Harvey, 2004). The first molecular-level evidence of BC-like structures in DOM was observed by Kim et al. (2003a). Specifically, ultrahigh-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry was used to analyze DOM from the Rio Negro, and a blackwater stream from the New Jersey Pine Barrens, revealing the presence of hydrogen-deficient molecules that are likely to be the soluble products of BC degradation (Kim et al., 2004).

We present evidence that directly links these hydrogen-deficient molecules to the degradation of charcoal BC in the soils of a fire-impacted watershed. We use ultrahigh-resolution electrospray ionization (ESI) FT-ICR to resolve BC degradation products within the soil pore water DOM at the site, and introduce a new structural criterion for identifying the condensed aromatic ring structures (CARS) that are directly linked to BC.

Section snippets

Experimental

Soil and water samples were collected during summer 2002 at the University of Michigan Biological Station (UMBS), in Cheboygan County, Michigan, where intense logging and biomass burning during the years 1890–1920 deposited a visible layer of BC to the soils. Soils at the station are excessively-drained, medium sand under a poorly developed, acidic O horizon (∼5 cm thick, pH 4.4). Soils are spodsols, specifically, Emmet medium, frigid, Typic Orthods. Development of the hard ornstein layer of

Results and discussion

ESI-FTICR mass spectrometry with dual electrospray ionization provided sufficient mass accuracy (<0.5 ppm) and resolving power (600,000 at 300 m/z) to assign unique empirical formulas to 95% of the 4000 signals in the mass spectrum of each sample, without a priori physical separation. Fig. 1 shows the ESI-FTICR mass spectrum of C18-extractable charcoal leachates, and several micrographs of soil charcoal surfaces. The 100-year-old charcoal particles from UMBS soils are inhabited by microorganisms

Conclusions

We report on the molecular composition of water-soluble products extracted directly from charcoal particles after 100 years of natural degradation in a forest soil. ESI FT-ICR mass spectrometry reveals condensed aromatic ring structures extensively substituted with oxygen-containing functional groups. The calculation of carbon normalized double bond equivalents from mass spectral data serves as a means to recognize BC degradation products by identifying species with condensed aromatic ring

Acknowledgements

We thank Dr. H. Knicker and an anonymous reviewer for their insightful comments that led to significant improvements of this manuscript. This work was supported by the National Science Foundation, Ohio State Environmental Molecular Science Institute (CHE-0089147 and CHE-0089172). We thank Dr. Alan Marshall and Dr. Ryan Rodgers at the National High Magnetic Field laboratory, Florida State University, for making available their 9.4 T FTICR-MS (funded through their NSF Grant CHE-9903528).

References (46)

  • S. Kim et al.

    High resolution electrospray ionization mass spectrometry and 2D NMR for the analysis of DOM extracted by C18 solid phase disk

    Organic Geochemistry

    (2003)
  • S. Kim et al.

    Hydrogen-deficient molecules in natural riverine water samples-evidence for the existence of black carbon in DOM

    Marine Chemistry

    (2004)
  • E.B. Kujawinski et al.

    Probing molecular-level transformations of dissolved organic matter: insights on photochemical degradation and protozoan modification of DOM from electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

    Marine Chemistry

    (2004)
  • C.A. Masiello

    New directions in black carbon organic geochemistry

    Marine Chemistry

    (2004)
  • M. Schnitzer et al.

    Some chemical characteristics of paleosol humic acids

    Chemical Geology

    (1985)
  • P.M. Trompowsky et al.

    Characterization of humic-like substances obtained by chemical oxidation to eucalyptus charcoal

    Organic Geochemisty

    (2005)
  • M.S. Akhter et al.

    The structure of hexane soot: I. Spectroscopic studies

    Applied Spectroscopy

    (1984)
  • G.F. Andrews et al.

    Bacterial film growth in adsorbent surfaces

    American Institute of Chemical Engineers Journal

    (1981)
  • M.I. Bird et al.

    Stability of elemental carbon in savanna soil

    Global Biogeochemical Cycles

    (1999)
  • Boose, D.L., 1986. Long-term changes in soil chemistry with forest succession. M.S. Thesis, Cornell University, Ithaca,...
  • D.E. Catcheside et al.

    Biological processing of coal

    Applied Microbiology and Biotechnology

    (1999)
  • C.I. Czimczik et al.

    How surface fire in Siberian Scots pine forests affects soil organic carbon in the forest floor: stocks, molecular structure, and conversion to black carbon (charcoal)

    Global Biogeochemical Cycles

    (2003)
  • A.F. Dickens et al.

    Reburial of fossil organic carbon in marine sediments

    Nature

    (2004)
  • Cited by (299)

    View all citing articles on Scopus
    View full text