Ž .Geoderma 104 2001 281–298www.elsevier.comrlocatergeoderma

Thermal resistance to high temperatures of differentorganic fractions from soils under pine forests

I. Fernandez), A. Cabaneiro, T. CarballasDpto. de Bioquimica del Suelo, Instituto de InÕestigaciones Agrobiologicas de Galicia, CSIC,´

Campus UniÕ Sur, Apartado de Correos 122, 15780 Santiago de Compostela, Spain

Received 25 September 2000; received in revised form 22 March 2001; accepted 22 May 2001

Abstract

Thermal resistance of the organic matter from two Humic Cambisols from the NW of Spain,developed over granite and under pine forest, was determined by heating samples of the upper 5cm of soils at 220 and 3508C for 30 min. Unheated samples of the same soils were used ascontrol. Lignin, holocellulose, lipids and water-soluble compounds as well as humic substanceswere determined by different chemical fractionation methods. At 2208C, the losses of holocellu-lose exceeded 75%, this being the most vulnerable organic fraction. At this temperature, thelipidic fraction exhibited different thermal resistance in both soils, showing losses of 73% in onecase and only 11% in the other. Lignin exhibited the lowest losses, increasing proportionally.Fulvic and humic acids decreased at 2208C but humin exhibited a net increase. At 3508C,holocellulose practically disappeared and lignin losses were significant, whereas lipid loss slightlyincreased compared with the loss observed at 2208C. All humic substances decreased at 3508C,but humin was the most resistant humic fraction. Heating the soils at 220 and 3508C increased theFe and Al extracted with Na P O and NaOH and the percentage of humification of the4 2 7

remaining soil organic matter.q2001 Elsevier Science B.V. All rights reserved.

Keywords: Controlled heating; Forest fires; Humic compounds; Soil organic matter; Thermalalteration

1. Introduction

Extensive pine reforestation has been carried out over the last two centuries inŽ . ŽGalicia NW Spain and at present, in this region, pine forests which are more

.prone to fire than the autochthonous forests can be found practically from sea

) Corresponding author. Tel.:q34-981-590958; fax:q34-981-592504.Ž .E-mail address: ifernandez@iiag.cesga.es I. Fernandez .

0016-7061r01r$ - see front matterq2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0016-7061 01 00086-6

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level to the most elevated zones. During the last few decades, forest fires,accidental or provoked, of varying intensity, have been extremely common inthese ecosystems. Soil organic matter is a very important component in Galicianforest ecosystems, the sustainability of which depends greatly on the quantityand composition of this soil organic matter, as clay is scarce. The relationshipbetween the temperature reached by the soil during a forest fire and its soilorganic matter alteration is of paramount importance, as the degree of alterationcontrols not only the immediate deterioration of the ecosystem, but also its

Ž .subsequent recovery Fernandez et al., 1999 . The residual effects of forest fires´may be extremely negative due to the destruction or composition alteration ofsoil organic matter. This is particularly evident in the upper 5 cm of soil, since

Ž .the temperature rarely surpasses 1508C below this depth Raison et al., 1986 .Therefore, it is of great interest to verify the thermal resistance of the differentorganic fractions contained in the upper 5 cm of soil when heated to hightemperatures. In spite of this, studies on the changes brought about by fire or

Žheating on forest soil organic matter are scarce Almendros et al., 1984;.Fernandez et al., 1999 . Changes in soil organic matter presumably depend on´

the intensity of burning; however, it is not easy to establish the temperaturereached by the soil during a fire, since many factors are involved: the amount,

Ž .quality, humidity and stacking of fuel litter, leaves, branches, stems , the windŽ . Žconditions direction and speed , the soil characteristics soil moisture and

. Ž .organic matter content and the duration of fire Tothill and Shaw, 1968 . Otherfactors, such as the vegetation stage and the topography, affect the impact offires on soil, which means that each fire is a particular case. For this reason, it isvery useful to heat the soil under controlled laboratory conditions in order tostudy the effects of heating at given temperatures on the soil organic matter in aprecise manner. The aim of this study was to contribute to a better knowledge ofthe effect of fire on forest-soil organic matter, determining in which form

Ž .high-temperature soil heating 220 and 3508C affects the composition of soilorganic matter. In order to do this, Humic Cambisols from two Galician pineforest ecosystems located at different altitudes were heated in laboratory condi-tions, and the thermal resistance of their organic fractions were studied usingdifferent chemical fractionation methods.

2. Material and methods

2.1. Soil sampling

Ž .The soils selected for study, located in Galicia NW Spain within the humidtemperate zone, were two Humic Cambisols developed under pine forests over

Žgranite. Soil 1, from Monte Xiabre in the province of Pontevedra UTM:.29TNH2 1 , was collected in February at an altitude of 190 m a.s.l. where the7 9

annual rainfall was approximately 1800 mm and the annual mean temperature

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Ž .14 8C with minimum of 5 8C and maximum of 268C . Pinus pinaster Aitonwas the dominant tree species. Soil 2, located in Cabeza Grande de Manzaneda

Ž .in the province of Orense UTM: 29TPG3 7 , was collected in October at an9 9

altitude of 1740 m a.s.l. where the annual rainfall was approximately 2000 mmŽand the mean temperature 68C with a minimum ofy4 8C and maximum of 22

.8C . The forest was composed ofP. sylÕestris L. The undergrowth was differentin both locations due to the different climatic conditions at each altitude. Theundergrowth in soil 1 was composed ofUlex europaeus L., Pteridium aquil-

Ž .inum L. Kuhn and diverse herbaceous plants, whereas in soil 2, it wasŽ .composed ofChamaespartium tridentatum L. P. Gibbs,Vaccinium myrtillus

L., Erica spp. and species of Poaceae.After the removal of the litter and the duff, soil samples from a depth of 0–5

cm were collected from the A horizon taking up to 50 subsamples at random andgathering more than 10 kg of each soil. Below this depth, soils are rarely

Ž .affected by natural fires Raison et al., 1986 .

2.2. Heating process

Ž .Soil heating was performed on sieved-4 mm and homogenised soilsamples in an oven with a timer and a heating rate programme. Heatingconditions were: an increase of 38C miny1 to prevent the sudden combustionproduced when reaching the soil ignition temperature and 30 min of heatingafter reaching the selected temperature. The sample thickness was less than 0.5cm to avoid a temperature gradient. The soil was heated at two temperatures:220 8C, which is approximately the soil organic matter ignition temperatureŽ .Salgado et al., 1995 , and 3508C, which produces large C losses. Soiltemperature was controlled by a thermocouple located into the bulk of soil.Unheated subsamples were used as controls. The heating time and the tempera-tures used for this study were selected according to the results obtained from aprevious work, in which the soils under study were heated at different tempera-tures between 100 and 5008C, to choose the laboratory heating temperatures atwhich soil organic matter losses were comparable to those found immediately

Ž .after forest fires in the region Fernandez et al., 1999 . At 1508C, there were no´significant changes in the main soil characteristics and C losses were negligible,whereas at 5008C, almost all organic matter disappeared.

2.3. Analysis of soil properties

Ž .The methods described by Guitian Ojea and Carballas 1976 were used to´Ž .determine the following soil properties: pH in H O 1:2.5 and extractable Fe2

Ž .and Al oxides by extraction with a mixture of Na S O and Tamm’s reagent .2 2 4

Organic C was determined by dry combustion and measurement of the CO2Ž .evolved in a Carmhograph 12 Wosthoff, Germany , with the combustion oven¨

at 900 8C and the postcombustion oven at approximately 4008C. The factor

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Žused to convert organic C into organic matter was 1.724 Guitian Ojea and´.Carballas, 1976 . Total N was determined by modified Kjeldahl digestion

Ž .Guitian Ojea and Carballas, 1976 . The results are the average of the replicate´determinations in three subsamples of the same material.

2.4. Organic matter fractionation

Two organic matter fractionation methods have been implemented in thisstudy to monitor the possible changes produced by high temperatures in the

Ž .whole soil organic matter that, according to Kononova 1982 , can be systema-tised into two groups: soil biochemical fractions and humic compounds.

Organic matter fractionation to estimate the quantitative importance of soilŽ .biochemical fractions was performed by Kononova’s 1961 method, except for

water-soluble compounds that were extracted as recommended by StevensonŽ .1965 . The water-soluble fraction was extracted by refluxing with boiling waterfor 2 h. Lipids were extracted with ethanol–benzene for 24 h in a Soxhlet. The

Ž .residual soil was treated with H SO 72% in order to solubilise holocellulose2 4Ž .celluloseqhemicellulose , leaving lignin and mineral material as residue. Afterignition of this residue in an oven at 5008C for 6 h, the lignin content was

Žcalculated by difference from the final residue weight Gonzalez-Prieto et al.,´.1993 . The results were expressed as the percentage of both soil weight and total

organic matter. For the interpretation of the results, it must be taken into accountthat the fractions obtained are heterogeneous and include several substances, andthus, the fraction referred as lignin can also include humified organic matterŽ . Ž .HOM Kogel, 1986 .¨

For extraction of the humic substances, a modified method of DuchaufourŽ .and Jacquin 1966 , which is proven suitable for the study of organic matter of

Ž .these forest soils Carballas et al., 1980, 1983 , was used. After a densimetricŽ .removal of the unhumified organic matter UOM using a mixture of bromo-

y3 Ž .form–ethanol with a density of 1.8 g cm , the fulvic acids FA and the humicŽ . Žacids HA were extracted successively at pH 9.8 with 1% Na P O FA I, HA4 2 7

. Ž .I and at pH 13 with 0.1N NaOH FA II, AH II . The extraction was performedŽ .by shaking the soil with the corresponding reagent 1:100 for 1 h, centrifuga-

tion and repetition of the cycle until total extraction. The final residue, whichŽ .constitutes the humin fraction H , was washed, dried at 408C, ground and

homogenised, and its C content was assessed by the Carmhograph method. Ineach extract, HA were separated from FA by precipitation at pH 1 with H SO2 4

Ž .and centrifugation after 4 h of standing Gonzalez-Prieto et al., 1993 . Carbon´content was determined for each extract by slightly modified Sauerlandt’s

Ž .method Guitian Ojea and Carballas, 1976 , which uses K Cr OqH SO for´ 2 2 7 2 4Ž . Ž . Ž .digestion 1158C during 90 min and further titration with NH Fe SO4 2 4 2

until a 590-mV final point, indicated by a redox electrode of platinum connectedŽ .to a Metrohm 682 titrator Metrohm, Switzerland . Extracted Fe and Al were

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also determined in Na P O and NaOH extracts by atomic spectrophotometry.4 2 7Ž . Ž .The % extraction C rtotal C , %humification C rtotal C andFAqHA FAqHAqH

Ž . ŽŽ . .indices of polymerisation FArHA and metal complexation FeqAl rCwere calculated.

2.5. Statistical analysis

For statistical comparison of the results obtained in the organic matterfractionations, thex 2 test was used. A multivariate analysis: cluster formationby agglomerative hierarchical clustering, using the absolute squared Euclidean

Ž .dissimilarity coefficient, in the computer software SPSS 6.0 1993 , was alsoapplied.

3. Results and discussion

Prior to the heating process, the organic matter of both soils studied could beconsidered, according to the classification of temperate forest humusŽ .Duchaufour, 1977 , an intermediate type betweenmoder and mull. Soil 1, with

Ž .a lower CrN ratio, had a higher C concentration than soil 2 Table 1 . Therewere also some differences between the humic fractions of both soils. Soil 1contained more humic substances, had a higher humification percentage andpresented more FA and a higher FArHA ratio than soil 2, which indicates that

Ž .the humus in soil 1 was less polymerised than in soil 2 Tables 2 and 3 . The FAŽIrFA II and HA IrHA II ratios were also higher in soil 1 2.9 and 11.6,

. Ž .respectively than in soil 2 1.5 and 3.1, respectively , which means that thepolymerisation of the FA fraction, as well as of the HA fraction, was also lowerin soil 1. Practically no differences between the two soils were observed in the

Ždistribution of biochemical fractions; lignin was the largest fraction about. Ž . Ž .60% , followed by holocellulose 30–31% , water-soluble compounds 6–7%

Ž . Ž .and lipids 4–5% Figs. 1 and 2 .

Table 1Main characteristics of soils 1 and 2 and of the samples from both soils heated at 220 and 3508CŽ .mean values"standard deviation

Sample pH H O C CrN N Fe O Al O2 2 3 2 3y1 a y1 y1 y1Ž . Ž . Ž . Ž .g kg d.s. g kg d.s. g kg d.s. g kg d.s.

1 4.2"0.0 196.8"1.9 14 14.3"0.2 12.6"0.1 18.2"0.11 3.8"0.0 106.7"0.4 9 12.4"0.1 7.7"0.2 10.8"0.2220 8C

b1 4.9"0.0 15.2"0.0 nd 4.6"0.1 17.8"0.4 28.1"0.5350 8C

2 4.4"0.0 102.9"1.1 19 5.5"0.1 5.7"0.1 6.6"0.12 4.1"0.0 64.8"0.1 12 5.4"0.0 5.0"0.2 6.2"0.0220 8C

b2 5.7"0.1 10.7"0.0 nd 1.4"0.0 5.4"0.1 7.1"0.0350 8C

aDry soil.bNot determined due to the high proportion of inorganic N.

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Table 2Ž . Ž . aMass and C content of lightd-1.8 and heavyd)1.8 fractions and characterisation indices of the soil organic matter for soils 1 and 2, unheated

and heated at 220 and 3508Cc d Ž .Soil Sample Fractiond-1.8 Fractiond)1.8 % Extraction % Humification FArHA FeqAl rC

y1 b y1 y1 y1g 100 g d.s. g C 100 g C g 100 g d.s. g C 100 g C

1 unheated 40.1 61 59.9 39 63 76 0.91 0.191 2208C 10.7 36 89.3 64 40 75 0.38 0.591 3508C 0.4 10 99.6 90 20 90 0.63 9.532 unheated 22.1 78 77.9 22 44 54 0.46 0.162 2208C 6.8 50 93.2 50 48 67 0.52 0.292 3508C 0.7 30 99.3 70 8 70 0.60 12.89

FA, fulvic acids; HA, humic acids; H, humin.a Ž .Using the fractionation method of Duchaufour and Jacquin 1966 .bDry soil.cC rtotal C.FAqHAdC rtotal C.FAqHAqH

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Table 3Ž . Ž . Ž . Ž . Ž .Unhumified organic matter UOM content and distribution of fulvic acids FA and humic acids HA extracted by Na P O at pH 9.8 I and NaOH at pH 13 II from soils 1 and 2,4 2 7

unheated and heated at 220 and 3508C

Soil Sample UOM FA I FA II HA I HA II

y1 a y1 y1 y1 y1 y1 y1 y1 y1 y1g C kg d.s. g C 100 g C g C kg d.s. g C 100 g C g C kg d.s. g C 100 g C g C kg d.s. g C 100 g C g C kg d.s. g C 100 g C

1 unheated 46.4 23.6 43.9 22.3 15.4 7.8 60.1 30.5 5.2 2.61 2208C 27.1 25.4 10.3 9.7 1.3 1.2 14.5 13.6 16.3 15.31 350 8C 1.5 9.9 1.1 7.2 0.1 0.7 0.1 0.7 1.8 11.62 unheated 47.2 45.9 8.7 8.5 5.8 5.6 23.6 22.9 7.6 7.42 220 8C 21.2 32.8 7.6 11.7 3.0 4.6 12.2 18.9 8.2 12.72 350 8C 3.3 29.9 0.3 2.8 0.0 0.0 0.1 0.9 0.4 3.7

aDry soil.

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Fig. 1. Total content and percent distribution of different biochemical fractions in soil 1, unheatedand heated at 220 and 3508C. Effect of heating with respect to the unheated soil. TOM: totalorganic matter; ds: dry soil.

The pH of both soils decreased slightly when the sample temperature reachedŽ .220 8C, whereas at 3508C, it increased by approximately 1 unit Table 1 , so

that both soils changed from beingÕery acid to moderately acid, according toŽ . ŽDuchaufour’s 1975 classification. Other authors Nishita and Haug, 1972;

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Fig. 2. Total content and percent distribution of different biochemical fractions in soil 2, unheatedand heated at 220 and 3508C. Effect of heating with respect to the unheated soil. TOM: totalorganic matter; ds: dry soil.

.Giovannini et al., 1990; Dıaz-Ravina et al., 1992 had found that the lowest pH´ ˜values appeared in soils heated at temperatures between 200 and 2208C, and apH increase over 2208C. The reason for the slight diminution of pH at 2208C

Ž .has not yet been clarified, although Sertsu and Sanchez 1978 have pointed out´that in acid soils, it was probably related to the release of exchangeable Al from

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Ž .organic matter. Ulery et al. 1993 attributed the increase of pH at highertemperatures to the loss of organic acids during the burning, and to thedeposition of hydroxides and carbonates with the ash from vegetation and soilorganic matter combustion. In the soil heating process under controlled labora-tory conditions, the quantity of ash is lower since ash results only from thecombustion of the soil organic matter; therefore, in this case, the loss of organicacids could be the main cause for the increase of soil pH. In agreement with

Ž .this, Schnitzer and Hoffman 1964 found dramatic losses of carboxyl groups attemperatures higher than 2508C during the pyrolysis of the organic matter fromtwo soil horizons.

The behaviour of Fe and Al extracted by Na S O and Tamm’s reagent was2 2 4

similar to that of pH, as they also decreased at 2208C and increased at 3508CŽ .Table 1 . However, the increase of both elements was only a relative enrich-ment due to the decrease of the soil weight as a direct consequence of theorganic matter loss.

ŽAt 220 8C, the losses of C were very high in both soils 46% and 37% of total.C in soils 1 and 2, respectively , especially in the soil with higher initial C

Žconcentration. About 90% of the total C was lost at 3508C in both soils Table.1 .

The CrN ratio decreased at 2208C by 5 units in soil 1 and 7 units in soil 2Ž .Table 1 ; this suggests a different C and N volatilisation as well as changes inthe organic matter characteristics. The CrN ratio decrease was also shown by

Ž .Fassbender 1975 , the magnitude of the decrease varying according to tempera-ture, burning duration and type of soil. At 3508C, the CrN ratio was notcalculated, due to the low concentrations of organic C and N that remained inthe soil. Nearly 40% of the organic N was converted into inorganic forms.

3.1. Effect of heating on water-soluble, lipid, holocellulose and lignin fractions

The thermal resistance at 2208C of the different soil biochemical fractionsŽ .showed similar behaviour in both soils Figs. 1 and 2 ; in spite of this, the

destruction of organic matter was slightly higher in soil 1 than in soil 2. Therewas an important loss of holocellulose, which was the most affected by theheating. Lignin, water-soluble compounds and lipids were also lost but to alesser degree. As a result of the heating at this temperature, the amount of ligninincreased relatively, accounting for more than 80% of the total soil C, whereasholocellulose represented only 12% and 7%, and the water-soluble compounds2% and 5% of the total C for soils 1 and 2, respectively. After heating, theproportion of the lipidic fraction decreased in soil 1 whereas it increased in soil2, suggesting that lipids from soil 2 have a higher thermal stability than lipidsfrom soil 1. This indicates that the initial lipid compositions were differentbetween the two soils, probably due to the type of vegetation, the climaticconditions or the influence of sampling time. Differences in the thermal stability

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Ž .of the lipidic fraction were found by Almendros et al. 1988 , who suggestedthat lipids associated to HA were comparatively more stable when heated. Whilestudying the lipidic fraction of a soil in which the proportion of the largest chain

Ž .acids )C was important, they also found an increase of shorter chain lipids20Ž .-C after heating the soil at 2108C.20

At 350 8C, only 10% of the initial soil organic matter remained; all fractionsshowed great decreases, except for lipids whose reduction was practically thesame as at 2208C. Consequently, the soil samples heated at 3508C showed aproportional enrichment in lipids, from 4% to 15% of the total organic matter

Ž .for soil 1 and from 5% to 33% for soil 2 Figs. 1 and 2 . Therefore, in bothsoils, there was a proportion of the lipid fraction that disappeared at a moderatetemperature, and another proportion of lipids with a higher thermal stabilitywhich remained in the soils even after heating at 3508C. This thermal stabilityof the recalcitrant lipidic fraction was exhibited at the highest temperatureparticularly in soil 2. At 3508C, the cellulosic fraction essentially disappeared,and only 10% of the initial lignin remained. As a consequence of the differentthermal stability of these fractions at high temperatures, the soil organic matterwas enriched in lipids and impoverished in cellulosic compounds, while main-taining the same relative proportion of lignin and water-soluble compoundsŽ .Figs. 1 and 2 .

The x 2 test was applied to compare the percent distribution of the differentfractions between the heated samples and the corresponding unheated samplefrom each soil, between the same soil heated at different temperatures andbetween the samples with the same treatment from different soils. For each soil,the composition of the organic matter from the heated samples at 220 and 3508C was significantly different from that of the corresponding unheated sample ata level of PF0.01 in all cases. The differences of composition between the

Ž .samples heated at 220 and 3508C were also significantPF0.01 in both soils.On the other hand, the differences between the two unheated soils were notsignificant; the same occurred when samples of both soils heated at 2208C werecompared. On the contrary, at 3508C, the differences between the distribution ofthe different fractions of soils 1 and 2 were significant atPF0.01 level. Thisindicates that the effect of soil heating at a high temperature on soil biochemicalfractions depends on the soil considered and it suggests that the previousdifferences between the two soils are related to their fractions of higher thermalresistance, particularly the lipid and lignin fractions, rather than the less resistantones.

3.2. Effect of heating on the humic compounds

Ž .At 220 8C, the proportion of unhumified UOM and humified organic matterŽ . Ž .HOM that disappeared differed between soils 1 and 2 Fig. 3 . For soil 1 with

Ž .higher C concentration and % humification Tables 1 and 2 , the heating

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Ž . Ž .Fig. 3. Total content and percent distribution of the unhumified UOM and the humified HOMorganic matter fractions in soils 1 and 2, unheated and heated at 220 and 3508C. FA: fulvic acids;HA: humic acids; H: humin.

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Ž . Ž .provoked high but similar losses of HOM 47% and UOM 42% . In soil 2, theŽ .loss of HOM was lower 23% and this fraction was clearly more resistant to

Ž .heating than the UOM 55% , in accordance with the lower loss of the)1.8 gy3 Ž .cm fraction Table 2 . Therefore, at 2208C, the percentage of humification of

soil 1 was virtually unchanged, whereas in soil 2, an important increase of thisindex was observed. In both soils, extracted Fe and Al in alkaline extracts

Ž .increased clearly at 2208C Fig. 4 , especially the latter.At 220 8C, FA and HA decreased dramatically in both soils, whereas there

Ž .was a high net and percentage increase of humin Fig. 3 , resulting in a humusŽ .with an increase of the more humified fractions. Kumada 1987 reported a

Ž .progression of the HA humification after heating. Almendros et al. 1990 foundchanges on solubility when FA and HA were heated at 3508C and related these

Ž .changes to an increase of aromaticity. Shindo and Urabe 1993 found that theeffect of heating on the degree of humification depends on the heating tempera-ture and the initial composition of the organic matter. After heating, the FArepresented 14% and 24% of the HOM in soils 1 and 2, respectively, and thedecrease was much greater in soil 1 with losses of 80% of the total FA fractionŽ .Fig. 3 . In this soil, the FA fraction seemed to be less polymerised, since it was

Ž . Ž .mainly extracted with Na P O FA I Table 3 . Although to a lesser degree4 2 7

than FA, the absolute and relative HA values also decreased at this temperature,Žfrom 43% to 39% of the HOM for soil 1, and from 56% to 47% for soil 2 Fig.

.3 . In both soils, the HA IrHA II ratio decreased after the heating, from 11.6 to0.9 for soil 1 and from 3.1 to 1.5 for soil 2, in accordance with Almendros et al.Ž .1988 who also found increases of the concentration of aromatic compounds in

ŽFig. 4. Amount of Fe and Al complexed with the alkali-extractable organic matter fulvic.acidsqhumic acids in soils 1 and 2, unheated and heated at 220 and 3508C. Me: metal; ds: dry

soil.

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soils heated under laboratory conditions. The percentage of extraction and theŽ .FArHA ratio decreased in soil 1 and increased in soil 2 Table 2 . Almendros et

Ž .al. 1984 reported decreases of both parameters after the heating. Differentbehaviour after heating three volcanic soils at 2508C were also found by Shindo

Ž .and Urabe 1993 , with a decrease of the extracted humic compoundsrtotalhumic compounds ratio in two soils and an increase in another one. In bothforest soils studied, the only fraction which increased after heating at 2208C

Ž .was the humin fraction Fig. 3 , which exhibited a net increase of 11.4 and 2.6 gC kgy1 d.s. in soils 1 and 2, respectively. As a consequence of this increase,humin represented 47% and 29% of the HOM in the heated soils 1 and 2,respectively, whereas in the unheated soils, it only represented 17% and 18% of

Ž .the HOM, respectively Fig. 3 . The net increase of humin indicates that heatinghas led to the insolubilisation of certain organic substances. Thus, Fernandez´Ž .1997 , studying samples collected immediately after an intense forest fire and

Ž . Ž .applying an Na S O 3% and HFrHCl 1 M 1:1 treatment to the humin2 2 4

fraction, found a decrease of solubilizable humin and a net increase of insolublehumin as a consequence of burning. The different behaviour of all thesefractions at this temperature allows to differentiate between fractions relativelyresistant to the effect of heating, which did not demonstrate a high decrease, or

Ž .which even increased at 2208C H , and fractions of a more thermolabileŽ .character that presented a high decrease e.g., FA . This seems to be confirmed

by the fact that the greatest heating effect was observed in soil 1, with a higherorganic C concentration and a higher proportion of FA which are very sensitiveto increased temperatures.

At 350 8C, the -1.8 g cmy3 fractions essentially disappeared in both soilsand the humification percentage increased by 14 units in soil 1 and by 16 units

Ž .in soil 2 Table 2 . The extraction percentage diminished by 43 units in soil 1,and by 36 units in soil 2, with respect to the unheated soils. The extracted Fe

Žand Al in alkaline extracts increased dramatically at 3508C, particularly Al Fig..4; Table 4 . The high increase of extracted Al could be attributed to thermal

Table 4Iron and aluminium complexed with fulvic acidsqhumic acids solubilised by different extractantsŽ .I: Na P O at pH 9.8; II: NaOH at pH 13 from soils 1 and 2, unheated and heated at 220 and4 2 7

350 8C

Soil Sample Extractant I Extractant II

FerC AlrC FerC AlrC

1 unheated 0.08 0.12 0.01 0.121 2208C 0.17 0.43 0.08 0.491 3508C 0.53 1.79 0.26 13.832 unheated 0.07 0.11 0.01 0.092 2208C 0.13 0.22 0.03 0.142 3508C 1.05 1.65 0.15 22.93

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alteration of the inorganic forms of this element, which became soluble at highŽ .pH NaOH at pH 13 solubilised 92% of the extractable total Al .

At 350 8C, the thermal lability of FA and HA was similar, with more thanŽ .95% of these two fractions disappearing Table 3 . At this temperature, the

FArHA ratio had values close to 0.6 in both soils, but had quite different valuesŽ . Ž .before heating 0.91 in soil 1; 0.46 in soil 2 Table 2 . Humin, the most

resistant fraction, showed only losses of 59% in soil 1 and 32% in soil 2;consequently, the remaining organic matter was almost exclusively composed of

Ž .humin Fig. 3 . At this temperature, there was no net increase of any humicfractions, only a relative increase of humin, which represented 77% and 89% ofthe HOM in soils 1 and 2, respectively, after the heating.

The x 2 test confirmed the different behaviour of the two soils studied whencomparing either all the fractions of the entire soil organic matter or thehumified fractions only. When unheated samples from soil 1 were comparedwith the samples from the same soil heated at 2208C, the differences were

Ž .significant PF0.01 , whereas no significant differences were found for soil 2.The profound alteration of the soil organic matter at 3508C was also confirmedsince for both soil differences between the heated samples at this temperature

Ž .and their controls were always significantPF0.001 . For each soil, samplesheated at 2208C were significantly different from samples heated at 3508CŽ . ŽPF0.001 . The organic matter composition was statistically differentPF

.0.01 between the two unheated soils when all fractions were taken into account,but not when only the humified fractions were considered. At 2208C, theinverse was true and the significant differences induced by the heating in thehumified organic matter from soils 1 and 2 were in accordance with the differentdegrees of FA and HA polymerisation in the two soils. Finally, at 3508C, theorganic matter composition of the heated sample from soil 1 always significantly

Ž .differed of that from soil 2 PF0.05 , once again showing the importance ofthe differences in the previous organic matter composition on the changesproduced by the heating process at high temperatures.

3.3. Hierarchical cluster analysis

An analysis of hierarchical classifications was applied to the unheated soilsand the samples heated at 220 and 3508C. The variables included in thisanalysis were C, CrN, pH , Fe O , Al O , water-soluble organic matter,H2O 2 3 2 3

lipids, holocellulose, lignin, UOM, FA, HA, H, % extraction, % humificationŽ .and FeqAl rC.

This analysis, which considers all the characteristics studied together, led to agreater valuation of the global effect of heating on the soil organic matter. Fig. 5shows the dendrogram obtained, in which two groups can be clearly identified:one group represented the unheated soils and the samples heated at 2208C, andthe other group consisted of the samples heated at 3508C. This indicates that

( )I. Fernandez et al.rGeoderma 104 2001 281–298296

Fig. 5. Dendrogram obtained by applying hierarchical cluster analysis using average linkageŽ .between groups to selected properties of soils 1 and 2, unheated and heated at 220 and 3508C.

heating at the latter temperature produced dramatic changes on soil organicmatter composition. Within the first group, the unheated samples and thesamples heated at 2208C were separated into two subgroups, indicating that thechanges produced at 2208C on soil organic matter composition were alreadyconsiderable. A decrease of the previous differences between soils 1 and 2 intheir organic matter composition was produced with heating at 2208C; thus, the

Ž .absolute squared Euclidean dissimilarity coefficient DC between both soilsdecreased from 17.53, for unheated samples, to 4.94, for samples heated at 2208C. This could be attributed to the fact that the prior differences between bothsoils with regard to their thermolabile fractions content diminished. On thecontrary, although both soil samples heated at 3508C constituted a group thatwas distinguishable from the others, the difference in their organic matter

Ž .composition DCs27.45 was higher than that exhibited by the unheated soils.This again seems to suggest that the previous soil organic matter compositioninfluenced the different soil behaviour vs. the heating and, in this case, thedifferent intrinsic composition of the most thermoresistant fractions was themain factor responsible for the differences between soils 1 and 2 after heating at

Ž350 8C. The stability of the groups distribution obtained Bosque and Moreno,.1994 was confirmed when the hierarchical cluster analysis was applied to each

soil separately.

4. Conclusions

The magnitude of the changes produced by heating on the soil organic mattercomposition was strongly dependent on the temperature reached during theheating process. Although the initial differences in the organic matter composi-tion between the unheated soils were small, the response of their soil organicmatter to heating was different between the two soils, particularly at 3508C.

The behaviour of the different organic fractions at 2208C allowed to identifythe less resistant fractions to heating. Thus, holocellulose and FA presented amore thermolabile character whereas lignin and humin were more resistant.

( )I. Fernandez et al.rGeoderma 104 2001 281–298 297

Other fractions, such as the lipids, show a great diversity on their thermalresistance, with different proportions of relatively labile lipids that disappearedat 2208C and resistant lipids that remained at 3508C. On the other hand, the FAand HA with a higher degree of polymerisation exhibited a higher resistance toheating. At 2208C, the soil organic matter losses were similar to those found as

Ža consequence of forest wildfires in soils of the same region Fernandez et al.,´.1999 . At 3508C, the destruction of soil organic matter was much higher than in

natural wildfires and the changes produced on the composition of the soilorganic matter were striking, remaining only the most resistant fractions.

Acknowledgements

The authors thank Mr. J. Salmonte and Ms. B. Arnaiz for technical assistance.´This research was supported by the Consellerıa de Educacion y Ordenacion´ ´ ´Universitaria de la Xunta de Galicia and by the Comision Interministerial de´

Ž .Ciencia y Tecnologıa CICYT , Spain.´

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