Organic Geochemistry 41 (2010) 690–697

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Organic Geochemistry

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Thermal alteration of organic matter during a shrubland fire: A field study

M.A. Alexis a,*, C. Rumpel a, H. Knicker b, J. Leifeld c, D. Rasse d, N. Péchot a, G. Bardoux a, A. Mariotti a

a Laboratoire de Biogéochimie et Ecologie des Milieux Continentaux, Centre INRA Versailles-Grignon, 78850 Thiverval-Grignon, Franceb Lehrstuhl für Bodenkunde, Technische Universität München, D-85350 Freising-Weihenstephan, Germanyc Swiss Federal Research Station for Agroecology and Agriculture, Reckenholzstrasse 191, CH-8046 Zürich, Switzerlandd Bioforsk, Norwegian Institute for Agricultural and Environmental Research, Frederik Dahls Vei, 1432 Ås, Norway

a r t i c l e i n f o

Article history:Received 12 June 2009Received in revised form 4 March 2010Accepted 9 March 2010Available online 11 March 2010

0146-6380/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.orggeochem.2010.03.003

* Corresponding author at: Université Pierre et MTour 56-66, 4 place Jussieu, 75252 Paris cedex 5, Franfax: +33 (0) 144274164.

E-mail address: marie.alexis@upmc.fr (M.A. Alexis

a b s t r a c t

Vegetation fires profoundly alter the C cycle of terrestrial ecosystems, notably through the potential for-mation of highly stable pyrogenic structures. Fire-induced changes in the structure of organic matter(OM) have been studied mainly under controlled laboratory conditions. The objective of this work wasto characterise changes in OM chemistry occurring in the litter layer of a scrub–oak ecosystem subjectedto a prescribed fire. Maximum temperatures reached during the fire were monitored with thermo-sensi-tive paint. Litter samples collected before and after the fire were subjected to size fractionation, each sizefraction being divided on the basis of visual observation into burnt and unburnt components, i.e. blackand brown, respectively. All fractions were analysed for C and N contents and stable carbon isotopic com-position. Shifts in the composition of >2 mm fraction components were evaluated using solid-state 13Cnuclear magnetic resonance (NMR) spectroscopy and differential scanning calorimetry (DSC).

The effects of charring were evaluated through comparisons of black vs. brown post-fire litter in>370 �C plots. Comparison of bulk pre- vs. post-fire litter proved unreliable because of the complicatingeffect of fresh litter fall during the fire event. Charring significantly increased the litter C content by 115–142 mg g�1 and significantly decreased the d13C value by an average of 0.8‰. The NMR and DSC analysesindicated that O-alkyl compounds were preferentially lost vs. aryl and alkyl compounds. This suggests apreferential loss of cellulose components and a relative preservation of lignin and lipids. However, thecharred litter samples had a low degree of condensation vs. a graphitic-like model. The findings suggestthat leaf-derived charcoal produced during natural vegetation fires does not contribute much to thehighly stable fraction of pyrogenic OM.

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1. Introduction

Vegetation fires profoundly alter the C cycle of terrestrial eco-systems. During fire events, a substantial amount of C is releasedto the atmosphere as greenhouse gases and aerosols, thereby mod-ifying the radiative forcing (Hansen et al., 2000; Novakov et al.,2003). At the same time, pyrogenic OM is produced (Jones et al.,1997). The production occurs mainly in the litter layer, while themineral soil is often little affected by fire (Certini, 2005). PyrogenicC is often considered highly stable in soil and sediments (Masiello,2004). Some studies report that it accumulates in soils, while oth-ers report lower than expected contributions to stable soil C pools(Czimczik et al., 2003; Rumpel et al., 2008). Quantification andcharacterisation of charcoal in soil compartments is therefore cru-cial for understanding the mean residence time of pyrogenic C inthe environment.

ll rights reserved.

arie Curie, Paris VI, 4� étage,ce. Tel.: +33 (0) 144275027;

).

Exposure of OM to temperatures >150 �C induces dehydrationand aromatic core formation, both leading to increased C and Ncontent and decreased H and O content (Baldock and Smernik,2002). Above 200 �C, the OM contains fewer O-alkyl structures,such as cellulose, and more aromatic C, as evidenced from solid-state 13C nuclear magnetic resonance spectroscopy with crosspolarisation-magic angle spinning (13C CPMAS NMR) and diffusereflectance infrared Fourier transform (FTIR) (Almendros et al.,1992; Pastorova et al., 1994; Knicker et al., 1996; Bilba and Ouen-sanga, 1996; Pietikainen et al. 2000; Baldock and Smernik, 2002;Czimczik et al., 2002). Above 250 �C, a slight increase in the car-bonyl region and a marked increase in the aromatic region of thespectra are observed. This enrichment in aromatic C results frompreferential O-alkyl degradation and neoformation of aromaticstructures (Almendros et al., 2003). Above 350 �C, polyaromaticcompounds are formed (Paris et al., 2005).

Effects of fire on OM d13C values are poorly understood andinconsistent (Beuning and Scott, 2002; Czimczik et al., 2002; Krullet al., 2003; Turney et al., 2006; Rumpel et al., 2007). The contrast-ing results suggest that multiple processes are involved at differentstages of the thermal alteration.

M.A. Alexis et al. / Organic Geochemistry 41 (2010) 690–697 691

Most of the above studies were conducted using artificial heat-ing under laboratory conditions. It is not clear whether results ob-tained during such laboratory studies directly apply to OMtransformation under natural fire conditions. Laboratory burningusually entails heating fresh material at different temperatures ina furnace under controlled conditions. As a result, oxygen availabil-ity, maximum and mean temperatures, exposure duration and airmoisture content are highly stable during the experiments. In con-trast, these parameters are highly variable in space and time dur-ing a natural fire event (Marion et al., 1991; Kuhlbusch et al.,1996; Schulze et al., 2000; Fearnside et al., 2001; Gimeno-Garciaet al., 2004; Certini, 2005).

In laboratory studies, temperature is often comparatively low andduration of exposure long (Knicker et al., 1996; Pietikainen et al.,2000; Baldock and Smernik, 2002; Almendros et al., 2003). The useof high temperature is generally associated with limited oxygen con-ditions (Bilba and Ouensanga, 1996; Czimczik et al., 2002; Krull et al.2003; Turney et al., 2006). These conditions reduce both mass lossand C loss, and increase the aromatic C content of the resulting pyro-genic OM. In contrast, low combustion temperatures are expected togenerate pyrogenic OM with low C and aromatic C contents.

Specific burning conditions prevailing during natural fires arenot the only difference between laboratory and field studies. In aprevious study, we showed that a substantial proportion of post-fire litter originates from non-senescent leaves falling during thefire (Alexis et al., 2007). These fallen leaves differ from those inpre-fire litter since the latter have been altered via senescence.Such fire-induced transfer of OM among ecosystem compartmentsis not considered in laboratory experiments. The objective of thepresent study was to follow changes in the chemistry of litter-layerOM induced by a prescribed fire under field conditions.

2. Materials and methods

2.1. Study site

The site is located on the Merritt Island Wildlife National Refugein Florida. The ecosystem is an oak scrub composed mainly of oaksand palm trees: Serenoa repens, Quercus myrtifolia, Quercus chapma-nii, Quercus geminata, Lyonia ferruginea and Myrica cerifera are themain species. In the past, fire occurred naturally during the dryseason that extends from November to April. Since the 1950s thescrub ecosystem has been managed with prescribed fires to protectNASA facilities and to conserve the habitat of an endangered birdspecies. In 2004, a prescribed fire was conducted in a 30 ha areacalled Shiloh (details in Alexis et al., 2007, Fig. 1). The fire returninterval is �11 yrs, and shrubs were 2–3 m high. Before the Shilohfire, we prepared a transect of 20 experimental plots (each 1 m2)about 5 m apart. We paid special attention not to disturb the plantand litter structure before the prescribed burn.

During the fire, living vegetation dried, the leaves fell from thetrees to the ground and contributed to >50% of the post-fire litterC (Alexis et al., 2007). We collected samples from ‘standing vegeta-tion leaves’ on living trees before the fire and the litter layer before(‘pre-fire litter’) and after (‘post-fire litter’) the fire. The post-fire lit-ter consisted of black and brown material. With the brown litterbeing located mainly under the black layer, we hypothesised thatbrown material was unaffected by the fire, whereas black materialcorresponded to charred litter. In the following, these fractions arecalled ‘brown post-fire litter’ and ‘black post-fire litter’, respectively.

2.2. Temperature measurement

Sixteen samples of thermo-sensitive paint for different temper-atures (Tempil Division, Big Three Industries, Inc., So. Plainfield, NJ,

USA), applied on copper tags, were used to assess the maximumtemperature reached during the fire. Paint melting point rangedfrom 80 to 870 �C. The day before the fire, the paint tags wereplaced on the litter surface at the centre of the post-fire litter sam-pling ring. They were recovered 3 days after the fire. It is importantto notice that the paint only reflects the local maximum tempera-ture reached during the fire. Moreover, the difficulty in interpret-ing the meaning of partial melting of the paint stripes generateduncertainty in fire temperature assessment. We assessed that theerror due to paint reading was equal to the maximum gap betweentwo different paint samples, i.e. 56 �C.

2.3. Sampling

For each of the six main ecosystem species, six samples ofstanding vegetation leaves were randomly collected from trees ofthe Shiloh area, outside the plots. They were pooled to constituteone typical fresh leaves sample per species.

Six litter samples were collected using a 27 cm diameter plasticring before and after the fire (Fig. 1). To avoid disturbing the fireconditions, 1 m was chosen as the distance to separate the pre-firesample position from the post-fire sample and temperature tag’sposition. Samples were oven dried at 75 �C over 1 week. Size frac-tionation was conducted by way of dry sieving pre- and post-firelitter samples. Fractions were >2 mm, 1–2 mm, 0.5–1 mm, 0.25–0.5 mm and <0.25 mm. They were weighed to assess their relativecontribution to total biomass.

In addition, four post-fire litter samples were collected with thesame method in plots representative of a range of temperatureexposures. They were divided into three size fractions (>2 mm,1–2 mm and <1 mm) and black and brown particles were visuallyidentified and handpicked from each fraction. Partially charredparticles were not selected.

2.4. Elemental and isotopic composition

All vegetation and leaf litter fractions were analyzed for C and Ncontents and stable carbon isotopic composition using an elemen-tal analyser (Fisons Carlo Erba NA1500) coupled to an isotope ratiomass spectrometer (MICROMASS-OPTIMA). Instrumental precisionwas 1% for elemental composition and 0.3‰ for d13C value. Aweighted average was calculated, using the relative contributionof each species to total ecosystem biomass (%) for the standing veg-etation leaves (data from Alexis et al. (2007)) and the relative con-tribution of each size fraction (%) for total pre- and post-fire litter.The distribution of brown and black post-fire litter C between sizefractions could not be directly measured and values were calcu-lated using the black particles proportion in each size fraction(Alexis et al., 2007) and the contribution of each size fraction to to-tal post-fire litter.

2.5. Solid-state 13C CPMAS NMR spectroscopy

Carbon distribution in litter OM was investigated using 13CCPMAS NMR for the size-fractions of pre-fire litter, brown post-firelitter and black post-fire litter (n = 1). As shown below (Sec-tion 3.3.1), no significant differences appeared in the compositionof the size fractions and we concentrated on the most abundantfraction (>2 mm) to study the influence of temperature. Four sam-ples of >2 mm black post-fire litter that had been exposed to differ-ent temperatures were analysed. Spectra were recorded using aBruker DSX 200 NMR spectrometer with a resonance frequencyof 50.32 MHz, and a 7 mm zirconium rotor. CP–MAS was appliedwith a spinning speed of 6.8 kHz. The contact time was 1 ms andthe pulse delay was 300 ms for post-fire litter and 800 ms forpre-fire litter.

ba

Fig. 1. Sampling location in Shiloh area. (a) Picture of area before fire event in 2004. The 20 plots of the transect are at the bottom right hand side of the picture (scale 100 m).(b) Scheme of sampling set-up for each sampling plot (scale 1 m). Squares show measurement plot, circles litter sampling and crosses soil sampling position. Dotted lineslocate pre-fire sampling and continuous lines post-fire sampling.

692 M.A. Alexis et al. / Organic Geochemistry 41 (2010) 690–697

Spectra were divided into four regions of chemical shift: alkyl C(0–45 ppm), O-alkyl C (45–110 ppm), aromatic C (110–160 ppm)and carbonyl C (160–220 ppm). The proportion of these differentC types was assessed by integrating the area under the spectrum,as indicated by Knicker and Ludemann (1995). A correction forspinning side bands in samples with high aromatic C contentwas carried out after the procedure of Knicker (2004) and Knickeret al. (2005a,b).

2.6. Differential scanning calorimetry

Thermal properties of >2 mm brown and black post-fire littercollected after the fire were analysed using a differential scanningcalorimeter (DSC100, TA Instruments). Between 2 and 3 mg mate-rial were weighed into an open Al pan and heated in a stream of50 ml min�1 synthetic air from ambient temperature to 600 �C at20 �C min�1 with an empty pan serving as a reference. The runswere analyzed for their 50% burnoff temperature after integrationof the whole thermogram and for the height and the temperatureof the last exotherm (W g�1 sample) occurring at temperatures>500 �C (see Leifeld, 2007). The ‘‘50% burnoff” parameter reflectsthe thermal resistance of the material and its increase can showa loss of labile C as well as an increase in thermally stable OM (suchas aromatic C). The height of the peak at temperatures >500 �C isdirectly proportional to the amount of thermally stable materialin the samples (Leifeld, 2007), whereas the temperature of thepeak reflects the thermal resistance of the stable C.

2.7. Statistical analysis

Differences among fresh vegetation leaves, pre-fire litter andpost-fire litter in mean C and N concentrations and d13C weretested with the non-parametric Mann–Whitney method, whichaccommodates comparisons among independent groups withnon-Gaussian distributions. The P value assessed the chance thata random sampling would result in medians as far apart as ob-served in the experiment. Difference was considered significantfor P < 0.05.

Paired t-tests of pre-fire litter vs. post-fire litter and black vs.brown post-fire litter were conducted for C and N contents andd13C values (Microsoft Excel� software). In this case, data werecompared for the total litter and inside of each size fraction pairingsamples from the same plot. Differences were considered signifi-cant for P < 0.05.

3. Results and discussion

3.1. Maximum fire temperature

Litter layer temperature reached during the prescribed fire ran-ged between 370 �C and 760 �C, with 30% of the plots experiencingtemperatures >600 �C. The mean temperature was 490 ± 110 �C(data not shown; see Alexis et al., 2007). In the six plots randomlyselected for analysis before and after the fire, the temperaturereached ca. 430 �C in five plots and ca. 650 �C in one plot, the meanbeing ca. 470 �C. The four plots chosen as representative of a rangeof fire exposure reached temperatures of 370, 430, 590 and 650 �C,respectively. As expected, the complete temperature range ob-served during this natural fire was wider than that generally usedin laboratory heating experiments (Bilba and Ouensanga, 1996;Krull et al. 2003; Turney et al., 2006).

3.2. Elemental and isotopic composition

C and N contents significantly decreased in the following order:standing vegetation leaves > post-fire litter > pre-fire litter (Table 1;P < 0.05). At the size fraction level, significant differences in ele-mental and isotopic composition of pre-and post-fire litter wereobserved only for some size fractions (Fig. 2). The comparison be-tween the total pre- and post-fire litter reflected only a trend to in-crease in N content (P < 0.01).

Black post-fire litter had a significantly higher C content(+134 mg g�1; P < 0.001) and a significantly lower d13C value(�0.6‰; P < 0.01) than brown post-fire litter (Fig. 3a and c). Thesedifferences were significant for all the size fractions. In contrast, N

Table 1Biomass distribution (%) in the six main ecosystem species for leaves and in the size fractions for litter, C and N content (mg g�1) and d13C (‰) of leaves and litter fraction [meanvalues were obtained by weighting each variable by the proportion of either ecosystem species for fresh vegetation leaves (n = 6) or litter size fractions for pre- and post-fire litter(n = 6); standard deviations are given between brackets.

Material Biomass distribution (%) Analysis

Origin Fraction C content (mg g�1) N content (mg g�1) d13C (‰)

Before fireStanding vegetation leaves of six main species Serenoa repens 9.9A 465 13 �29.2

Quercus myrtifolia 41.9A 500 16 �27.8Quercus geminata 37.1A 497 18 �27.1Quercus chapmanii 5.3A 485 19 �29.2Lyonia ferruginea 5.5A 531 15 �28.9Myrica cerifera 0.3A 493 20 �29.1

Mean (standard deviation) 496 (22)a 17 (3)c �27.8 (0.9)e

Total pre-fire litter (n = 6) >2 mm 84 461 10 �28.91–2 mm 7 427 15 �28.40.5–1 mm 4 479 18 �28.40.25–0.5 mm 2 464 20 �28.1<0.25 mm 2 443 20 �27.9

Mean (standard deviation) 459 (14)b 11 (1)d �28.8 (0.2)e

After fireTotal post-fire litter (n = 6) >2 mm 67 474 13 �28.7

1–2 mm 15 463 16 �28.80.5–1 mm 8 499 19 �28.60.25–0.5 mm 5 487 21 �28.4<0.25 mm 5 451 19 �28.3

Mean (standard deviation) 474 (30)a 15 (2)c �28.7 (0.4)e

Brown post-fire litter (n = 4) >2 mm 47B 466 14 �28.41–2 mm 10B 453 16 �28.9<1 mm 43B 441 17 �28.7

Black post-fire litter (n = 4) >2 mm 71B 593 14 �29.01–2 mm 16B 594 16 �29.9<1 mm 13B 556 19 �29.5

a,b,c,d,eResults of the Mann–Whitney tests (P < 0.05).A Derived from data in Alexis et al. (2007).B Calculated using the black particles proportion in each size-fraction (Alexis et al., 2007) and the contribution of each size-fraction to total post-fire litter.

M.A. Alexis et al. / Organic Geochemistry 41 (2010) 690–697 693

content was not significantly affected by the fire (Fig. 3b). Differ-ences in elemental and isotopic composition between brown andblack post-fire litter result from two factors: (i) fire-inducedmolecular transformations and (ii) litter fall during the fire event.In the latter case, we have demonstrated that the ‘‘fresh litter” fallcontributed most to the black post-fire litter, as standing vegeta-tion leaves were generally charred when reaching the soil surface(Alexis et al., 2007). The difference in C content between standingvegetation leaves and pre-fire litter (37 mg g�1, Fig. 3a) was nothigh enough to solely explain the strong C enrichment for the blackvs. brown post-fire litter (115–142 mg g�1, Fig. 3a). Therefore, fire-induced molecular transformations appear to be the main driverfor the enrichment in black vs. brown post-fire litter.

Cellulose is enriched in 13C relative to lignin and lipids (Benneret al., 1987; Loader et al., 2003). The differential loss of compoundsduring heating could therefore explain the lower d13C value ofthermally altered OM (Fig. 3). In contrast, during artificial burningwith no or limited O2, the carbon of thermally sensitive com-pounds may be incorporated into aromatic compounds, rather thanreleased as gas. The result would be reduced isotopic discrimina-tion, together with a high C concentration. Above 370 �C, labora-tory pyrolysis studies report a C content of wood charcoalranging from 68% to 91% (Czimczik et al., 2002; Turney et al.,2006; Hammes et al., 2006). These values are substantially higherthat the charcoal C content observed in the present study (Fig. 3),supporting the hypothesis formulated above. In contrast, the de-crease in 13C content measured for our charcoal is in the samerange as that obtained under O2-free conditions (Czimczik et al.,

2002; Hammes et al., 2006) and lower that that obtained for O2-limited conditions (Turney et al., 2006). This result could suggestthat the differential loss of compounds does not control the d13Cvalue for charcoal. However, the effect of charring on the isotopiccomposition that we observed (�0.6‰ to�1.0‰, Fig. 3c) was likelyto be an underestimate for black post-fire litter. Indeed, standingvegetation leaves had a higher d13C value than pre-fire litter(+1‰; Table 2, Fig. 3c) and its significant contribution to blackmaterial may have attenuated a greater contrast between thed13C value of charred litter and unaltered litter.

Our data underline the importance of separating charred litterfrom bulk sample when information on chemical transformationis needed. Because of the low proportion of charred leaves (Alexiset al., 2007) and the high contribution from non-senescent vegeta-tion leaves to post-fire litter, isotopic and elemental changes in-duced by temperature rise did not appear significant in totalpost-fire litter when compared to pre-fire litter.

3.3. Solid-state 13C NMR spectroscopy

3.3.1. Litter characteristicsThe 13C CPMAS NMR spectrum of pre-fire litter >2 mm was

characterised by a dominant signal at 72 ppm in the O-alkyl C re-gion (45–110 ppm; Fig. 4a). The signal is generally attributed topolysaccharide material such as cellulose (Kögel-Knabner, 1997).In the same region, we observed a 56 ppm signal, which potentiallyrepresents methoxyl C of lignin and/or contributions from N-alkylC in peptide structures, as suggested by Skjemstad et al. (1983).

* **

**

**

*

C c

onte

nt (

mg

g-1)

δ13

C (‰

)N

con

tent

(m

g g-1

)

350

400

450

500

550

600

10

15

20

25

-29.5

-29.0

-28.5

-28.0

-27.5

-27.0

b

a

c

Size-fractions (mm)

0

5

>2 1-2 0.5-1 0.25-0.5 <0.25

* **

**

**

*

P < 0.01

Fig. 2. C content (a), N content (b) and d13C value (c) of total pre-fire (circles) andpost-fire (squares) litter. Standard deviation is represented (n = 6). Stars indicatesize fractions with a significant change considering paired t-tests pairing sixrepetition plots [levels of significance: �P < 0.05; ��P < 0.01; ���P < 0.001)]. P value isadded when the difference between total pre- and post-fire litter is significant.

*

*** ******

*

*** ***

C c

onte

nt (m

g g-1

)N

con

tent

(mg

g-1)

>2 1-2 <1size fractions (mm)

δ13C

(‰)

0

5

10

15

20

25

-30.5-30.0-29.5-29.0-28.5-28.0-27.5-27.0

350

400

450

500

550

600

650

b

c

a

*

*** ******

*

*** ***

P < 0.001

P < 0.01

Fig. 3. C content (a), N content (b) and d13C value (c) of brown (triangle) and black(diamond) post-fire litter. Standard deviation is represented (n = 4). Stars indicatesize fractions with a significant change considering paired t-tests pairing fourrepetition plots [levels of significance: �P < 0.05; ��P < 0.01; ���P < 0.001]. P value isadded when the difference between total brown and black post-fire litter issignificant. Continuous line shows fresh vegetation leaf values, fine dotted line totalpre-fire litter values and large dotted line total post-fire litter.

Table 2DSC results for black and brown post-fire litter (the amount of the fourth sample didnot allow measurements).

Peak 3 (last exotherm)

Temperature(�C)

Height(W g�1)

50% Burnoffvalue (�C)

Brown particles >2 mm (n = 3)

694 M.A. Alexis et al. / Organic Geochemistry 41 (2010) 690–697

Two signals at 19 and 30 ppm occur in the alkyl C region and couldreflect the acetyl C and methyl C in lipid, cutin, suberin or aminoacids (Golchin et al., 1996). Signals from lignin-derived phenols oc-cur mainly in the aryl region (110–160 ppm; Knicker, 1993). Inparticular, we observed signals at 119, 130, 150 ppm that mayoriginate from lignin compounds and represent protonated, C-substituted and O-substituted aromatic C respectively (Knickerand Ludemann, 1995). Tannin and tannin-like structures may alsocontribute to the signal intensity in this region. The characteristicsplit peak at 144 and 154 ppm was found to be consistent with tan-nin content as determined from wet chemical analysis (Lorenzet al., 2000). The signal at 172 ppm in the carbonyl C region isindicative of acids or amides (Kögel-Knabner, 1997).

Considering the proportion of C allocated to the four regions,the litter size fractions did not prove to be different (Fig. 5a–c). Nei-ther was the bulk composition of pre-fire litter and post-fire brownlitter (Fig. 5a and b), though we noticed a slight decrease in O-alkylC content associated with an increase in other compounds inbrown post-fire litter. In contrast, black post-fire litter had lowercontent of alkyl C and O-alkyl C and a higher content of aromaticC than brown post-fire litter (Fig. 5c).

525 2 400520 4 397521 2 399

Mean (standard deviation) 522 (3) 3 (1) 399 (1)

Black particles >2 mm (n = 3)518 10 436510 11 431508 13 433

Mean (standard deviation) 512 (6) 11 (1) 433 (2)

3.3.2. Effect of temperature on composition of litterConsidering the >2 mm fractions isolated from post-fire litter,

which experienced different temperatures, no change in aromaticC contribution was apparent for brown particles, whereas blackparticles clearly reflected the carbonisation process. We noticed asignificant correlation (r2 = 0.94; P < 0.01) between increasing tem-perature and aromatic C contribution (Fig. 6). The latter varied be-

tween 16% of total organic carbon (TOC) for pre-fire litter and 58%TOC for black particles collected in a plot where the temperaturereached 650 �C.

For black post-fire litter, the aromatic peak at 125–129 ppm be-came the major component above 370 �C (Fig. 4b–e). From this rel-atively low temperature, we also noticed a shift in the dominantsignal in this region from 129 ppm before the fire, to 125–127 ppm in black post-fire litter (Fig. 4). This reflected the aroma-tisation process, as reported previously (Freitas et al., 1999; Bal-dock and Smernik, 2002; Czimczik et al., 2002). The effect isattributed to the transition from protonated aromatics to polyaro-

LLAc/a

mL L

C

CPs

L/Pr Lip

Lip

a. Before fire

300 200 100 0ppm

Lip

Ac/a

m

L/Char (125ppm)

L/Char (125ppm)

C

CL/

Prb. 370°C

300 200 100 0ppm

Ac/a

m

L/Char (126ppm)

C CPs

L/Pr

Lipc. 430°C

300 200 100 0ppm

CL/

Pr

Lip

d. 590°C

ppm300 200 100 0

Ac/a

m

L/Char (127ppm)

L/Char (126ppm)

L/Pr

Ac/a

m

C

Lip

e. 650°C

300 200 100 0ppm

Fig. 4. NMR spectra of litter sample >2 mm collected before fire (a) and four blacklitter samples >2 mm collected after fire (b–e). Temperatures recorded in corre-sponding plot litter were 370 �C (b), 430 �C (c), 590 �C (d) and 650 �C (e). Ac/Am,acids and amides; L, lignin; C, cellulose; Ps, polysaccharides; Pr, proteins; Lip, lipids;L/Char, lignin or charcoal; L/Pr, lignin or protein. Vertical dotted lines giveboundaries between the four chemical shift regions used to integrate area inspectrum for alkyl C (0–45 ppm), O-alkyl C (45–110 ppm), aromatic C (110–160 ppm) and carbonyl C (160–220 ppm).

carbonyl

aromatic

O-alkylalkyl

a

0%

20%

40%

60%

80%

100%

>2 1-2 0.5-1 0.25-0.5 <0.25

%O

C

6(3)%16(2)%

54(7)%

20(6)%

b

0%

20%

40%

60%

80%

100%

>2 1-2 <1

>2 1-2 <1%

OC

11(3)%22(4)%

43(8)%

24(2)%

c

0%

20%

40%

60%

80%

100%

Size fractions (mm)

%O

C

9(0)%

51(6)%

23(4)%

17(1)%

Fig. 5. C distribution from solid-state 13C CPMAS NMR for pre-fire litter (a), brownpost-fire litter (b) and black post-fire litter (c). The proportions of C (%) allocated todifferent chemical shift regions are mean values (standard deviation) of the sizefractions (nq indicates spectrum with poor signal/noise ratio, which was notintegrated).

temperature recorded in litter (°C)

Con

tribu

tion

from

arom

atic

C (%

OC

)

0

10

20

30

40

50

60

70

0 100 200 300 400 500 600 700

r2 = 0.94

P < 0.01

Fig. 6. Relationship between aromatic C contribution and temperature increase forbrown (white triangles) and black (black triangles) >2 mm post-fire litter; r2 and Pvalues correspond to linear regression obtained for correlation between black post-fire litter and temperature.

M.A. Alexis et al. / Organic Geochemistry 41 (2010) 690–697 695

matic structures (Knicker et al., 2005b). However, it is not clearwhether the protonated aromatic peak totally disappeared orwas simply hidden by the polyaromatic one, i.e. whether plant-de-rived aromatic molecules, such as lignin or tannin, were actuallypreserved or altered. The decreasing contribution from the72 ppm peak was particularly obvious and can be attributed tothe thermal sensitivity of polysaccharides (Fernandez et al.,2001). In contrast, alkyl-C appeared to be relatively preserved until

severe heating. From 430 to 650 �C, the contribution from alkyl-Cand O-alkyl-C decreased vs. that of aromatic C. However, even at650 �C (Fig. 4e), some signals in the alkyl C and O-alkyl C regionswere still identifiable. They were most likely related to ligno-cellu-lose structures and aliphatic compounds.

The increase in aromatic C with increasing temperature was notrelated to changes in carbonyl or alkyl C contributions (Fig. 7;P = 0.8 and 0.25, respectively). In contrast, the aromatic C contentwas highly correlated with the progressive decrease in O-alkyl C(Fig. 6; r2 = 0.98, P < 0.01). This negative correlation suggests thatthe neoformed aromatic structures originated from cellulose C.However, the low d13C value of charred OM does not support this

Aromatic C content (%OC)

Che

mic

alsh

ift re

gion

cont

ribut

ion

(%O

C)

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70

r2 = 0.98

P < 0.01

Fig. 7. Contributions of carbonyl C (diamonds), alkyl C (squares) and O-alkyl C(circles) depending on aromatic C contribution (% of TOC) in black > 2 mm post-firelitter; r2 and P values correspond to linear regression obtained for correlationbetween O-alkyl C content and aromatic C content. Alkyl C and carbonyl C did notsignificantly correlate with aromatic C content.

696 M.A. Alexis et al. / Organic Geochemistry 41 (2010) 690–697

hypothesis (Section 3.2). Cellulose C and/or lignin C potentially suf-fered partial mineralisation in parallel with cyclisation during nat-ural charcoal formation.

Charcoal is supposed to have a mineralisation rate related to thetransformations induced by heating and especially to the decreasein O-alkyl C and the increase in aromatic C contribution (Baldockand Smernik, 2002). Our leaf charcoal has a C content and an aro-matic C proportion (%) in the same range as grass char producedafter 1 min heating under oxic conditions (Hilscher et al., 2009).The latter authors showed that microbial mineralisation rates ofthis type of charcoal in soil can be as high as that of non-pyrogenicOM. This suggests that the charcoal produced during our scrub–oak ecosystem fire is probably not stable in soil in the long term.Similarly, Knicker (2007) recently suggested that charcoal pro-duced during natural burning does not contribute to the stable Cpool in soil.

3.4. Differential scanning calorimetry

The greater aromaticity of black vs. brown particles was consis-tent with DSC analysis of the >2 mm fractions. Indeed, thermalalteration of the black fraction was evidenced by the thermogramincrease in the 50% burnoff point and the height of the third peak(Table 2; Fig. 8). Brown post-fire particles had a 50% burnoff of399 ± 1 �C and the height of the third peak was 3 ± 1 W g�1, whilethese parameters were ca. 433 ± 2 �C and 11 ± 1 W g�1 for black

-10

0

10

20

30

40

0 100 200 300 400 500 600 700temperature (°C)

heat

flow

(W/g

)

Fig. 8. Comparison of typical DSC thermograms of brown (solid) and black (dashed)post-fire particles >2 mm. Arrows indicate positions of temperature maxima.Dashed and solid vertical lines indicate 50% burnoff of black and brown post-firelitter, respectively.

particles. The shift in 50% burnoff temperature was mainly dueto loss of the thermally most labile compounds at 348–357 �C,whereas the evolution of thermally more stable material burningat temperatures >500 �C played a minor role (Fig. 8). The third peakwas almost four times higher for black than brown post-fire parti-cles. However the temperature of the third peak in black particles(512 ± 6 �C) was below that of brown particles (522 ± 3 �C). Thissuggests that brown post-fire litter contained traces of materialwith an even higher thermal stability than black litter. This mate-rial could potentially be soot, which could have condensed out ofthe gas phase and accumulated on the brown litter.

For both black and brown particles (Fig. 8), the temperature ofthe peak that corresponds to thermally altered and condensed mol-ecules is, however, below that of wood-derived charcoal (Leifeld,2007) or polycondensed aromatic carbon (Plante et al., 2009). Thisis in agreement with the NMR data and supports observations byKnicker et al. (2005a) showing that, under wildfire conditions, mostof the aromatic structures produced consist of small clusters ratherthan highly condensed graphite-like structures. One important fac-tor determining the nature of charcoal is known to be its precursormaterial (Hammes et al., 2007). The litter we studied was entirelycomposed of leaves, which have been shown to produce less stableblack carbon upon charring than wood (Hammes et al., 2007;Rumpel et al., 2007). It is likely that wood is able to suffer a greaterdegree of alteration than leaf material, without being entirely con-sumed. The thickness and density of wood pieces may induce loweroxygen availability during burning. The resulting wood charcoalcould thus have greater stability than leaf charcoal.

4. Conclusions

The relationship between the maximum temperature reachedduring a natural fire and the chemical transformation of litterOM appeared consistent with previous results obtained under con-trolled conditions. The main differences between artificial and nat-ural burning conditions are most probably related to the oxygenavailability, which could influence the mass and the C loss andthe aromatic C content, as well as the isotopic composition of theremaining OM. The visual identification and separation of burntand unburnt material constituted an essential preliminary stepfor the characterisation of thermally altered OM. Quantitativeand qualitative analyses highlighted the great influence of unburntlitter and non-senescent leaves on the composition of post-fire lit-ter. Both contributions somewhat balanced the temperature effectson the bulk litter samples. In contrast, C content and cyclisationwere observed for black litter that had been exposed to tempera-tures >370 �C. A significant correlation between the production ofaromatic structures and the decrease in O-alkyl C contributionwith increasing temperature probably resulted from the completecombustion of thermally sensitive cellulose-like C, as well as itsincorporation into newly formed pyrogenic compounds. Such leafcharcoal, produced during natural burning, presents a relativelylow content of highly condensed structures and appears quite sim-ilar to low-stability grass charcoal.

Acknowledgements

The project was financially supported by the Institut Nationaldes Sciences de l’Univers, a department of the Centre National dela Recherche Scientifique under the framework of the ECCO pro-gram, and by a grant from the Smithsonian Environmental Re-search Center. We would like to thank the teams at the FloridaCO2 site and Dynamac Corporation for fieldwork. Many thanks goto S. Derenne for a helpful review and to two anonymous reviewersfor contributions which improved the manuscript.

M.A. Alexis et al. / Organic Geochemistry 41 (2010) 690–697 697

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.orggeochem.2010.03.003.

Associate Editor—G.D. Abbott

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