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Black carbon yields and types in forest and cultivated sandy soils (Landes de Gascogne, France) as determined with different methods: Influence of change in land use
Organic Geochemistry Organic Geochemistry 37 (2006) 1185–1189 www.elsevier.com/locate/orggeochem
Note
Black carbon yields and types in forest and cultivated sandy soils (Landes de Gascogne, France) as determined with different methods: Influence of change in land use K. Que´ne´a a
e
a,b
, S. Derenne a,*, C. Rumpel b, J.-N. Rouzaud c, O. Gustafsson d, C. Carcaillet e, A. Mariotti b, C. Largeau a
LBCOP, UMR CNRS 7618 BIOEMCO, ENSCP, 11 rue P. et M. Curie, 75231 Paris cedex 05, France b BIOEMCO, INRA-CNRS-UPMC, baˆt EGER, 78850 Thiverval-grignon, France c Laboratoire de Ge´ologie, ENS, 24 rue Lhomond, 75231 Paris cedex 05, France d Department of Applied Environmental Science, Stockholm University, Sweden Centre de Bio-Arche´ologie et d’Ecologie (UMR5059), EPHE, Institut de Botanique, Montpellier, France Received 9 November 2005; received in revised form 27 April 2006; accepted 17 May 2006 Available online 7 July 2006
Abstract Black carbon (BC) was isolated from sandy soils of a pine forest reference plot and an adjacent plot used for maize cropping since forest clearing 22 years ago. This was performed by: (i) isolation of a refractory organic macromolecular fraction (ROM) using strong hydrolysis followed by chemo-thermal oxidation (CTO) and (ii) direct hand-picking of the untreated soils. Much lower BC contents, ca. ·300, were obtained with the ROM–CTO approach. Experiments on reference chars from the ‘‘international BC-ring trial’’ and high resolution, transmission electron microscopy (HRTEM) observations showed that this large difference was not due to BC component losses resulting from the strong hydrolysis during ROM isolation but was due primarily to complete removal of char/charcoal upon CTO. BC is heavily dominated by char/ charcoal and soot only affords a very low contribution in both soils. Calculations showed that BC accounts for a substantial part, ca. 13%, of total ROM and change in land-use resulted in a large loss of BC relative to the forest soil, ca. 60% after 22 years, thus supporting recent questions raised about BC persistence in soil. 2006 Elsevier Ltd. All rights reserved.
1. Introduction Black carbon (BC), the carbonaceous residue from incomplete combustion of biomass or fossil fuels, appears to be ubiquitous in soil and sediments * Corresponding author. Tel.: +33 1 4427 6716; fax: +33 1 4325 7975. E-mail address: [email protected] (S. Derenne).
(e.g. Kuhlbusch, 1998). It is believed to play an important role in some major geochemical processes, such as global carbon cycling, and organic pollutant and heavy metal transport in natural environments (e.g. Schmidt and Noack, 2000). Nevertheless, its abundance in soil is still largely unknown (e.g. Golchin et al., 1997). These uncertainties are related to the fact that BC is a generic term for a broad continuum of more or less
0146-6380/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2006.05.010
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condensed and carbonised products, ranging from slightly charred materials to graphitic soot (e.g. Schmidt and Noack, 2000). Various methods have been developed for BC measurement (reviewed by Schmidt and Noack, 2000; Gustafsson et al., 2001; Nguyen et al., 2004; Elmquist et al., 2004), including chemical, thermal and optical methods. The former two methods aim at eliminating all the non-BC components for subsequent BC measurement, while the last is based on microscopic observations for counting or hand-picking of BC particles. Different methods often detect different parts of the BC continuum (e.g. Schmidt et al., 2001); hence the inherent discrepancies in quantitative data. Previous studies showed the presence of BC in the refractory organic macromolecular fraction (ROM), the insoluble fraction exhibiting a conspicuous resistance to strong laboratory hydrolysis, isolated from various soils (Poirier et al., 2000, 2002, 2003). BC occurrence was recently reported in the ROMs from two sandy soils of Cestas, Landes de Gascogne, S.W. France (Que´ne´a, 2004; Que´ne´a et al., 2005a,b). These ROMs were isolated from soil from two adjacent plots: a reference plot, where the pine forest was preserved and a plot cleared 22 years ago and since then continuously used for intensive maize cropping. Direct evidence for BC occurrence in the ROMs was obtained using HRTEM (high resolution transmission electron microscopy), but this method provides no information on BC abundance. The present study was aimed at deriving quantitative information about BC content in both soils and its possible variation, due to change in land use. To this end: (i) BC was obtained using two markedly distinct methods: chemo-thermal oxidation (CTO) of the isolated ROMs and hand-picking of the untreated soils, (ii) two reference chars from the ‘‘international BC ring trial’’ were treated as for ROM isolation so as to better understand the significance of these measurements and (iii) the materials obtained after CTO were re examined using HRTEM.
ne´a et al., 2005a,b). In brief, a bulk representative sample was obtained from each plot by mixing ca. 100 sub-samples, and the sampling depth for the forest soil was corrected for differences in bulk density. For ROM isolation: (i) lipids, fulvic and humic acids were removed by extraction, (ii) the insoluble residue was submitted to stepwise hydrolysis using trifluoroacetic acid and HCl and (iii) demineralisation was carried out using concentrated HCl and HF. Transparency of the supernatant was checked after centrifugation, performed to separate the solid material left following the different steps, so as to ensure complete recovery. CTO was performed on the ROMs under optimised conditions ensuring access to excess oxygen throughout the sample (Gustafsson et al., 2001). The ROMs were heated at 375 C for 24 h under a 200 ml.min 1 air flow. BC in the CTO residues was determined using a Carlo Erba 1106 elemental analyser. HRTEM observations on the CTO residues (002 lattice fringe mode) were performed as described by Que´ne´a et al. (2005b), using a Philips CM 20 microscope working at 200 kV. BC isolation by hand-picking was performed directly on the whole soils after sieving at 350 lm (Carcaillet and Talon, 2001). Organic particles were separated by flotation with an ascending water flow, and charred particles were sorted using a microscope (·40). Observations at higher magnification (·500) were performed to ensure that the material thus isolated only contained BC particles. BC amount was determined by weighing after drying. The reference chars, a pine wood char and a rice grass char, were prepared for the BC ring trial from large batches obtained under controlled conditions and thoroughly homogenised (Schmidt et al., 2003). They were submitted to the same treatment as above for ROM isolation. Weight loss was determined for each step after drying. 3. Results and discussion 3.1. BC yield from ROMs
2. Materials and methods Extensive pine plantations were set up ca. 1850 on the Gascogne moors. Huge fires occurred in 1937 and 1949, destroying 450,000 ha (i.e. half of the whole forest). Reforestation was carried out in the early 1950s and some plots were cleared in the 1980s for maize cropping. Soil sampling and ROM isolation were as previously carried out (Que´-
BC isolated using CTO showed a very low content of total organic carbon (TOC): 0.016% and 0.008% for the forest and cultivated soil, respectively (Table 1). Its abundance relative to total ROM was also calculated based on ROM content for each soil (34% and 20% of TOC for the forest and cropped soil, respectively). Similar values in the 0.040–0.050% range were thus obtained (Table
K. Que´ne´a et al. / Organic Geochemistry 37 (2006) 1185–1189
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Table 1 BC yields from Cestas soils by chemo-thermal oxidation of refractory organic fraction and direct hand-picking of untreated soil Forest soil
CTO Hand-picking
Cultivated soil
BC (% soil TOC)
BC (% ROM)
BC (% soil TOC)
BC (% ROM)
0.016 4.6
0.048 13.8
0.008 2.5
0.040 12.5
1). Much higher BC contents were obtained through hand-picking. Indeed, hand-picking afforded values around three hundred times greater than those for the CTO results (e.g. 13.8% relative to ROM for the forest soil instead of only 0.048% with CTO). Two hypotheses could be considered for explaining such a large discrepancy: First, part of the BC components would be lost upon the treatment used for ROM isolation, since CTO is applied on the isolated ROM, whereas hand-picking is directly performed on the untreated soil. Second, CTO would result not only in the removal of non-BC compounds, but also in the elimination of the less resistant BC components. Indeed, HRTEM (Que´ne´a, 2004; Que´ne´a et al., 2005b) showed the presence of two main types of BC particles in the Cestas ROM, i.e. char/charcoal and soot. The former corresponds to relatively large, irregularly shaped, particles composed of poorly ordered polyaromatic basic structural units. The latter corresponds to smaller spherical particles, ca. 20 nm in diameter, with highly organised basic units forming a concentric ‘‘onion-like’’ microtexture. Although quantitative information on BC abundance can hardly be provided by HRTEM observations, these soot particles seemed to represent a much smaller fraction of BC than char/charcoal. Previous studies (e.g. Gustafsson et al., 2001) showed that BC resistance to oxidation increases, along with the degree of carbonisation, from char to charcoal and soot. As a result, soot appears stable during CTO whereas char/charcoal can undergo partial to complete elimination (e.g. Gustafsson et al., 2001; Nguyen et al., 2004). Experiments on reference chars from the BC-ring trial and HRTEM observations on CTO residues were carried out to test the above assumptions, respectively. 3.2. BC losses upon ROM isolation Only weak to negligible losses occurred for the wood char when submitted to the different steps in
Table 2 Loss (wt%) from reference chars of BC-ring trial submitted to different steps for treatment used for ROM isolation
Wood char Grass char
‘‘Lipid’’ extraction
‘‘Humic’’ extraction
Acid hydrolysis
Total recovery
0.2 2.8
1 3
<0.1 13.4
98.7 80.8
the isolation treatment (Table 2) and the final recovery was 99 wt%. On the contrary, substantial weight losses occurred for the grass char, especially after acid hydrolysis, with a final recovery of ca. 80 wt%. Elemental analysis showed a lower level of carbonisation, hence less aromatisation, for the grass char than for the wood char. This difference is reflected in a higher H/C atomic ratio for the grass char (0.66 vs. 0.54). It therefore appears that the strong hydrolysis used or ROM isolation may result in char BC elimination. However, such elimination would preferentially affect weakly aromatized char and, even in that case, would remain moderate. Accordingly, the loss of BC components during ROM isolation is, at most, a minor factor and is far from accounting for the huge difference in BC content noted between ROM-CTO and hand-picking measurements. 3.3. BC losses upon CTO HRTEM observations on the BC retained after CTO of the ROMs showed only soot ‘‘onion-like’’ particles (Fig. 1), while char/charcoal particles predominated in the two ROMs before oxidation. The CTO treatment at 375 C for 24 h, optimised for ensuring entire removal of the non-BC compounds, thus also resulted in complete elimination of the char/charcoal components of BC. This feature accounts for the bulk of the marked decrease observed in BC content obtained via CTO vs. hand-picking. Such a strong degradation is consistent with recent studies showing complete removal of laboratory-produced wood chars (Nguyen et al., 2004) and of char/charcoal in soil and sediment samples (Gelinas et al., 2001) with the CTO approach.
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Fig. 1. HRTEM observation of residue obtained after CTO of ROM from cultivated soil: only ‘‘onion-like (soot) particles, generally ca. 20 nm in diameter, like the one showed here, often aggregated in strands, are retained (same for cropped soil).
Taken together, the above quantitative and microscopic data indicate that BC in these two forest fire-impacted soils is almost exclusively composed of char/charcoal while soot only affords an extremely low contribution. This is consistent with the much greater contribution to ground residue from large char/charcoal than fine soot after biomass burning (Suman et al., 1996). The soot contents obtained for the Cestas soils using CTO, from ca. 0.01% to 0.02% of soil TOC, are comparable to, but still lower than, the values reported by Gelinas et al. (2001), 0.06–0.1%, for various Australian soils. However, the latter were probably more frequently affected by fire than the Cestas soils. Char/charcoal accounts for a substantial part (ca. 12% and 14% of the total ROM) for both soils, as shown by the measurements after hand-picking (Table 1). Moreover, these values are probably somewhat underestimated since the finest char/charcoal particles (<350 lm) are not measured using the hand-picking method (Carcaillet and Talon, 2001). 3.4. BC losses following change in land use Forest clearing and extensive cropping resulted in a TOC decrease from 2.6 wt% for the forest soil to 2 wt% for the cropped soil after 22 years cultivation.
From these values and those reported in Table 1 for soot content (% TOC, measured with CTO), it can be calculated that ca. 60% of the soot in the cropped soil was lost after 22 years compared to the forest soil. A similar calculation for char/charcoal, measured using hand-picking, showed a similar level of loss of ca. 58%. This similarity was somewhat unexpected since it is generally considered that soot is more resistant than the less aromatized BC forms. However, BC biodegradation is not only related to its chemistry but also to the physical features among char BCs whose resistance increases as the particles become larger (e.g. Baldock and Smernik, 2002). In fact, in the present case, since a hand-picking method was used, char/charcoal measurements were concerned with relatively large particles. The observed losses may also reflect, at least in part, greater BC erosion and/or translocation to deep horizons due to cultivation. However, erosion should be limited in such flat areas and there are no indications of important charcoal translocation to deeper parts of the cultivated soil. BC has often been considered as exhibiting a great resistance to degradation in soil and thus to largely contribute to the slow cycling carbon pool in soil organic matter (e.g. Schmidt and Noack, 2000). However, BC persistence in soil has recently been questioned (e.g. Hamer et al., 2004) and our observations suggest that substantial BC degradation took place in the Cestas soil, on a time scale of decades, following change in land use. Acknowledgements Drs. D. Arrouays (Inra, Orle´ans) and M. Schmidt (Zurich University) are gratefully acknowledged for providing soil and char samples and general information on the samples. We thank Sarah Ivorra for help in the laboratory with charcoal hand-picking, and Dr. E. Krull and an anonymous reviewer for constructive comments. Associate Editor—I. Ko¨gel-Knabner References Baldock, J.A., Smernik, R.J., 2002. Chemical composition and bioavailability of thermally altered Pinus resinosa (red pine) wood. Organic Geochemistry 33, 1093–1109. Carcaillet, C., Talon, B., 2001. Soil carbon sequestration by Holocene fires inferred from soil charcoal in the dry French Alps. Arctic, Antarctic, and Alpine Research 33, 282–288.
K. Que´ne´a et al. / Organic Geochemistry 37 (2006) 1185–1189 Elmquist, M., Gustafsson, O., Anderson, P., 2004. Quantification of sedimentary black carbon using the chemothermal oxidation method: an evaluation of ex-situ pretreatments and standard additions approaches. Limnology and Oceanography: Methods 2, 417–427. Gelinas, Y., Prentice, K.M., Baldock, J.A., Hedges, J.I., 2001. An improved thermal oxidation method for the quantification of soot-graphitic black carbon in sediments and soils. Environmental Science and Technology 35, 3519–3525. Golchin, A., Clarke, P., Baldock, J.A., Higashi, T., Skjemstad, J.O., Oades, J.M., 1997. The effects of vegetation and burning on the chemical composition of soil organic matter in a volcanic ash soil as shown by 13C NMR spectroscopy. I. Whole soil and humic acid fraction. Geoderma 76, 155–174. Gustafsson, O., Bucheli, T.D., Kukulska, Z., Anderson, M., Largeau, C., Rouzaud, J.-N., Reddy, C.M., Eglinton, T.I., 2001. Evaluation of a protocol for the quantification of black carbon in sediments. Global Biogeochemical Cycles 15, 881– 890. Hamer, U., Marschner, B., Brodowski, S., Amelung, W., 2004. Interactive priming of black carbon and glucose mineralisation. Organic Geochemistry 35, 823–830. Kuhlbusch, T.A.J., 1998. Black carbon and the carbon cycle. Science 280, 1903–1904. Nguyen, T.H., Brown, R.A., Ball, W.P., 2004. An evaluation of thermal resistance as a measure of black carbon content in diesel soot, wood char, and sediment. Organic Geochemistry 35, 217–234. Poirier, N., Derenne, S., Rouzaud, J.-N., Largeau, C., Mariotti, A., Balesdent, J., Maquet, J., 2000. Chemical structure and sources of the macromolecular, resistant, organic fraction isolated from a forest soil (Lacade´e, France). Organic Geochemistry 31, 813–827. Poirier, N., Derenne, S., Balesdent, J., Rouzaud, J.-N., Mariotti, A., Largeau, C., 2002. Abundance and composition of the refractory organic fraction of an ancient, tropical soil (Pointe Noire, Congo). Organic Geochemistry 33, 383–391.
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Poirier, N., Derenne, S., Balesdent, J., Mariotti, A., Massiot, D., Largeau, C., 2003. Isolation and analysis of the non-hydrolysable fraction of a forest soil and an arable soil (Lacade´e, southwest France). European Journal of Soil Science 54, 243– 255. Que´ne´a K., 2004. Etude structurale et dynamique des fractions lipidiques et organiques re´fractaires de sols d’une chronose´quence foreˆt/maı¨s (Cestas, Sud-Ouest France). PhD thesis, Universite´ Pierre et Marie Curie, Paris. Que´ne´a, K., Derenne, S., Largeau, C., Rumpel, C., Mariotti, A., 2005a. Spectroscopic and pyrolytic features and abundance of the macromolecular refractory fraction in a sandy acid forest soil (Landes de Gascogne, France). Organic Geochemistry 36, 349–362. Que´ne´a, K., Derenne, S., Gonza´lez-Vila, F.J., Mariotti, A., Rouzaud, J.N., Largeau, C., 2005b. Study of the composition of the macromolecular refractory fraction from an acidic sandy forest soil (Landes de Gascogne, France) using chemical degradation and electron microscopy. Organic Geochemistry 36, 1151–1162. Schmidt, M.W.I., Noack, A.G., 2000. Black carbon in soils and sediments: analysis, distribution, implications, and current challenges. Global Biogeochemical Cycles 14, 777–793. Schmidt, M.W.I., Skjemstad, J.O., Czimczik, C.I., Glaser, B., Prentice, K.M., Gelinas, Y., Kuhlbusch, T.A.J., 2001. Comparative analysis of black carbon in soils. Global Biogeochemical Cycles 15, 163–167. Schmidt, M.W.I., Masiello, C.A., Skjemstad, J.O., 2003. Final recommendations for reference materials in black carbon analysis. EOS 84, 582–583. Suman, D.O., Kuhlbusch, T.A.J., Lim, B., 1996. Marine sediments: a reservoir for black carbon and their use as spatial and temporal records of combustion. In: Clark, J.S., Cachier, H., Goldhammer, J.G., Stocks, B. (Eds.), Sediment Records of Biomass Burning and Global Change. Springer Verlag, New York, pp. 271–293.
Note
Black carbon yields and types in forest and cultivated sandy soils (Landes de Gascogne, France) as determined with different methods: Influence of change in land use K. Que´ne´a a
e
a,b
, S. Derenne a,*, C. Rumpel b, J.-N. Rouzaud c, O. Gustafsson d, C. Carcaillet e, A. Mariotti b, C. Largeau a
LBCOP, UMR CNRS 7618 BIOEMCO, ENSCP, 11 rue P. et M. Curie, 75231 Paris cedex 05, France b BIOEMCO, INRA-CNRS-UPMC, baˆt EGER, 78850 Thiverval-grignon, France c Laboratoire de Ge´ologie, ENS, 24 rue Lhomond, 75231 Paris cedex 05, France d Department of Applied Environmental Science, Stockholm University, Sweden Centre de Bio-Arche´ologie et d’Ecologie (UMR5059), EPHE, Institut de Botanique, Montpellier, France Received 9 November 2005; received in revised form 27 April 2006; accepted 17 May 2006 Available online 7 July 2006
Abstract Black carbon (BC) was isolated from sandy soils of a pine forest reference plot and an adjacent plot used for maize cropping since forest clearing 22 years ago. This was performed by: (i) isolation of a refractory organic macromolecular fraction (ROM) using strong hydrolysis followed by chemo-thermal oxidation (CTO) and (ii) direct hand-picking of the untreated soils. Much lower BC contents, ca. ·300, were obtained with the ROM–CTO approach. Experiments on reference chars from the ‘‘international BC-ring trial’’ and high resolution, transmission electron microscopy (HRTEM) observations showed that this large difference was not due to BC component losses resulting from the strong hydrolysis during ROM isolation but was due primarily to complete removal of char/charcoal upon CTO. BC is heavily dominated by char/ charcoal and soot only affords a very low contribution in both soils. Calculations showed that BC accounts for a substantial part, ca. 13%, of total ROM and change in land-use resulted in a large loss of BC relative to the forest soil, ca. 60% after 22 years, thus supporting recent questions raised about BC persistence in soil. 2006 Elsevier Ltd. All rights reserved.
1. Introduction Black carbon (BC), the carbonaceous residue from incomplete combustion of biomass or fossil fuels, appears to be ubiquitous in soil and sediments * Corresponding author. Tel.: +33 1 4427 6716; fax: +33 1 4325 7975. E-mail address: [email protected] (S. Derenne).
(e.g. Kuhlbusch, 1998). It is believed to play an important role in some major geochemical processes, such as global carbon cycling, and organic pollutant and heavy metal transport in natural environments (e.g. Schmidt and Noack, 2000). Nevertheless, its abundance in soil is still largely unknown (e.g. Golchin et al., 1997). These uncertainties are related to the fact that BC is a generic term for a broad continuum of more or less
0146-6380/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2006.05.010
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condensed and carbonised products, ranging from slightly charred materials to graphitic soot (e.g. Schmidt and Noack, 2000). Various methods have been developed for BC measurement (reviewed by Schmidt and Noack, 2000; Gustafsson et al., 2001; Nguyen et al., 2004; Elmquist et al., 2004), including chemical, thermal and optical methods. The former two methods aim at eliminating all the non-BC components for subsequent BC measurement, while the last is based on microscopic observations for counting or hand-picking of BC particles. Different methods often detect different parts of the BC continuum (e.g. Schmidt et al., 2001); hence the inherent discrepancies in quantitative data. Previous studies showed the presence of BC in the refractory organic macromolecular fraction (ROM), the insoluble fraction exhibiting a conspicuous resistance to strong laboratory hydrolysis, isolated from various soils (Poirier et al., 2000, 2002, 2003). BC occurrence was recently reported in the ROMs from two sandy soils of Cestas, Landes de Gascogne, S.W. France (Que´ne´a, 2004; Que´ne´a et al., 2005a,b). These ROMs were isolated from soil from two adjacent plots: a reference plot, where the pine forest was preserved and a plot cleared 22 years ago and since then continuously used for intensive maize cropping. Direct evidence for BC occurrence in the ROMs was obtained using HRTEM (high resolution transmission electron microscopy), but this method provides no information on BC abundance. The present study was aimed at deriving quantitative information about BC content in both soils and its possible variation, due to change in land use. To this end: (i) BC was obtained using two markedly distinct methods: chemo-thermal oxidation (CTO) of the isolated ROMs and hand-picking of the untreated soils, (ii) two reference chars from the ‘‘international BC ring trial’’ were treated as for ROM isolation so as to better understand the significance of these measurements and (iii) the materials obtained after CTO were re examined using HRTEM.
ne´a et al., 2005a,b). In brief, a bulk representative sample was obtained from each plot by mixing ca. 100 sub-samples, and the sampling depth for the forest soil was corrected for differences in bulk density. For ROM isolation: (i) lipids, fulvic and humic acids were removed by extraction, (ii) the insoluble residue was submitted to stepwise hydrolysis using trifluoroacetic acid and HCl and (iii) demineralisation was carried out using concentrated HCl and HF. Transparency of the supernatant was checked after centrifugation, performed to separate the solid material left following the different steps, so as to ensure complete recovery. CTO was performed on the ROMs under optimised conditions ensuring access to excess oxygen throughout the sample (Gustafsson et al., 2001). The ROMs were heated at 375 C for 24 h under a 200 ml.min 1 air flow. BC in the CTO residues was determined using a Carlo Erba 1106 elemental analyser. HRTEM observations on the CTO residues (002 lattice fringe mode) were performed as described by Que´ne´a et al. (2005b), using a Philips CM 20 microscope working at 200 kV. BC isolation by hand-picking was performed directly on the whole soils after sieving at 350 lm (Carcaillet and Talon, 2001). Organic particles were separated by flotation with an ascending water flow, and charred particles were sorted using a microscope (·40). Observations at higher magnification (·500) were performed to ensure that the material thus isolated only contained BC particles. BC amount was determined by weighing after drying. The reference chars, a pine wood char and a rice grass char, were prepared for the BC ring trial from large batches obtained under controlled conditions and thoroughly homogenised (Schmidt et al., 2003). They were submitted to the same treatment as above for ROM isolation. Weight loss was determined for each step after drying. 3. Results and discussion 3.1. BC yield from ROMs
2. Materials and methods Extensive pine plantations were set up ca. 1850 on the Gascogne moors. Huge fires occurred in 1937 and 1949, destroying 450,000 ha (i.e. half of the whole forest). Reforestation was carried out in the early 1950s and some plots were cleared in the 1980s for maize cropping. Soil sampling and ROM isolation were as previously carried out (Que´-
BC isolated using CTO showed a very low content of total organic carbon (TOC): 0.016% and 0.008% for the forest and cultivated soil, respectively (Table 1). Its abundance relative to total ROM was also calculated based on ROM content for each soil (34% and 20% of TOC for the forest and cropped soil, respectively). Similar values in the 0.040–0.050% range were thus obtained (Table
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Table 1 BC yields from Cestas soils by chemo-thermal oxidation of refractory organic fraction and direct hand-picking of untreated soil Forest soil
CTO Hand-picking
Cultivated soil
BC (% soil TOC)
BC (% ROM)
BC (% soil TOC)
BC (% ROM)
0.016 4.6
0.048 13.8
0.008 2.5
0.040 12.5
1). Much higher BC contents were obtained through hand-picking. Indeed, hand-picking afforded values around three hundred times greater than those for the CTO results (e.g. 13.8% relative to ROM for the forest soil instead of only 0.048% with CTO). Two hypotheses could be considered for explaining such a large discrepancy: First, part of the BC components would be lost upon the treatment used for ROM isolation, since CTO is applied on the isolated ROM, whereas hand-picking is directly performed on the untreated soil. Second, CTO would result not only in the removal of non-BC compounds, but also in the elimination of the less resistant BC components. Indeed, HRTEM (Que´ne´a, 2004; Que´ne´a et al., 2005b) showed the presence of two main types of BC particles in the Cestas ROM, i.e. char/charcoal and soot. The former corresponds to relatively large, irregularly shaped, particles composed of poorly ordered polyaromatic basic structural units. The latter corresponds to smaller spherical particles, ca. 20 nm in diameter, with highly organised basic units forming a concentric ‘‘onion-like’’ microtexture. Although quantitative information on BC abundance can hardly be provided by HRTEM observations, these soot particles seemed to represent a much smaller fraction of BC than char/charcoal. Previous studies (e.g. Gustafsson et al., 2001) showed that BC resistance to oxidation increases, along with the degree of carbonisation, from char to charcoal and soot. As a result, soot appears stable during CTO whereas char/charcoal can undergo partial to complete elimination (e.g. Gustafsson et al., 2001; Nguyen et al., 2004). Experiments on reference chars from the BC-ring trial and HRTEM observations on CTO residues were carried out to test the above assumptions, respectively. 3.2. BC losses upon ROM isolation Only weak to negligible losses occurred for the wood char when submitted to the different steps in
Table 2 Loss (wt%) from reference chars of BC-ring trial submitted to different steps for treatment used for ROM isolation
Wood char Grass char
‘‘Lipid’’ extraction
‘‘Humic’’ extraction
Acid hydrolysis
Total recovery
0.2 2.8
1 3
<0.1 13.4
98.7 80.8
the isolation treatment (Table 2) and the final recovery was 99 wt%. On the contrary, substantial weight losses occurred for the grass char, especially after acid hydrolysis, with a final recovery of ca. 80 wt%. Elemental analysis showed a lower level of carbonisation, hence less aromatisation, for the grass char than for the wood char. This difference is reflected in a higher H/C atomic ratio for the grass char (0.66 vs. 0.54). It therefore appears that the strong hydrolysis used or ROM isolation may result in char BC elimination. However, such elimination would preferentially affect weakly aromatized char and, even in that case, would remain moderate. Accordingly, the loss of BC components during ROM isolation is, at most, a minor factor and is far from accounting for the huge difference in BC content noted between ROM-CTO and hand-picking measurements. 3.3. BC losses upon CTO HRTEM observations on the BC retained after CTO of the ROMs showed only soot ‘‘onion-like’’ particles (Fig. 1), while char/charcoal particles predominated in the two ROMs before oxidation. The CTO treatment at 375 C for 24 h, optimised for ensuring entire removal of the non-BC compounds, thus also resulted in complete elimination of the char/charcoal components of BC. This feature accounts for the bulk of the marked decrease observed in BC content obtained via CTO vs. hand-picking. Such a strong degradation is consistent with recent studies showing complete removal of laboratory-produced wood chars (Nguyen et al., 2004) and of char/charcoal in soil and sediment samples (Gelinas et al., 2001) with the CTO approach.
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Fig. 1. HRTEM observation of residue obtained after CTO of ROM from cultivated soil: only ‘‘onion-like (soot) particles, generally ca. 20 nm in diameter, like the one showed here, often aggregated in strands, are retained (same for cropped soil).
Taken together, the above quantitative and microscopic data indicate that BC in these two forest fire-impacted soils is almost exclusively composed of char/charcoal while soot only affords an extremely low contribution. This is consistent with the much greater contribution to ground residue from large char/charcoal than fine soot after biomass burning (Suman et al., 1996). The soot contents obtained for the Cestas soils using CTO, from ca. 0.01% to 0.02% of soil TOC, are comparable to, but still lower than, the values reported by Gelinas et al. (2001), 0.06–0.1%, for various Australian soils. However, the latter were probably more frequently affected by fire than the Cestas soils. Char/charcoal accounts for a substantial part (ca. 12% and 14% of the total ROM) for both soils, as shown by the measurements after hand-picking (Table 1). Moreover, these values are probably somewhat underestimated since the finest char/charcoal particles (<350 lm) are not measured using the hand-picking method (Carcaillet and Talon, 2001). 3.4. BC losses following change in land use Forest clearing and extensive cropping resulted in a TOC decrease from 2.6 wt% for the forest soil to 2 wt% for the cropped soil after 22 years cultivation.
From these values and those reported in Table 1 for soot content (% TOC, measured with CTO), it can be calculated that ca. 60% of the soot in the cropped soil was lost after 22 years compared to the forest soil. A similar calculation for char/charcoal, measured using hand-picking, showed a similar level of loss of ca. 58%. This similarity was somewhat unexpected since it is generally considered that soot is more resistant than the less aromatized BC forms. However, BC biodegradation is not only related to its chemistry but also to the physical features among char BCs whose resistance increases as the particles become larger (e.g. Baldock and Smernik, 2002). In fact, in the present case, since a hand-picking method was used, char/charcoal measurements were concerned with relatively large particles. The observed losses may also reflect, at least in part, greater BC erosion and/or translocation to deep horizons due to cultivation. However, erosion should be limited in such flat areas and there are no indications of important charcoal translocation to deeper parts of the cultivated soil. BC has often been considered as exhibiting a great resistance to degradation in soil and thus to largely contribute to the slow cycling carbon pool in soil organic matter (e.g. Schmidt and Noack, 2000). However, BC persistence in soil has recently been questioned (e.g. Hamer et al., 2004) and our observations suggest that substantial BC degradation took place in the Cestas soil, on a time scale of decades, following change in land use. Acknowledgements Drs. D. Arrouays (Inra, Orle´ans) and M. Schmidt (Zurich University) are gratefully acknowledged for providing soil and char samples and general information on the samples. We thank Sarah Ivorra for help in the laboratory with charcoal hand-picking, and Dr. E. Krull and an anonymous reviewer for constructive comments. Associate Editor—I. Ko¨gel-Knabner References Baldock, J.A., Smernik, R.J., 2002. Chemical composition and bioavailability of thermally altered Pinus resinosa (red pine) wood. Organic Geochemistry 33, 1093–1109. Carcaillet, C., Talon, B., 2001. Soil carbon sequestration by Holocene fires inferred from soil charcoal in the dry French Alps. Arctic, Antarctic, and Alpine Research 33, 282–288.
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