Soil Color as an Indicator of Slash-And-Burn Fire Severity

7/30/2019 Soil Color as an Indicator of Slash-And-Burn Fire Severity

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1826 SOIL SCI. SOC. AM. J., VOL. 64, SEPTEMBEROCTOBER 2000

Wheatley, R., K. Ritz, and B. Griffiths. 1990. Microbial biomass and Zou, X., D.W. Valentine, R.L. Sanford, and D. Binkley. 1992. Resin-mineral N transformations in soil planted with barley, ryegrass, core and buried-bag estimates of nitrogen transformations in Costapea or turnip. Plant Soil 127:157167. Rican lowland rainforests. Plant Soil 139:275283.

Zak, D.R., G.E. Host, and K.S. Pregitzer. 1989. Regional variabilityin nitrogen mineralization, nitrification, and overstory biomass innorthern Lower Michigan. Can. J. For. Res. 19:15211526.

Soil Color as an Indicator of Slash-and-Burn Fire Severityand Soil Fertility in Sumatra, Indonesia

Quirine M. Ketterings and Jerry M. Bigham*

ABSTRACT properties (e.g., Nye and Greenland, 1960; Palm et al.,1996). More severe burns may alter such fundamentalFire is widely used as a tool for converting forest to agriculturalcharacteristics as texture, mineralogy, and cation-ex-land in many developing countries, and correlations are thought to

exist between fire severity, burned soil color, and soil fertility. To change capacity (Sertsu and Sanchez, 1978; Ulery andtest this hypothesis, field experiments were conducted in Sepunggur, Graham, 1993; Ulery et al., 1996; Ketterings et al., 2000).Jambi Province, Sumatra, Indonesia. Field burning a slashed 12- to The severity of a fire varies widely and depends on such15-yr-old secondary forest caused Munsell values and chromas to variables as fuel load, fuel moisture, climatic conditions,decrease and hues to become yellower with increasing heat severity, and size of the area being burned (Wells et al., 1979;especially in the top 5 cm of the soil. However, at peak surface

Brown, 1988; Lobert and Warnatz, 1993).temperatures600 C, soil C was mostly depleted and the soil matrixPostburn color patterns and the presence or absence

was reddened. Laboratory studies showed similar results with staticof wood ash have previously been used as indicators ofheating. Moreover, color changes were highly dependent on the dura-fire severity (e.g., Ulery and Graham, 1993; Romanyation of exposure at a given temperature. Fire induced the formationet al., 1994; Van Noordwijk et al., 1998b). Lightly burnedof aggregates with exteriors that had lower values and chromas and

slightly redder hues than the interiors. Laboratory removal of organic areas (short exposure at 100 to 250C) were character-matter from burned samples by chemical oxidation did not alter the ized by incompletely combusted organic material andcolor. Soil exchangeable Ca, Mg, and K increased with fire severity, blackened soil. Moderate burns (300400C exposurewhile exchangeable acidity and Al decreased 2 wk after the burn. Soil for longer periods of time) reportedly consumed plantC and N were reduced at high burn severity only. Phosphorus showed material (little residual white ash) without altering theanincreasein availabilityat lowto mediumfire severityand a decrease underlying soil (Wells et al., 1979), and very severein availability at the most intense burn levels. Colors of burned areas

burns (long exposure to 500C) left white ash andin the field did not change significantly during the 12 wk following

reddened the soil.the burn. However,within 12 wk followingthe fieldburn exchangeableDarkened soils that resulted from low- and medium-Ca had decreased to preburn levels and Al saturation had increased

severity burns were preferred for agricultural produc-markedly. Using postburn color measurements to predict the spatialpatterns in soil fertility was limited by the fact that fertility changed tion by farmers in Sumatra, Indonesia, because theyrapidly following the burn, whereas color parameters did not. were associated with higher crop yields and quicker

crop establishment (Ketterings et al., 1999). In the samestudy, areas of reddened topsoil were classified as unde-sirable due to their perceived low fertility status andFire is still widely used for land clearing in the poor water-holding capacity. Areas of reddened topsoiltropics even though traditional shifting cultivationhave been commonly observed in field burns (e.g., Sree-is declining due to increases in population pressure.nivasan and Aurangabadkar, 1940; Boyer and Dell,Burning slash introduces within-field variability in many1980; Ulery and Graham, 1993), but the proportion ofsoil properties due to uneven distribution of ash andland affected by high fire severity is thought to be onlyexposure to heat (Brouwer et al., 1993; Van Noordwijk2 to 8% (Dyrness and Youngberg, 1957; Ulery and Gra-et al., 1998b; Rodenburg, 1999; Ketterings, 1999). Rec-ham, 1993). Although unburned, ash-covered and redognizing and acting on this fire-induced microvariabilitycombusted soils can easily be distinguished in a recentlycould be an asset to the small farmer who is typically

burned field, it is unknown how color patterns developresource poor and working on marginal lands with fewor relate to soil fertility over time.external inputs (Brouwer et al., 1993).

Soil color is heavily influenced by the type andTemperature maximum and duration of exposure areamount of organic matter (Shields et al., 1968; Schulzemajor indicators of fire severity (Wells et al., 1979). Lowet al., 1993) and Fe oxides (Bigham et al., 1978; Schwert-severity fires (short exposure to 250C) have beenmann, 1993), both of which are fire-sensitive compo-shown to temporarily affect soil biological and chemicalnents. Although correlations between color and burnseverity should exist, there have been relatively fewQ.M.Ketterings, Environmental Science Graduate Program,and J.M.

Bigham, School of Natural Resources, Ohio State Univ., 2021 Coffey attempts to quantitatively evaluate changes in soil colorRoad, Columbus OH 43210. Received 6 Oct. 1999. *Correspondingauthor ([email protected]).

Abbreviations: CBD, citrate-bicarbonate-dithionite; REML, residualmaximum likelihood analysis option.Published in Soil Sci. Soc. Am. J. 64:18261833 (2000).


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KETTERINGS & BIGHAM: SOIL COLOR AND SOIL FERTILITY AFTER SLASH-AND-BURN 1827

Table 1. Characterization of unburned forest soil in Sepunggur, Sumatra, Indonesia.

Experiment Mineralogy Depth Sand Silt Clay pH (H2O) C CBD Fe

cm % g kg1

Primary burn Kaolinite with gibbsite, 05 1 40 59 4.5 67 12goethite, quartz, and a trace 515 1 43 56 4.6 20 24of hydroxy-Al-vermiculite

Secondary burn Kaolinite with gibbsite, 05 4 9 87 4.4 52 41goethite, quartz, and anatase 515 4 20 76 4.6 32 45

CBD Fe is citrate-bicarbonate-dithionite extractable Fe.

on top of the mineral soil prior to the burn. Samples of com-with heating, perhaps due to uncertainties associatedpletely combusted, brick-like topsoil were also taken fromwith the use of field color charts. Portable tristimuluslocations where high fuel loads had produced temperaturescolorimeters now offer the potential for more precisethat exceeded those measurable with the crayons (600C)color measurements from large numbers of samplesfor considerable amounts of time. Samples were taken 2 wk(Post et al., 1993; Ulery and Graham, 1993). The use ofafter the burn at 10 different locations and were composited

color transformations, such as redness rating (Hurst, in the field. In addition, bulk soil samples (05 and 515 cm1977; Torrent et al., 1983) or redness susceptibility depth) were taken in the surrounding forest and in the slashed(LaFleur, 1970), could also provide improved relation- field directly prior to burning. All samples were dried at 60Cships between color and burn severity. and passed through a 2-mm sieve before further analysis.

In this study, we investigated the properties of someSecondary Burn Experimentpre- and postburn soils in Sumatra, Indonesia, where

slash-and-burn agriculture is still commonly used (Ket- Three wood piles were constructed following a low-severityterings et al., 1999). Our objectives were to evaluate (i) broadcast burn of a 12- to 15-yr-old forest by the farmer. Eachthe effects of fire severity on soil color; (ii) the depen- pile consisted of 400 kg of field dried wood on a 3 by 3 m

area. The piles were ignited and surface temperatures weredence of soil color changes on the maximum surfaceestimated using heat-sensitive crayons placed beneath eachtemperature achieved and the duration of exposure topile. To determine the effect of heat intensity on soil fertilitythis temperature; (iii) the dependence of postburn colorand soil color parameters, samples were taken at locationson soil grain size, organic matter content, and iron oxidewhere the topsoil temperature had been 600, 300, and 100C,content; and (iv) the potential use of soil color parame-thelatterbeingoutside theburn spot. Forcomparison,samples

ters for assessing fire-induced changes in soil C, N, andwere also taken in the surrounding forest resulting in a total

exchangeable Ca, Mg, K, Na, and Al with time. of four treatments. Samples were collected during a 3-moperiod at 0- to 5- and 5- to 15-cm depths starting 1 d after theburn, then 1, 2, 4, 8, and 12 wk after burning. ImmediatelyMATERIALS AND METHODSprior to the burn, the centers of the three burn piles and the

All studies were performed using soil samples from thesurrounding forest were sampled. All samples were taken at

Sepunggur area, Jambi Province, Sumatra, Indonesia (102

two depths (05 and 515 cm) resulting in 120 measurements14 E, 1 29 S). A description of the climate, vegetation, and for the entire experiment. Of these 120 samples, a subset (38land use in this region is given in Van Noordwijk et al. (1995,

samples) was separately (i) ground to pass a 250-m sieve,1998a). Soils of the study area are classified as Hapludox or

(ii) heated in a muffle furnace at 550C for 8 h, and (iii)Kandiudox according to U.S. soil taxonomy (Soil Survey Staff,

treated with 30% (w/w) H2O2. These treatments were done1999). Basic characterization data for forest soils adjacent toto determine changes in soil color due to grinding, reheating

the primary and secondary burn sites are presented in Table 1.and organic matter removal, respectively.

Field Studies Laboratory Study

Plot research was conducted using the fields of two local The soil samples (05 and 515 cm depth) taken from thefarmers. In one case, the farmer performed only a primary, forest adjacent to the primary burn experiment were passedbroadcast burn. In the second case, stacks of wood were pre- through a 2-mm sieve and homogenized in the laboratory.pared and ignited in a field that had already experienced a Duplicate samples of 15 g of each of the soil layers werebroadcast burn to simulate the practice of performing second- placed in glass vials with a diameter of 2 cm and a height ofary burns for removing residual woody material (see Ketter- 4 cm (soil column height of 3.5 cm). One batch of samples

ings et al., 1999, for a description of farming practices in was exposed to 600C for 1, 2, 4, 6, 10, 15, 30, 45, 60, 75, 90,the area).105, 120, 180, 240, 300, 360, 420, 480, 540, 660, and 930 minand then cooled to room temperature. Identical samples were

Primary Burn Experiment exposed to 300C for 1, 2, 4, 6, 10, 15, 30, 45, 90, 120, 240, 360,and 660 min. The experiment was conducted in two replicates.For this experiment, a 20-yr-old secondary forest was se-Colors were measured on the dried samples after heat ex-lected. This 5-ha area was slashed and burned by the farmerposure.without relocating any of the slashed wood. Bulk soil samples

(45 kg) were taken at 0- to 5- and 5- to 15-cm depths inSoil Analyseslocations (one per sample) where surface temperatures of

100, 300, and 600C were reached during the burn. SurfaceSoil Color

temperature regimes were estimated in the field using heat-Soil color was measured in triplicate from air dried samplessensitive crayons (Cole Parmer, Vernon Hills, IL) mounted

on ceramic tiles, wrapped in aluminum foil, andplaceddirectly with a CR-300 colorimeter (Minolta Corp., Ramsey, NJ). Al-


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expressed as means with standard deviations. Linear regres-phanumeric hues obtained in the Munsell Renotation Systemsion analyses with color parameters and iron oxide data werewere converted to numerical values by using a Munsell hueperformed using Genstat 5 for Windows, Release 3 (Genstatcircle (Chamberlin and Chamberlin, 1980) (in this conversion5, 1993).10R 10, 10YR 20, 10Y 30, 10GY 40). Redness sus-

ceptibilities were calculated after heating some samples toSecondary Burn Experiment550C for 8 h according to the formula of LaFleur (1970): R

hue25hue550, where hue25 is the converted hue of field-burned This experiment was analyzed using the residual maximumsoil and hue550 is the converted hue after 8 h exposure to 550C.

likelihood analysis option (REML) of Genstat 5, Release 3(Genstat, 1993) with a fixed treatment and time interactionSoil Chemical Analyses effect (FIXED Treatment*Time). Burn piles (replicates)

and pile, treatment and time interactions were treated as ran-Field samples were analyzed for total C by the Walkleydom effects (RANDOM Pile/Treatment/Time). AnalysesBlack procedure (Walkley, 1947). Total N was determinedwere done for each sampling depth (05 and 515 cm) individ-using the Kjeldahl procedure (Bremner and Mulvaney, 1982).ually. Wald statistics were calculated to determine the signifi-Iron oxide contents were determined using dithionite-citratecance of the models. Standard errors of difference and ttestsbuffered with sodium bicarbonate at pH 7.3 (Mehra and Jack-were used to determine if treatment means were significantlyson, 1960). Final extracts were analyzed for Fe by atomicdifferent at P 0.05. When the initial statistical analysesabsorption spectrophotometry. Exchangeable K, Mg, Ca, and

Na were obtained by extraction with 1 MNH4-acetate at pH showed no significant effects from time of sampling, the time7.2. Exchangeable acidity and Al were determined in 1MKCl series were combined and an average for each replicate perby titration with 0.05MNaOH and 0.05 MHCl (after addition treatment per layer was calculated. The experiment was thenof 1 M NaF), respectively. Exchangeable Ca and Mg were analyzed using the same REML option but without samplingmeasured by atomic absorption spectrophotometry, while K time as a factor.and Na were measured by flame emission. The Bray-1 proce-dure was used to extract labile inorganic P, and extracts were Laboratory Studycolorimetrically analyzed for total P using the ammonium

Means and standard deviations were calculated. Nonlinearmolybdate method (Bray and Kurtz, 1945).regression analyses were computed with Tablecurve 2D forWindows, v. 2.03. No further statistical analyses were per-Statistical Analysesformed on these results.

Primary Burn Experiment

RESULTS AND DISCUSSIONThis experiment did not have true replicates because com-posite bulk samples were taken in the field. Each treatment Changes in Soil Fertility with Burningwas separated into four pseudo replicates (subsamples of

Table 2 shows soil fertility parameters for the samplesthe bulk sample) that were analyzed separately to assess preci-sion of the measurements. Results of this experiment were from the primary burn and from the 2-wk sampling of

Table 2. Effects of burn intensity on selected soil fertility parameters. Samples were taken 2 wk after the burn.

Treatment C N Bray-1 P Ca Mg K Na Al Hg kg1 mg kg1 cmolc kg

1

Primary burn

05 cm

Forest 66.5 6.4 60.1 0.85 0.38 0.34 0.04 4.93 0.81100 C 37.2 3.1 39.1 0.60 0.37 0.22 0.03 4.80 0.80300 C 49.3 3.6 50.7 1.02 0.72 0.31 0.05 4.20 0.64600 C 51.4 4.4 70.8 7.03 2.10 1.41 0.04 0.22 0.09600 C 4.3 0.6 10.8 9.35 2.70 1.43 0.05 0.00 0.00

515 cm

Forest 19.5 2.0 9.1 0.54 0.65 0.13 0.06 6.07 0.85100 C 20.8 1.9 16.8 0.58 0.28 0.12 0.07 3.96 0.67300 C 22.9 2.2 18.0 0.56 0.24 0.12 0.08 3.68 0.69600 C 25.6 2.8 24.4 1.41 0.56 0.33 0.02 2.07 0.51

Secondary burn

05 cm

Forest 57.0 4.4 43.1 0.98 0.76 0.29 0.06 4.00 0.85100 C 58.1 4.7 69.0 2.19 1.27 0.66 0.06 2.85 0.67300 C 48.2 4.0 76.0 2.97 1.55 0.97 0.08 1.21 0.35600 C 39.7 3.5 62.5 4.02 2.51 1.86 0.10 1.01 0.20s.e.d. 6.1 0.7 7.0 1.2 0.76 0.46 0.03 0.55 0.12Wald 12.1 3.7 20.5 7.1 5.5 12.8 3.5 39.6 34.3

515 cm

Forest 32.6 2.4 19.7 0.54 0.41 0.09 0.04 3.99 0.79100 C 45.9 3.2 40.1 1.43 0.83 0.35 0.06 3.42 0.75300 C 29.1 2.3 21.6 0.69 0.55 0.22 0.06 3.37 0.67600 C 25.4 2.3 25.8 1.47 0.90 0.55 0.10 1.60 0.47s.e.d. 7.8 0.5 10.3 0.52 0.24 0.10 0.03 0.66 0.14Wald 7.8 5.9 4.8 5.2 5.4 21.9 4.0 14.7 6.8

s.e.d. is standard error of difference. Wald statistics 7.8 indicate significant differences at P 0.05 (3 df).


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the secondary burn sites. The exchangeable cation (Ca, the most severe burn. Exchangeable Ca (Fig. 1b) andMg, K, and Na (results not shown) declined rapidlyMg, and K) concentrations and Bray-1 P at 0- to 5-cm

depth increased with intensification of the fires to 600C throughout the 90-d duration of the experiment, whileAl saturation (Fig. 1c) increased with time following thein the primary burn. Combustion at higher temperatures

reduced Bray-1 P to below the preburn level but further low and medium severity burns. The low Al saturation84 d after the severe burn was probably the result of aincreased exchangeable Ca, Mg, and K concentrations.

Exchangeable Na was unaffected by fire exposure. Ex- marked increase in exchangeable K due to ash reloca-

tion (results not shown). With the disappearance of ashchangeable Al and H, on the other hand, decreasedwith increasing fire severity. The Al saturation was ef- from the surface with time, increases in Al saturationare expected. These changes indicate a decline in soilfectively reduced from 67 to 0%. Carbon and N concen-

trations were variable and showed severe reductions fertility that could be due to leaching losses (Uhl andJordan, 1984; Khanna et al., 1994) and that has been aonly at temperatures 600C. Similar results were ob-

tained for the secondary burn. Effects were most pro- major reason for field abandonment and opening ofnew land for agricultural production in tropical regionsnounced in the surface layer (05 cm) and were absent

(soil C, N, Bray-1 P, exchangeable Ca, Mg, Na, H) or (see, e.g., reviews in Nye and Greenland, 1960; San-chez, 1976).less severe (exchangeable K and Al) at 5- to 15-cm

depth. These results are similar to short-term burn ef-fects reported in the literature (e.g., Nye and Greenland, Color Observations in the Field Studies1960; Andriesse and Koopmans, 1984; Andriesse and

Colors of the topsoil (05 cm) and subsoil (515 cm)Schelhaas, 1987; Holscher et al., 1997) and support farm-layers at the two locations were similar (Table 3). Aters observations on the enhanced fertility of ash-cov-the primary burn site, dry colors of the two layers before

ered black soil resulting from medium severity burns burning were 0.1Y 4.9/2.8 (10YR 5/3) and 0.1Y 5.5/(Ketterings et al., 1999).3.8 (10YR 6/4), respectively. Corresponding colors forSoil fertility parameters in the secondary burn experi-forest soil near the secondary burn were 0.1Y 3.3/2.7ment showed significant interactions between time and(10YR 3/3) and 0.1Y 3.6/3.3 (10YR 4/3). Soil color didfire severity. Soil organic C (Fig. 1a) and N (resultsnot change significantly during the 12-wk time periodnot shown) were unaffected by the low-severity firefollowing the burn, but was affected by fire severity.throughout the experiment. Exposure to the moderate

burn reduced soil C and N (surface soil only) during Burning of the topsoil (05 cm) at both locations causedthe first week following the burn, whereas reduced C Munsell values and chromas to decrease with increasingand N concentrations were detected up to 28 d following temperature to 600C (Table 3). The darkening of the

soil could be due to charring of the organic matter(Ulery and Graham, 1993). Similar trends were ob-served for value and chroma of the deeper layer in theprimary burn experiment. No change in color was noted

at 5 to 15 cm beneath the wood piles of the secondaryburn, suggesting that elevated temperatures in the sec-ondary burn did not penetrate deep into the soil andwere probably maintained for a shorter period. Pro-nounced reddening was only visible when topsoil hadbeen exposed to temperatures exceeding 600C in por-tions of the primary burn. This color change was dueto the thermal conversion of goethite (yellow) to maghe-mite (reddish brown) and hematite (red) (Ketterings etal., 2000). Reddening was accompanied by an increasein both the value and chroma of the fused soil material(Table 3), most likely due to more thorough removalof organic matter (Sertsu and Sanchez, 1978; Ulery andGraham, 1993). Such areas comprised 30 to 35% of

the burned field and were much more extensive thanreported by others (e.g., Dyrness and Youngberg, 1957;Ulery and Graham, 1993; Ulery et al., 1996), probablydue to differences in soil properties, types of slash, slashloading, and burn conditions.

Laboratory Study

Redness SusceptibilityFig. 1. Effect of fire and climatic exposure with time (190 d following A significant increase in redness susceptibility oc-

the secondary pile burn) on (A) soil C, (B) exchangeable Ca, andcurred with increasing temperature of exposure for the(C) Al saturation at the 0- to 5-cm depth. Numbers indicate the

standard error of difference for each sampling time. topsoil (05 cm) samples (Fig. 2). A similar but nonsig-


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Table 3. Effect of burn severity on field colors of2-mm sieved soil.

Forest 100 C 300 C 600 C 600 C s.e.d. Wald statistic

Primary burn

05 cm

Hue 0.1Y 0.2Y 0.6Y 0.6Y 7.6Y NA NAValue 4.9 3.6 3.2 3.2 4.4 NA NAChroma 2.8 2.8 2.2 2.1 3.5 NA NA

515 cm

Hue 0.1Y 0.1Y 0.1Y 0.1Y ND NA NAValue 5.5 4.1 3.7 3.7 ND NA NAChroma 3.8 3.2 2.8 2.8 ND NA NA

Secondary burn

05 cm

Hue 0.1Y 0.5Y 0.4Y 1.1Y ND 0.2 27.1Value 3.3 3.3 3.3 3.2 ND 0.1 6.2Chroma 2.7 2.5 2.5 2.2 ND 0.2 14.0

515 cm

Hue 0.1Y 0.1Y 0.2Y 0.1Y ND 0.1 5.3Value 3.6 3.7 3.7 3.7 ND 0.1 7.1Chroma 3.3 3.3 3.4 3.3 ND 0.1 1.5

s.e.d. is standard error of difference. Wald statistics 7.8 indicate significant differences at P 0.05 (3 df). NA indicates that no statistical analyses were done due to the fact that samples were composited. ND indicates that no samples were taken.

Changes in Color with Time of Exposurenificant trend was observed for the deeper materials.to Elevated TemperaturesAn increase in redness susceptibility confirms that the

samples became yellower with exposure to increasing Trends in the field colors and redness susceptibilityfield temperatures to 600C. However, this result seems can be understood by examining changes with time withcounterintuitive if redder colors are expected with burn- static heating of forest soil from the primary burn ating (LaFleur, 1970; Sertsu and Sanchez, 1978; Ulery and 300 and 600C in a laboratory oven. In this experiment,Graham, 1993; Ulery et al., 1996; Anda et al., 1998). topsoil (05 cm) hues initially became yellower withTemperatures of 400C were high enough to redden the heating, and the effect was much more pronounced atsoil in a study by Sertsu and Sanchez (1978), while in our 600C than at 300C (Fig. 3a). The yellowing also per-study surface temperatures of600C were necessary to sisted longer at the higher temperature (250 vs. 125cause reddening. Although the mineralogical composi- min). Eventually, the heated samples became reddertion and organic matter content of the soil will play an than the starting soil at both temperatures. Similar re-important role in fire-induced Fe oxide conversion and sults were obtained with the subsoil (515 cm) at 600C;

color changes (Ketterings et al., 2000), the duration of however, duration of the yellowing was shorter and theexposure to elevated temperatures must also be con- final hue was redder. At 300C, no change in subsoil

hue was observed.sidered.

Fig. 2. Redness susceptibility (R hue25 hue550, where hue25 is hue of field-burned soil and hue550 is the hue after 8 h of exposure to 550 C)of surface (05 cm) and subsurface soil (515 cm) exposed to different peak surface temperatures during a broadcast field burn. AlphanumericMunsell hues were converted to numerical values by using a Munsell hue circle (Chamberlin and Chamberlin, 1980) before calculating rednesssusceptibility. With this conversion 10R 10, 10YR 20, and 10Y 30. Different letters (surface soil only) indicate significant differencesat P 0.05.


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The samples rapidly darkened (lower Munsell value)with heating (Fig. 3b). No recovery over time was notedat 300C; however, samples heated at 600C becamelighter (higher Munsell value) as organic C was volatil-ized. Once again, this effect was delayed in the topsoilrelative to the subsoil. A similar delay was observedwith chroma of the 600C samples, and the final chromas

obtained after 930 min of heating differed significantlybetween the two layers (Fig. 3c).The pronounced changes observed at static tempera-

tures in the oven experiments demonstrate that the du-ration of exposure to elevated temperatures must beconsidered to understand the evolution of soil colorover short time periods (4 h) at moderate to hightemperatures in the field. Other factors affecting com-busted soil colors may include aggregate formation, or-ganic matter content, and mineralogy of the soil.

Effect of Aggregate Formationon Combusted Soil Color

The textures of combusted soils usually become

coarser as a result of the melting and fusion of primarymineral grains into aggregates (e.g., Sertsu and Sanchez,1978; Ulery and Graham, 1993; Ketterings et al., 2000).If aggregate formation shields primary particles fromcombustion, grinding should result in color changes.Table 4 shows the effect of grinding aggregates from 38samples representing all temperature treatments andboth layers at the secondary burn site. The results sug-

Fig. 3. Effect of static heat exposure over time on (A) Munsell hue,gest that the aggregates were coated with pigments that (B) Munsell value, and (C) Munsell chroma. Alphanumericdecreased the value and chroma. Increases in value and Munsell hues were converted to numerical values by using a

Munsell hue circle (Chamberlin and Chamberlin, 1980). With thischroma with grinding were largest for the forested sitesconversion 5YR 15, 5Y 25, and 5GY 35.and decreased with increasing heat intensity (data not

shown). Hues of the field burned samples were unaf-The same sample set contained 12 to 48 g kg1 offected by grinding.

citrate-bicarbonate-dithionite (CBD)-extractable Fe,Increases in the Munsell values of ground sampleswhich is commonly taken as a measure of the Fe-oxideafter reheating at 550C in an oven were similar to thosecontent. Although Fe oxides are important pigmentingof the field-burned samples. Chroma increases due toagents, CBD-extractable Fe produced very low correla-grinding were generally twice as large in the oven-com-tion coefficients with color parameters for the field sam-busted samples than in the field-burned samples. Huesples (data not shown). On the other hand, additionalincreased slightly with the largest increases at the high-exposure of field samples to 550C for 8 h resultedest fire intensities (data not shown). These results indi-in strong correlations between color parameters of thecate that pigmentation within the aggregates was notcombusted samples and CBD-extractable Fe contentuniform, even when heating was performed under con-prior to combustion in the laboratory (Fig. 4). The ob-trolled conditions. Aggregation apparently resulted inserved trends (decreasing hue and value but increasingphysical protection of the soil material. Therefore,chroma) with increasing Fe content are also consistentgrinding of combusted soils may result in lost or modi-with the results shown in Fig. 3 for surface and near-fied information.surface materials after heating at 600C for long time

periods.Effect of Organic Matter and Iron Oxideson Soil Color

Table 4. Effects of grinding (to pass a 250-m sieve) and organicTreatment of a subset of ground samples from the matter removal (treatment with 30% H2O2) on colors of field-

burned and oven-reheated 2-mm sieved soil (n 38).secondary burn with H2O2 to remove organic matter didnot significantly change soil color parameters (compare Hue Value Chroma250mto30%H2O2 samples in Table 4). These results Field burnedsuggest that the color was caused by the Fe oxides rather

2 mm 0.6Y 0.7 3.4 0.3 2.7 0.6than the organic matter. However, treatment with H2O2 250 m 0.4Y 0.3 5.4 0.5 3.8 0.5

30% H2O2 0.2Y 0.5 5.0 0.3 4.0 0.3could not remove all residual C; on average, 11 g kg1

Reheated (550 C, 8 h)C remained. This fraction was probably dominated by2 mm 4.9YR 0.4 3.8 0.3 5.9 0.2charcoal that could not be further oxidized and may250 m 5.1YR 0.2 5.4 0.2 7.5 0.3have affected the overall color of the soil.


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Fig. 4. Soil color parameters of field burned soil after recombustion at 550 C for 8 h as a function of the amount of citrate-bicarbonate-dithionite-extractable Fe. Alphanumerical Munsell hues were converted to numerical values by using a Munsell hue circle (Chamberlin and Chamberlin,1980). With this conversion 5YR 15 and 10YR 20.

SUMMARY AND CONCLUSIONS to soil fertility, colors of burned areas in the field did notchange significantly during the 12 wk following the burn.

Surface temperatures 600C were achieved in bothThe usefulness of postburn soil color measurements

field experiments, but the primary broadcast burn pro-for identifying heat-induced changes in soil fertility is

duced higher average intensities in this study. Mostlimited by the fact that fire severity is determined both

likely, locations selected under the primary burn wereby peak temperature and the duration of exposure.

exposed to elevated temperatures longer than the soilTherefore, one color measurement could represent dif-

under the secondary burn. Burning caused measurableferent combinations of time and temperature. Because

changes in soil color and fertility.soil fertility parameters do not react to different temper-

Concentrations of most nutrient elements (exchange-atureduration combinations as soil color does, the use-

able Ca, Mg, K, and Bray-1 P) taken 2 wk after the fulness of field color measurements for predicting soilsecondary field burns were significantly higher than in fertility is limited. Rapid changes in soil fertility (butthe forested controls at corresponding depths. Effects

not color) following the burn further limit the predictivewere most pronounced within 5 cm of the soil surface value of color measurements.and were strongly attenuated or lost within 15 cm. Simi-lar trends were observed in the primary burn. Exchange-

ACKNOWLEDGMENTSable Al and extractable acidity, by contrast, were re-

This research was funded by a Scholarship from the Mervinduced by fire exposure. Soil C and N were significantlyG. Smith International Studies Fund, an Ohio State Universityreduced in combusted (reddened) soil only. Within 2 moGraduate School Alumni Research Award, and two projectsafter the burn, exchangeable cations had decreased towithin the International Center for Research in Agroforestry

preburn levels, whereas Al saturation had increased Southeast Asia Regional Program: the Smallholder Rubbermarkedly. Agroforestry Project (CIRAD/GAPKINDO/ICRAF) and the

Munsell values and chromas decreased and hues be- Alternatives-to-Slash-and-Burn project supported by the Globalcame yellower with heating to 600C. At higher temper- Environment Facility with United Nations Development Pro-

gram sponsorship.We aregrateful to Mr.Sahroni,Mr. Zulkifli,atures, soil C was mostly depleted and reddening of theand the Kepala Desa of Sepunggur for allowing us to conductsoil matrix occurred. Laboratory studies showed thatour study in their fields and to their families for their support,pronounced changes in color parameters developedmeals, and water supply that prevented one of us (QMK)with time when the soil was subjected to static heating. from reaching skin color hues approaching 10R.As in the field, Munsell hues became yellower as values

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