Changes in Soil Mineralogy and Texture Caused by Slash-And-Burn Fires

1108 SOIL SCI. SOC. AM. J., VOL. 64, MAY–JUNE 2000

G. Ullmann. 1987. Quantitative x-ray powder diffraction method Taylor, J.C., and Z. Rui. 1992. Simultaneous use of observed andusing the full diffraction pattern. Powder Diff. 2:73–77. calculated standard profiles in quantitative XRD analysis of miner-

Soil Survey Staff. 1972. Soil survey of islands of Kauai, Oahu, Maui, als by the multiphase Rietveld method: The determination of pseu-Molokai, and Lanai, State of Hawaii. USDA-SCS in cooperation dorutile in mineral sands products. Powder Diff. 7:152–161.with the Univ. of Hawaii Agric. Exp. Stn. Tazaki, K.S., S. Kimura, T. Yashimura, J. Akai, and W.S. Fyfe. 1989.

Soil Survey Staff. 1973. Soil survey of island of Hawaii, State of Clay-organic complexes as a cementing agent in the Arahama SandHawaii. USDA-SCS in cooperation with the Univ. of Hawaii Agric. Dune, Japan. Clays Clay Miner. 37:219–226.Exp. Stn. Von Dreele, R.D., and J.P. Cline. 1995. The impact of background

Taylor, J.C. 1991. Computer programs for the standardless quantita- function on high accuracy quantitative Rietveld analysis (QRA):tive analysis of minerals using the full powder diffraction profile. Applications to NIST SRMs 676 and 656. Adv. X-Ray Anal. 38:Powder Diff. 6:2–9. 59–68.

Taylor, J.C., and C.E. Matulis. 1994. A new method for Rietveld clay Wada, K., and M.E. Harward. 1974. Amorphous clay constituents ofanalysis. Part I. Use of a universal measured standard profile for soil. Adv. Agron. 26:211–260.Rietveld quantification of montmorillonites. Powder Diff. 9: Young, K.R.A. 1995. Introduction to the Rietveld method. p. 1–38.119–123. In R.A Young (ed.) The Rietveld method. International Union of

Taylor, J.C., C.E. Matulis, I. Leipa. 1996. SIROQUANT Clay Package Crystallography. Oxford Science Publ., New York.Manual. CSIRO Division of Coal and Energy Technology, Can-berra, ACT, Australia.

Changes in Soil Mineralogy and Texture Caused by Slash-and-Burn Firesin Sumatra, Indonesia

Quirine M. Ketterings, Jerry M. Bigham,* and Valerie Laperche

ABSTRACT

We investigated the effect of fire intensity from slash-and-burn

Small-scale rubber farmers in Sepunggur, Sumatra,agriculture on the mineralogy of Oxisols in the Sepunggur area, JambiProvince, Sumatra, Indonesia, in both field and laboratory experi- Indonesia, indicated in a recent survey that fire isments. Samples were collected from two depths (0–5 and 5–15 cm) widely used to clear forests because it provides an easyat locations exposed to 100, 300, 600, and .6008C surface temperatures and economical means of increasing access to the fieldsduring the burns. Soils under forest and slashed vegetation were col- (Ketterings et al., 1999). Forest vegetation is slashedlected as controls. The pre-burn soil mineralogy was dominated by in the beginning of the dry season (March–April) andkaolinite, gibbsite, anatase, and goethite. Changes in soil properties burned from June through August. If a first-time broad-with burning were most pronounced in the 0- to 5-cm layer. Burning

cast burn does not remove enough aboveground bio-the topsoil led to coarser textures, especially at temperatures ex-mass, the remaining wood is piled in heaps of 200 toceeding 6008C. Heat reduced the gibbsite and kaolinite concentrations400 kg and burned a second time.and converted goethite into ultra-fine maghemite, thus increasing the

Peak temperatures measured at the soil surface dur-magnetic susceptibility of the samples. The conversion of goethite didnot take place until water in the samples had vaporized. Addition of ing broadcast and subsequent secondary burns varyorganic matter to soil with a low organic C content before heating widely. Intensities depend not only on the fuel load andincreased the magnetic susceptibility, indicating that organic matter the water content of the slashed biomass (Brown, 1988)was necessary (and limiting) for the complete conversion of goethite. but also on the stacking pattern, climatic conditionsCoarse-grained magnetite particles were present prior to and after the during the burn, and the size of the area being burned.burning and, therefore, were not pyrogenic. Magnetic susceptibility Hartford and Frandsen (1992) measured maximum sur-measurements were highly discriminatory among heat treatments,

face temperatures of 4008C in a ground (litter and duff)whereas x-ray diffraction (XRD) was much less sensitive to fire-fire, and Sertsu and Sanchez (1978) reported tempera-induced changes in mineralogy. Our research showed that severetures .5008C in shifting cultivation fires. Similar inten-burning had drastic effects on soil mineralogy, but changes shouldsities have been recorded by Wells et al. (1979) andalso be expected at lower fire intensities. Further research is neededChandler et al. (1983), whereas burns in Senegal report-to determine how important these changes in soil mineralogy are for

nutrient availability in the growing season after the burn. edly reached levels .7008C (Masson, 1948).Heat transfer into the soil depends not only on theWhenever he saw the sea, or a fire, he fell silent,

surface temperature but also on the duration of expo-impressed by their elemental force.sure, on the water content of the soil, and the soil porePaulo Coelho, The Alchemistdistribution (Steward et al., 1990). Ghuman and Lal(1989) measured temperatures of 2188C at 1 cm below

Q.M. Ketterings, Environmental Science Graduate Program; and J.M.Abbreviations: CBD, citrate-bicarbonate-dithionite; DTA, differen-Bigham and V. Laperche, School of Natural Resources, Ohio Statetial thermal analysis; FC%mass, fractional conversion factor for mag-Univ., 2021 Coffey Road, Columbus OH 43210. Received 1 Feb. 1999.netic susceptibility; TG, thermogravimetric analysis; XRD, x-ray dif-*Corresponding author ([email protected].).fraction; kfd, frequency-dependent magnetic susceptibility; xlf, lowfrequency mass specific magnetic susceptibility.Published in Soil Sci. Soc. Am. J. 64:1108–1117 (2000).

KETTERINGS ET AL.: SOIL CHANGES CAUSED BY SLASH-AND-BURN IN INDONESIA 1109

Table 1. Characterization of unburned forest soil from Field Experiments I and II.

Experiment Mineralogy Depth Sand Silt Clay pHwater OC† CBD-Fe‡

cm g kg21 g kg21

I Kaolinite with gibbsite, goethite, 0–5 40 90 870 4.4 52 41quartz, and anatase 5–15 40 200 760 4.6 32 45

II Kaolinite with gibbsite, goethite, 0–5 13 398 590 4.5 67 12quartz, and trace of hydroxy- 5–15 9 428 564 4.6 20 24Al-vermiculite

† OC is organic C.‡ CBD is citrate-bicarbonate-dithionite extraction.

the soil surface in windrows burned after clearing a netic susceptibility of the same soil after being exposedto 5508C for 8 h in an oven. High values of the fractionaltropical rainforest. Temperatures dropped to 150, 104,

and 708C at 5-, 10-, and 20-cm depths, respectively. De- conversion factor indicate that Fe oxide conversion tookplace prior to reheating. Lower values are obtainedBano et al. (1979), as reported in Brown (1988), found

that temperatures at the 5-cm depth did not change where prior burning was either absent or of low inten-sity. In situations where laboratory facilities for XRDsignificantly even when surface temperatures reached

7008C. are not available, Fe concentrations are very low, fieldmeasurements are needed, or the amount of sample isNumerous papers have shown that low-intensity

burns (,2508C) affect soil biological and chemical prop- limited, magnetic susceptibility may be a good tool formeasuring fire intensities and fire-induced changes inerties (e.g., Nye and Greenland, 1960), but the impactFe oxides.of burning on soil mineralogy has not been widely evalu-

We investigated the effect of heat intensity on theated even though moderate (250–5008C) and severe firesmineralogy of Oxisols in the Sepunggur area, Jambi(.5008C) have the potential to cause mineral transfor-Province, Sumatra, Indonesia, where slash-and-burnmations. Kaolinite, for example, has been shown to de-agriculture is still commonly used (Ketterings et al.,compose at temperatures between 500 and 7008C (Rich-1999). Our objectives were to evaluate (i) the effects ofardson, 1972). Gibbsite may be completely destroyedburn intensity on soil mineralogy, (ii) the use of mag-by heating in air at 2008C (Rooksby, 1972), and goethitenetic susceptibility as a tool for measuring burn inten-is altered to hematite at ≈3008C (Cornell and Schwert-sity, and (iii) factors influencing changes in magnetic sus-mann, 1996). Synthetic magnetite transforms to maghe-ceptibility.mite at temperatures as low as 2208C (Sidhu, 1988), and

pure maghemite may revert to hematite at temperaturesMATERIALS AND METHODSaround 3508C (Mullins, 1977). However, natural maghe-

mites are often stabilized by impurities giving rise to Two sites in the Sepunggur area (1028149 E, 18299 S) weretransformation temperatures exceeding 6008C (Mul- selected for Field Experiments I and II, respectively. Thelins, 1977). soils at both sites were classified as Hapludox and Kandiudox

according to U.S. soil taxonomy (Soil Survey Staff, 1999).Most studies of the heat stability of minerals havebeen performed in the laboratory with either synthetic

Field Experiment Ior purified natural minerals. Under field conditions, firehas been observed to collapse some 2:1 phyllosilicates Field Experiment I was a 12- to 15-yr-old secondary forestand destroy kaolinite (Ulery et al., 1996). The presence that was burned in July 1997. The entire field (0.75 ha) wasof maghemite in soils has often been attributed to the subjected to a low-intensity broadcast burn by the farmer.

Following the broadcast burn, three burn piles were estab-dehydroxylation of goethite or lepidocrocite by burninglished (400 kg of wood with stem diameters between 3.2 andin the presence of organic matter (Van der Marel, 1951;6.4 cm) on an area of 3 by 3 m. The piles were ignited, andTaylor and Schwertmann, 1974; Anand and Gilkes,temperatures were measured using heat-sensitive crayons1987; Stanjek, 1987). Maghemite formation is of interest(Cole Parmer, Vernon Hills, IL) situated directly on top ofbecause of its ferrimagnetic character and the potentialthe mineral soil and beneath any existing vegetative litter.for using magnetic techniques to evaluate the conver- Crayons were placed in the center, at the edge, and outside

sion process. Magnetic susceptibility (the total magnetic each pile. A control sample was taken in a 12- to 15-yr-oldforce in a material divided by the strength of the mag- secondary forest near the burned field (Table 1). The farmernetic field inducing magnetization) is one such method. indicated that the age of the forest surrounding the burned

plot was identical to the forest that was slashed, burned, andBesides aiding in the identification of Fe oxide trans-sampled during this experiment. Therefore, the samples takenformations due to fire, magnetic susceptibility measure-in the forest reflect the situation prior to slash-and-burn, thements may give an indication of fire intensity whensamples taken outside the piles reflect the situation after aresults are expressed as frequency-dependent magneticlow-intensity broadcast burn, and the samples taken under-susceptibility (kfd) or as a fractional conversion factorneath the piles reflect the effect of high-intensity secondary(FC%mass). Dearing (1994) has suggested that soils with fires at two temperatures. All locations were sampled at depths

kfd between 10 and 15% are dominated by ultrafine of 0 to 5 cm (hereafter referred to as topsoil) and 5 to 15 cmferrimagnetic particles that are often products of high- (hereafter referred to as subsoil). Ash, charcoal, and unburnedintensity burning. The FC%mass is calculated as the ratio wood remaining on the field after the secondary burn were

carefully removed prior to soil sampling. Each location wasof the magnetic susceptibility of a field soil to the mag-


sampled before the burn and 1 d and 1, 2, 4, 8, and 12 wk is the high-frequency (4.6 kHz) volume-specific magneticsusceptibility 3 1025. The mean diamagnetic volume specificafter the burn. Soil water contents (1058C) were determined

at each sampling time. Samples were returned to the labora- magnetic susceptibility of the standard 10-cm3 pot was 20.4 31025. All volume-specific magnetic susceptibility measure-tory the same day, sieved through a 2-mm sieve, and air-dried

for further analysis. ments were adjusted for this value prior to calculating mass-specific susceptibility. Soils with kfd between 10 and 15% wereconsidered to contain ultrafine (,0.035 mm) ferrimagneticField Experiment IIminerals as reported by Dearing (1994).

For Field Experiment II, a 20-yr-old secondary forest was A second set of dried (608C) samples was exposed to 5508Cselected (Table 1). Bulk soil samples (4–5 kg) in this 5-ha for 8 h and analyzed for xlf after cooling to room temperature.field were taken after slashing and after a broadcast burn at Fractional conversion values, which may serve as indicatorslocations based on the temperature (100, 300, and 6008C) of prior burning (Dearing, 1994), were calculated as:reached during the burn as measured with heat-sensitive cray-

FC%mass 5 100(xlf/xlf @ 5508C) [2]ons. Samples were taken at depths of 0 to 5 and 5 to 15 cm.Subsamples of completely combusted, brick-like topsoil were where FC%mass is the fractional conversion value in percentage,taken in locations where high fuel loads had produced temper- xlf is the mass-specific low-frequency (0.46 kHz) magneticatures that exceeded those measurable with the crayons susceptibility 3 1028 m3 kg21, and xlf @ 5508C is the xlf of(.6008C). In addition, control samples were taken in the sur- 5508C exposed soil 3 1028 m3 kg21.rounding unslashed and unburned forest. All samples weretaken 2 wk after the burn, and sample processing was identical X-Ray Diffractionto that for the samples from Field Experiment I.

The sand and silt samples were ground in a ceramic mortarand pestle to pass a 250-mm sieve and backfilled to produceRoutine Soil Analysesrandomly oriented sample mounts. Two-hundred milligrams

Whole soil samples were analyzed for organic C according of each clay were mixed with 15 mL of deionized water, andto the Walkley–Black method (Walkley, 1947). The pH was one drop of 0.5 M NaOH was added to enhance dispersion.determined electrometrically in a 1:5 mass ratio of soil/water The suspensions were then ultrasonically dispersed for 30 s,after shaking the samples on a reciprocal shaker for 2 h. Parti- and 5 mL of the suspension were filtered through a 0.22-mmcle-size analyses were performed using standard sieve and Millipore filter (Millipore Corp., Bedford, MA). Filtration waspipette methods (Kilmer and Alexander, 1949) following re- stopped when the deposits were almost dry. The orientedmoval of organic matter with 30% (w/w) H2O2. The samples aggregates were then transferred onto glass slides for XRDwere extracted with citrate-bicarbonate-dithionite (CBD) analyses. The oriented clays were scanned after air drying and(Mehra and Jackson, 1960) to dissolve crystalline Fe oxides, after heating to 5508C for 4 h. All samples were analyzed withexcept magnetite. Final extracts were analyzed for Fe by a Philips diffractometer (Philips Elec. Inst., Mahwah, NJ) usingatomic absorption spectrophotometry. CuKa radiation (35 kV, 20 mA) and a graphite monochro-

mater. Measurements were made by step scanning with a fixedSoil Fractionation time of 4 s per 0.0582u for routine work and a fixed time of

6 s per 0.0182u for detailed analyses of Fe oxides. In most cases,Soil samples taken from Field Experiment II and from Field the XRD patterns were obtained from 2 to 7082u. MineralsExperiment I before the second burn were treated with 1 M occurring in the sand, silt, and clay fractions were identifiedNaOAc-HOAc buffer (pH 5) and 30% (w/w) H2O2 to remove on the basis of their diagnostic XRD peaks and their responseorganic matter (Jackson, 1975). The residual materials were to heating. Hydroxy-Al vermiculite was distinguished fromwashed free of excess salts, dispersed in water, and separated vermiculite and chlorite by the partial collapse of the 1.4-nminto sand (2–0.05 mm), silt (0.05–0.002 mm), and clay (,0.002 peak with heating to 5508C.mm) fractions using standard sieve and gravity sedimentationtechniques (Jackson, 1975). The sand and silt fractions were

Thermal Analysisdried at 608C, whereas the clays were Na-saturated, quickfrozen, and freeze-dried. Thermal analyses of clay and silt samples were conducted

to quantify kaolinite and gibbsite concentrations using a SeikoSSC 5020 instrument (Haake Inst., Paramus, NJ) that providedMagnetic Susceptibilitysimultaneous thermogravimetric (TG) and differential ther-

All whole soil samples and particle-size fractions were ana- mal analysis (DTA). Approximately 50 mg of sample werelyzed for magnetic susceptibility with a Bartington MS2 Sus- heated from 25 to 10008C at a rate of 208C min21 under aceptibility System (ASC Scientific, Carlsbad, CA). This system continuous flow (200 mL min21) of dry N2 gas. Calibration ofwas equipped with a dual-frequency sensor that accepted 10- the temperature signal was achieved using the melting pointscm3 samples in standard plastic pots. Mass-specific magnetic of In and Sn. Calibration of the thermal balance was performedsusceptibility was calculated as the volume-specific susceptibil- using a reference weight provided by the instrument manufac-ity (k) divided by the density of the sample in the 10-cm3 pots. turer. Thermal events associated with dehydroxylation of ka-Low-frequency (0.46 kHz) magnetic susceptibility (xlf, units olinite and gibbsite were assigned based on published litera-of m3 kg21) was measured for all samples prior to exposing ture (e.g., Dixon, 1966) and analyses of standard mineralthem to a high-frequency magnetic field (4.6 kHz). The dual- samples.frequency measurements permitted the calculation of fre-quency-dependent magnetic susceptibility as:

Oven Experiment I: Effect of Added Carbonkfd 5 100(klf 2 khf)/klf [1]

Duplicate, 10-g samples of forest soil (0–5 and 5–15 cm)from the control area of Field Experiment II were mixed withwhere kfd is the volume-specific frequency-dependent mag-

netic susceptibility in percentage, klf is the low-frequency (0.46 0, 20, 40, 80, 100, 150, and 200 g kg21 air-dried, processedIndian tea (Camellia sinensis L.) or 50 and 100 g kg21 granu-kHz) volume-specific magnetic susceptibility 3 1025, and khf


Table 2. Effect of heat on soil texture in Field Experiment II.†

Sand Silt Clay

Total Total TotalSample Depth VCS CS MS FS VFS sand CSI MSI FSI silt CC FC clay

cm g kg21

Forest 0–5 5 2 1 3 3 13 136 120 143 398 379 211 5905–15 3 2 1 2 2 9 147 151 131 428 355 208 564

Slashed 0–5 6 5 5 16 9 41 309 27 21 357 273 329 6025–15 3 6 4 14 10 36 231 35 18 283 326 356 682

100 8C 0–5 3 5 5 16 10 38 172 35 22 228 347 388 7355–15 2 4 5 16 10 36 109 37 25 170 417 378 794

300 8C 0–5 3 11 10 23 12 58 126 31 26 182 277 483 7615–15 1 4 5 14 8 31 131 33 20 184 367 419 786

600 8C 0–5 7 21 17 30 17 92 124 44 46 213 352 344 6965–15 3 6 5 16 10 39 129 28 22 179 397 387 783

.600 8C 0–5 365 234 80 106 45 829 19 31 14 63 55 54 108

† VCS, very coarse sand (1–2 m); CS, coarse sand (0.5–1 mm); MS, medium sand (0.25–0.5 mm); FS, fine sand (0.1–0.25 mm); VFS, very fine sand (0.05–0.1mm); total sand (0.05–2 mm); CSI, coarse silt (20–50 mm); MSI, medium silt (5–20 mm); FSI, fine silt (2–5 mm); total silt (2–50 mm); CC, coarse clay(0.2–2 mm); FC, fine clay (,0.2 mm), total clay (,2 mm).

lated sugar. Samples were exposed to 6008C for 930 min to of the 18 measurements and analyzed Field Experiment I asa two-factorial experiment (fire intensity at four levels andensure the complete conversion of antiferromagnetic to ferri-

magnetic minerals. Weight loss, water content, and magnetic depth of sampling at two levels) in three replicates. Analyses ofvariance were performed using the software package Genstat 5susceptibility were measured after heating. The tea used in

this experiment was combusted at 6008C for 8 h to determine for Windows, Release 3.2 (Genstat 5, 1987).Field Experiment II did not have independent replicatesthe ash content by weight loss.

due to the fact that bulk samples were taken. Each bulk samplewas separated into two subsamples that were analyzed sepa-Oven Experiment II: Kineticsrately and in duplicate in order to assess the precision of the

Duplicate 25-g soil samples (0–5 and 5–15 cm) from the measurements. Results were expressed as means and stan-control area of Field Experiment II were placed in 2-cm- dard deviations.diam. glass vials and heated in a muffle furnace. Samples wereexposed to 300 or 6008C for 1, 2, 4, 6, 10, 15, 30, 45, 90,120, 240, 360, and 660 min. Weight loss, water content, and RESULTS AND DISCUSSIONmagnetic susceptibility were measured after heating. Samples Texture Changeswere, in addition, analyzed for CBD-extractable Fe oxides.The magnetic susceptibility of the material remaining after The soil samples from Field Experiment II showedCBD extraction was measured in order to determine the con- some natural variability in texture but, in general, weretribution of the CBD-extractable Fe oxides to the total mag- high in clay (.55%) and low in sand (,5%) prior tonetic susceptibility. burning. Exposure of the soil to extreme temperatures

(.6008C) resulted in a sharp increase in the amount ofStatistical Analyses sand and a decrease in silt and, especially, clay (TableAll results of Field Experiment I were initially analyzed in 2). Increases in sand were also observed at lower tem-

a complete randomized block design in three replicates with peratures but were mostly limited to the 0- to 5-cmthree factors: treatment (forest, first burn estimated as 1008C depth. These results are similar to those obtained inexposure, 3008C exposure during second burn, and 6008C ex- laboratory studies by Dyrness and Youngberg (1957),posure during second burn), depth (0–5 and 5–15 cm), and Sertsu and Sanchez (1978), and Duriscoe and Wellstime (0, 1, 7, 14, 28, 56, and 84 d after burning). Parameters

(1982), and in field studies by Sreenivasan and Auranga-were considered time dependent when differences over timebadkar (1940) and Ulery and Graham (1993).were larger than the standard deviations of the individual

treatments, the uncertainty of the means when sampling intriplicate, or the standard deviations of the forest treatment. Mineralogical ChangesAnalyses of the 60 grid samples (0–5 cm depth) in the forest

The sand fraction of the forest soil from Field Experi-neighboring that of Field Experiment I showed that takingment II (both 0–5 and 5–15 cm) contained mainly quartzthree samples to calculate a mean (a 5 0.05) would result inand gibbsite with traces of kaolinite. Diagnostic XRDan uncertainty of the mean of 63.7 3 1028 m3 kg21 for xlf and

2.6% for kfd. Uncertainties for the subsoil (5–15 cm) means peaks for gibbsite in the topsoil sand fractions graduallywere less: 26.3 3 1028 m3 kg21 for xlf and 1.9% for kfd. Al- decreased in intensity with increasing burn temperaturethough several magnetic susceptibility parameters showed sig- and disappeared completely in the combusted (.6008C)nificant differences among sampling times, those differences samples (data not shown). Weakened intensities in thedid not follow a recognizable pattern. Nor were they larger subsoil samples were observed only at 6008C. Quartzthan the standard deviations of the parameters in the forest XRD peak intensities were unaffected except for a de-treatment or the uncertainty of the mean estimates when tak-

crease in the topsoil sands exposed to temperatures ofing three replicates. The time series samples of Field Experi-6008C or greater. Because quartz should not vitrify atment I were thus combined to calculate the average for eachthis temperature, the diminished intensities must bereplicate per treatment per layer for further heat exposurecaused by encapsulation of quartz particles in a glassycomparisons. Because the samplings over time cannot be con-

sidered completely independent, we calculated the averages matrix.


Table 3. Low frequency (0.46 kHz) mass–specific magnetic susceptibility (xlf) of sand, silt, and clay fractions of heat-exposed soil fromField Experiment II.

xlf of sand xlf of silt xlf of clay

0–5 cm 5–15 cm 0–5 cm 5–15 cm 0–5 cm 5–15 cm

1028 m3 kg21

Forest 11.2 (0%)‡ 19.2 (0%) 31.9 (19%) 41.3 (45%) 90.4 (81%) 37.8 (54%)Slashed 85.2 (2%) 86.2 (1%) 184.8 (30%) 215.7 (26%) 249.5 (68%) 245.7 (72%)100 8C 69.4 (2%) 59.0 (1%) 166.8 (24%) 165.0 (17%) 159.4 (74%) 173.9 (82%)300 8C 133.5 (3%) 94.0 (1%) 235.6 (17%) 223.8 (20%) 259.7 (80%) 212.5 (79%)600 8C 192.2 (2%) 99.7 (1%) 331.5 (9%) 250.7 (16%) 948.6 (88%) 294.7 (83%).600 8C 2583.9 (84%) ND§ 2329.1 (6%) ND 2397.3 (10%) ND

† Actual values equal reported values times the indicated factor.‡ Contribution of a particular fraction to the mass-specific magnetic susceptibility of the bulk soil.§ ND 5 no field observations.

The sand fraction contributed ,3% of the total soil at 6008C and were absent in the combusted (.6008C)topsoil (Fig. 1). Kaolinite concentrations in the silt frac-magnetic susceptibility after all heat treatments except

for the combusted topsoil, where it was responsible for tion of the topsoil were mostly unaltered by heating. Inthe subsoil, quartz peak intensities remained unchanged84% of the total (Table 3). The glassy, sand-sized parti-

cles showed high spontaneous magnetization, with xlf through 3008C but were lower after exposure to 6008C(data not shown). Kaolinite, gibbsite, and goethite con-values of 2584 3 1028 m3 kg21 (Table 3). The magnetic

susceptibility of the subsoil sand fraction was one-half centrations in the subsoil silt fractions were unaltered bythat of the overlying topsoil in areas exposed to 6008C the surface fires. Heat exposure increased the magneticsurface temperatures. Silt in the forest soil contained susceptibility of silt fractions from both the topsoil andquartz, gibbsite, kaolinite, goethite, anatase, and traces subsoil (Table 3).of hydroxy-Al-vermiculite. Once again, burning the top- The clay fraction at both depths contained some hy-soil resulted in a decrease in quartz peak intensities droxy-Al-vermiculite and anatase but was dominatedafter exposure to 6008C (Fig. 1). A decrease in gibbsite by kaolinite, gibbsite, and goethite. Losses of kaoliniteconcentration occurred at 3008C and was evident in both with heating and the formation of a glassy phase wereXRD (Fig. 1) and TG analyses (Table 4). No further pronounced only in the combusted topsoil where kaolin-reductions with heating were noted from thermal analy- ite concentrations were reduced from 750 g kg21 (1008C)sis, but XRD peaks for gibbsite decreased in intensity to 270 g kg21 (.6008C) (Fig. 2, Table 4). Losses of

gibbsite occurred at lower heat intensities. Gibbsite con-centrations in the topsoil clay decreased from 100 to 50g kg21 at 6008C, and clay-sized gibbsite disappeared inthe combusted topsoil (.6008C). An increase in mag-netic susceptibility of the clays upon heating (slashedto 6008C) indicated that ferrimagnetic minerals weregradually being formed by conversion of goethite tomaghemite or magnetite (Table 3). Hematite was alsoa combustion product. The appearance of these phaseswas difficult to confirm through conventional XRDanalysis of the clays except in the combusted topsoil.The persistence of ferrimagnetic phases at surface tem-peratures exceeding 6008C indicates that heat transferinto the soil is a fairly inefficient process. Moreover,

Table 4. Kaolinite and gibbsite contents of silt and clay fractionsfrom Field Experiment II as determined by thermogravime-tric analysis.

Topsoil (0–5 cm) Subsoil (5–15 cm)

Clay Kaolinite Gibbsite Kaolinite Gibbsite

g kg21

Clay100 8C 750 100 740 100300 8C 740 90 720 100600 8C 730 50 730 100.600 8C 270 0 ND† ND

Silt100 8C 220 240 260 250300 8C 210 170 210 220Fig. 1. X-ray diffraction patterns, showing anatase (A), gibbsite (Gb),600 8C 480 150 300 220goethite (Go), hematite (H), kaolinite (K), and quartz (Q), of the.600 8C 190 160 ND ND

silt fraction from topsoil exposed to different intensities of burnsin Field Experiment II. † ND 5 no field observations.


Fig. 2. X-ray diffraction patterns, showing anatase (A), gibbsite (Gb), goethite (Go), hematite (H), kaolinite (K), maghemite and/or magnetite(M), and quartz (Q), of the clay fraction prior to burning (slashed soil) and following combustion at .6008C in Field Experiment II.

trace impurities, such as Al, tend to inhibit thermal different types of magnetic particles, some that wereblack and others that were orange-brown. X-ray diffrac-transformations of these minerals (Sidhu, 1988).tion analyses showed that the black particles containedmaghemite and hematite in addition to magnetite (Fig.Source of Increased Magnetic Susceptibility:

Maghemite or Magnetite? 4a). Microprobe analyses yielded Ti/Fe ratios rangingfrom 0 to 1. These particles were also present in the soilThe percentage of the magnetic susceptibility lost

after CBD extraction of the samples from Field Experi-ment II increased from 64% in the slashed topsoil to93% in the combusted topsoil (Fig. 3). Hence, heatingcauses the CBD-extractable Fe oxides to become in-creasingly susceptible to spontaneous magnetization.Because magnetite is resistant to CBD extraction(McKeague et al., 1971; Gamble and Daniels, 1972;Walker, 1983; Fine and Singer, 1989), our results suggestthat maghemite is formed during the burning process.

Fractionation of ground subsamples of the field com-busted sand fraction using a hand magnet produced two

Fig. 4. X-ray diffraction patterns showing hematite (H), magnetiteFig. 3. Effect of fire intensity on the contribution of citrate-bicarbon- (Ma), and maghemite (Mh) of the black (a) and orange-brown

(b) ferrimagnetic fractions obtained from completely combustedate-dithionite (CBD)-extractable Fe oxides to the total bulk soilmagnetic susceptibility (Field Experiment II). Error bars indicate topsoil in Field Experiment II. X-ray diffraction d spacings (nm)

are given for an unidentified phase.standard deviations of the means.


Table 5. Effect of burning on magnetic susceptibility and derived parameters from ,2-mm soil material (Field Experiments I and II).†

Depth

0–5 cm 5–15 cm

Forest 100 8C 300 8C 600 8C .600 8C Forest 100 8C 300 8C 600 8C

Field Experiment I

xlf 91.8 86.1 101.9 109.9 ND‡ 89.7 73.9 75.5 77.7kfd 3.3 4.4 5.5 5.5 ND 3.4 3.4 3.5 3.5FC%mass 20.8 23.1 31.0 43.8 ND 56.2 58.9 72.2 70.0

Field Experiment II

xlf 14.9 68.7 117.7 231.2 2467.3 22.0 65.8 95.6 102.2kfd 1.0 7.5 9.0 11.2 13.5 1.7 7.5 8.0 7.7FC%mass 2.3 11.6 13.0 23.1 98.5 1.7 17.5 25.0 20.7

† xlf, mass-specific magnetic susceptibility measured at low frequency of soil dried at 60 8C in 1028 m2 kg21; kfd, frequency-dependent volume specificmagnetic susceptibility of soil dried at 60 8C in %; FC%mass, mass-specific fractional conversion at low frequency in percentage.

‡ ND 5 no field observations.

prior to burning and, therefore, were not pyrogenic. transfer did not reach below 5 cm in Field ExperimentThe orange-brown fraction could be dissolved in CBD, I but did at Field Experiment II. The location of Fieldwhereas the black particles were mostly unaffected by Experiment II (on top of a small hill), favorable climaticCBD treatment. X-ray diffraction analysis of the or- conditions, a longer period of drying after the slash, andange-brown particles (Fig. 4b) indicated the presence a larger burn area might have contributed to higher fireof both hematite and maghemite. The approximate xlf intensities and deeper heat penetration into the soil atof the maghemite can be calculated as 53870 3 1028 this site relative to Field Experiment I. An increase inm3 kg21 if we assume that these particles are mostly kfd indicates the formation of ultrafine superparamag-maghemite. This value corresponds well with the xlf of netic grains upon heating (Dearing, 1994). In the com-54000 3 1028 m3 kg21 found by Graham and Scollar busted (.6008C) topsoil from Field Experiment II, kfd(1976) and is slightly higher than those previously re- approached the upper limit of 15% (Dearing, 1994),ported for pure maghemites (Mullins, 1977; Dearing, indicating that ultrafine maghemite particles dominated1994). the magnetic fraction.

Fractional conversion factors (FC%mass) also in-creased with heat intensity. Combusted topsoilVariability of Fire Induced(.6008C) from Field Experiment II gave a FC%massMineralogical Changesvalue of ≈100%, indicating that maximum conversion

In Field Experiment I, the xlf of forest topsoil in- of goethite to maghemite had taken place in the fieldcreased upon exposure to fire (6008C) by a factor of (Table 5). The FC%mass values appear to be good indica-1.2, whereas in Field Experiment II the increase was by tors of fire intensity for the 0- to 5-cm depth samples.a factor of 15.5 (compare xlf for forest with xlf at 6008C In this regard, it is surprising that the 5- to 15-cm depthin Table 5). Furthermore, the magnetic susceptibility of samples showed higher FC%mass values for all heat inten-unburned forest soil in Field Experiment I was six times sities in Field Experiment I and for the 100 and 3008C(topsoil) and four times (subsoil) larger than the mag- treatments in Field Experiment II. It might be expectednetic susceptibility of the forest soil in Field Experiment that the subsoil FC%mass values would be lower becauseII. The FC%mass values for topsoil amounted to 20.8 and there was less exposure to the high surface tempera-2.3% for Field Experiments I and II, respectively. For tures. However, the total organic matter contents of thesubsoil, the FC%mass values were 56.2 and 1.7% for Field 5- to 15-cm depth samples were, on average, half thoseExperiment I and II, respectively. These observations of the 0- to 5-cm depth materials and may have limitedsuggest that the field in Field Experiment II had not the conversion of goethite to maghemite. We investi-been seriously impacted by prior burns; whereas, the gated the contribution of soil organic matter and Feforest soil in Field Experiment I had probably been oxide concentration, as well as the kinetics of changeburned previously. The topography of the field in Field in magnetic susceptibility at two temperatures (300 andExperiment I was flat, and the area was surrounded by 6008C), in two oven experiments using the forest soilforest. We therefore can assume that erosion did not from Field Experiment II (Table 1).play a major role in changing the magnetic susceptibilityof the soil so that the effects of burning may be detect-

Contribution of Soil Organic Matter to Increasesable for long periods of time.in Magnetic SusceptibilityIncreasing fire intensity in the 1997 burns resulted in

an increase in both xlf and kfd in the topsoil (0–5 cm) Adding additional organic material to the unburnedat both field sites (Table 5); however, the increases were topsoil of Field Experiment II prior to exposing it tomore dramatic in Field Experiment II. Subsoil (5–15 6008C for 930 min caused a linear decrease in magneticcm) effects were absent in Field Experiment I and pres- susceptibility (Fig. 5) according to the relationship:ent but less extreme than in the 0- to 5-cm depth atField Experiment II. These results suggest that heat xlf 5 702.6 2 0.709masstea r2 5 0.96, n 5 7 [3]


Fig. 5. Effect of organic material addition (Indian tea) on maximum achievable mass-specific magnetic susceptibility for initially unburned topsoil(0–5 cm) and subsoil (5–15 cm) in Oven Experiment I. Samples were heated at 6008C for 930 min to ensure complete combustion. OC isorganic C.

where xlf is the mass-specific magnetic susceptibility at the subsoil (24 g kg21 Fe in subsoil vs. 12 g kg21 Fe inthe forest topsoil).low frequency in 1028 m3 kg21, and masstea is the number

of grams of air-dry tea kg21 soil. Standard errorsamounted to 7.1 3 1028 m3 kg21 and 0.067 3 1028 m3 g21 Conversion Kineticsfor the intercept and the slope, respectively. The tea

The increase in xlf during the first 30 min of heatcontained 61 g kg21 ash, and this paramagnetic ash wasexposure in Oven Experiment II is shown in Fig. 6. Alsoapparently responsible for the decrease in magnetic sus-indicated in the same figure are the final levels of xlfceptibility due to a dilution effect.reached after 660 min of exposure. Soil water determi-By contrast, adding organic material to the subsoilnations (results not shown) indicated that pore waterincreased the xlf up to 50% (Fig. 5). The xlf increasedwas lost at 300 and 6008C after 10 and 1 min of exposure,until 105 g kg21 organic material were added and thenrespectively. The lag time in response to temperaturereached a plateau of 2147 31028 m3 kg21. Eighty-fivefor the 3008C curve can be explained by the initial vapor-percent of this increase was achieved with the additionization of soil water; that is, conversion of goethite toof 80 g kg21 organic material. The organic C content ofmaghemite did not take place until water in the samplesthe forest subsoil prior to burning was 20 g kg21. Addi-had vaporized. After water vaporization, xlf increasedtion of 105 g kg21 tea (with 61 g kg21 ash) to the subsoilsharply to 80% of the maximum level within 220, 115,made the organic matter level similar to that of the 300, and 75 min for topsoil at 6008C, subsoil at 6008C,topsoil. Apparently, 70 g kg21 organic C was sufficient topsoil at 3008C, and subsoil at 3008C, respectively.to keep this soil reduced during the burn and enabled The maximum xlf obtained by heating the topsoil forcomplete conversion of goethite to maghemite. an extended period of time at 6008C was ≈50% of thatAddition of 50 and 100 g kg21 granulated sugar in- observed for the subsoil. Part or all of this differencecreased the xlf of the topsoil to 654 and 696 3 1028 m3in obtainable xlf was probably due to the fact that thekg21, respectively. For subsoil the results were 1517 and forest subsoil contained 24 g kg21 CBD-extractable Fe;

1869 3 1028 m3 kg21 for 50 and 100 g kg21 sugar addition, whereas, the topsoil contained only 12 g kg21 CBD-respectively. These values deviated only slightly from extractable Fe. The CBD-extractable Fe in the samplesthe values obtained with the tea experiment. exposed to 6008C decreased rapidly with time of expo-

These observations support our earlier hypothesis sure. A similar decrease was observed in the combustedthat the amount of organic matter controlled the conver- topsoil in Field Experiment II and was most likely duesion of goethite to maghemite in the subsoil. Similar to the formation of glassy, sand-sized particles that en-results were obtained by Brown (1988) who observed capsulated and protected the ultrafine maghemite par-an increase in magnetic susceptibility when flour was ticles.added to a sample before igniting it at 5508C for 1 h.The relatively high FC%mass for subsoil as compared with

Field vs. Laboratory Levels of Magnetictopsoil (Table 5) could thus be explained by insufficientSusceptibilityorganic matter to keep the environment reduced during

burning. The final xlf of the subsoil material after addi- In both field experiments, burning produced measur-tion of extra organic matter was higher than for topsoil able changes in magnetic susceptibility, especially inwith the same organic matter content, which is probably the topsoil where fires lasted for a longer time (Field

Experiment II). The extent of change differed signifi-due to the higher CBD-extractable Fe oxide content of


Fig. 6. Mass-specific magnetic susceptibility (xlf) as a function of time and temperature of exposure for forest topsoil (0–5 cm, 67 g kg21 organicC, 12 g kg21 Fe) and subsoil (5–15 cm, 20 g kg21 organic C, 24 g kg21 Fe) in Oven Experiment II. On the right for each treatment is the xlfmeasured after 660 min exposure.

cantly from those obtained in oven experiments, where atures were between 470 and 5508C (data not pre-sented), a total of 30 to 35% of the surface of the burnedtemperature levels and duration of exposure were con-

trolled. Maximum achievable xlf levels with oven exper- field was covered by combusted (reddened) soil mate-rial. At places where tree trunks were present, under-iments using forest soil from Field Experiment II were

728 (topsoil) and 1333 3 1028 m3 kg21 (subsoil) at 6008C ground burning of roots led to the formation of holesthat could extend up to 50 cm deep. This severe burningand 543 (topsoil) and 1188 3 1028 m3 kg21 (subsoil)

at 3008C (Fig. 6). The values obtained at comparable was observed more frequently in our research area andcovered larger areas than reported by others.temperatures in the field were much lower. Topsoil ex-

posed to 3008C in the oven reached field-observed levels In a recent survey in the Sepunggur area (Ketteringset al., 1999), farmers indicated that reddened soil wasafter 12 min of exposure. For subsoil material, 3 min of

oven exposure was sufficient to reach field levels. For undesirable for crop growth, based upon their observa-tion that it could not hold water and was not fertile.exposure to 6008C in the oven, field-observed levels

were reached after 3 min (topsoil) and 15 s (subsoil). Our research showed that severe burning had drasticeffects on soil texture and mineralogy. Changes in soilBecause field samples had a water content of ≈450 g

kg21 directly before the burn (much larger than the mineralogy also take place at lower fire intensities andare still likely to influence the soil fertility status. Furtherwater content of the 608C dried soil that was used in

the oven experiments), an increase in time needed for research is needed to determine how important thesechanges in soil mineralogy are for nutrient availabilityevaporation of the remaining water probably accounts

for the substantial increase in time of exposure required in the growing season after the burn. Magnetic suscepti-bility should be an excellent tool for determining thein the field.

The highest magnetic susceptibility found in Field spatial variability of fire intensity for soil fertility evalu-ations.Experiment II (combusted topsoil) was 2467 3 1028 m3

kg21. The FC%mass approached 100% for the combustedtopsoil, indicating that this was the maximum obtainable ACKNOWLEDGMENTSmagnetic susceptibility. The magnetic susceptibility of This research was funded by a Scholarship from the Mervincombusted topsoil in Field Experiment II was higher G. Smith International Studies Fund, an Ohio State Universitythan the maximum achievable level for forest topsoil Graduate School Alumni Research Award, and by two proj-(670 3 1028 m3 kg21) in Oven Experiment II and also ects within the International Center for Research in Agro-of subsoil with the addition of extra organic matter forestry Southeast Asia Regional Program: the Smallholder

Rubber Agroforestry Project (CIRAD/GAPKINDO/(2147 3 1028 m3 kg21). Because topsoil from the origi-ICRAF) and the Alternatives-to-Slash-and-Burn project sup-nally unburned forest soil used in the oven experimentported by the Global Environment Facility with United Na-contained less than one-third of the CBD-extractabletions Development Program sponsorship. Salary and researchFe oxides (12 g kg21 Fe in forest topsoil, 41 g kg21 Fesupport for J.M.B. were provided by state and federal fundsin slashed topsoil prior to burning), a higher magneticappropriated to the Ohio Agricultural Research and Develop-susceptibility for the field-combusted topsoil should ment Center, The Ohio State University. We thank Sandy

be expected. Jones for running the texture analyses and are grateful to Mr.Zulkifli (Field Experiment I) and the chief of the village ofSepunggur (Field Experiment II) for allowing us to take sam-CONCLUSIONSples and to study their fields during the burning season of

Field observations showed that in intense burns, such 1997. Constructive criticisms from Dr. D.W. Ming, AssociateEditor, and three anonymous reviewers led to several im-as in Field Experiment II where average surface temper-


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