0038-075X/04/16909-663-674 September 2004

Soil Science Vol. 169, No.9

Copyright 0 2004 by Lippincott Williams & Wilkins, Inc. Prinited in U.S.A.

WEATHERING INTENSITY CONTROLLING SUSTAINA ILITYOF ULTISOLS UNDER SHIFTING CULTIVATION IN THE

CHITAGONG HILL TRACTS OF BANGLADESH

Abdul Gafurl, Christian Bender Koch2, and Ole K. Borsgaard2

Steep slopes and very high rainfall (nearly 3000 mm yr-1) concentratedwithin a few months (monsoon) suggest that the practice of slash-and-burn agriculture (known locally as Jhum) in the Chittagong Hill Tracts(CHT) region of Bangladesh will induce severe erosion losses. This is par-ticularly so in recent years because high population pressure has reducedthe fallow period from 10 to 15 years to 3 to 5 years. A 2-year study ofsoil composition and erosion comparing a Jhum cultivated catchmentand a neighboring non-burnt catchment showed additional loss by runoffof about 30 Mg ha-1 (41.1 from the burnt site minus 11.5 from the non-burnt site) of upland soil, containing substantial amounts of plant nutri-ents, as a result of burning. The main loss occurred within 2 to 3 monthsafter burning, but after 1 year, losses in the Jhum cultivated catchmentwere the same as in the non-burnt catchment. Furthermore, as almosthalf of the sediment was deposited in lower parts of the catchment, thenet loss (export) was 15.5 Mg ha-1, which corresponds to only 3 times theupper critical limit (about 5 Mg ha-1 yr-1) of sustainability, indicatingthat Jhum fallowed for more than 3 years may be sustainable. The ashfrom the burnt vegetation was found to add more available Ca, Mg, K,S, Fe, Mn, and Zn to the soil than was removed in runoff sediments,whereas soil C, N, P, and Cu contents decreased.

X-ray diffraction, Mossbauer and infrared spectroscopy, and totalchenmical analysis of the clay fractions from selected soil samples in profilesfrom upper, middle, and lower parts of a slope in the Jhum catchmentshowed composition and properties of the fractions that were almost iden-tical, irrespective of soil depth and site position. The high clay contents(31-51%), the rather unique clay mineralogy (gibbsite, goethite, halloysite,HlV/HIS, illite, kaolinite, verm-iiculite/smectite), and the horizontal andvertical uniformity are clear indicators of intensive and fast weathering.The fast and profound weathering results from a combination of an easilyweatherable parent rock (micaceous shale) exposed to a hot alternatingvery wet and very dry climate and vigorous vegetation. This combinationand the resulting soil development may, in turn, explain the observed highdegree of hydraulic and chemical resilience associated with Jhum in CHT.However, more studies over longer periods of time are needed to deter-mine whether currentJhum, with fallow periods of only 3 to 5 years, is sus-tainable in the long run. (Soil Science 2004;169:663-674)

Key words: Erosion, slash and burn, nutrients, resilience, weatherabil-ity, clay silicates, oxides.

'Soil Conservation and Watenhed Center, SRDI, Meghia, Bandarban, Bangladesh. 6i OILS in the monsoon-affected regions of Asia

'Dept. of Natural Sciences, The Royal Veterinary and Agricultural University, Thoor Sdevelop under seasonally very high rainfallvaldsensvel 40, DK-1871 Frederiks6erg C, Denmark; Dr. Borggaard is corresponding and general high soil temperatures which pro-author. E-mail: olkb@r6l.dk mote (i) soil acidification because of intense bio-

Received March 26, 2004; accepted June 23, 2004. logical activity and leaching, (ii) dissolution reac-

DOI: l0.1097101ss.0000142632.80160.5s tions causing release of essential plant nutrients,

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GAFUR, KOCH, AND BORGGAARD

(iii) transformations of primary to secondaryminerals, and (iv) selective removal of fines andnutrients by runoff on slopes (Borggaard and El-berling, 2004; Brammer, 1996; Brand and Pfund,1998; Farid et al., 1992; Gafur al., 2003). Soils inthese regions are often cultivated using traditionalshifting cultivation (slash-and-burn) systems(Borggaard et al., 2003; Ketterings et al., 1999;Rodenburg et al., 2003), the success of which de-pends on the ability of a fallow crop to collectsufficient plant nutrients over a number of sea-sons. The accumulated nutrients in the fallowcrops are released by slashing and burning (andsome are lost as gasses in the fire), and, for a shortperiod, the soils are sufficiently productive forfarming.

For many decades, the Chittagong Hill Tracts(CHT) region in Bangladesh has supported ashifting cultivation system, known locally asJhum, which was considered sustainable whenthe fallow periods were 10 to 15 years long(Borggaard et al., 2003; Gafur et al., 2000; Palm etal., 1996). However, in response to the growingpopulation pressure, farmers have now reducedthe fallow period to 3 to 5 years (Gafur et al.,2000), increasing the demand for fast release ofplant nutrients. Adequate mobilization of essen-tial plant nutrients through natural soil acidifica-tion and weathering processes is mandatory forproductivity, and, hence, the production capacityof the Jhum system. In addition to the climate,the extent of this mobilization depends on thesoil mineralogy and chemistry. Different weath-ering rates can cause fertility and crop productionto be high on a soil rich in easily weatherableminerals such as limestone, micas, olivine, and py-roxene, whereas a soil dominated by resistant, nu-trient-poor minerals such as quartz, kaolinite, andAl and Fe oxides will have low natural fertility(Borggaard and Elberling, 2004; Buol et al.,2003). In nonamended soil ecosystems, the com-position and weatherability of the minerals deter-mine soil sensitivity and resilience and, hence, thesoils' capacity to resist stress and to recover per-turbations, for example, in relation to variousland-use forms (Borggaard and Elberling, 2004;Lal, 1997; Sanchez et al., 2003).

Weathering typically transforms coarse pri-mary silicates into fine secondary minerals, i.e.the clay content will increase as weathering pro-ceeds. Moreover, the change from primary tosecondary minerals will affect soil properties be-cause the clay minerals are much more reactivethan the minerals of the coarser fractions, but thereactivity of the clay minerals depends on the

mineral form: e.g., very different chemical andphysical properties characterize soils dominatedby kaolinite and those dominated by smectite(Borggaard and Elberling, 2004; Buol et al.,2003). Soil properties and, therefore, the sustain-ability of the soils, depend strongly on the con-tent and composition of the clay and other parti-cle size fractions. In contrast to lowland soils inBangladesh, there seems to be almost no infor-mation on the composition and weatherability ofthe minerals in CHT soils, which are formed inan aggressive climate on shale to siltstone rockmaterials (Alam et al., 1993; Brammner, 1996; Ga-fur et al., 2003; Islam and Lotse, 1986).

In order to expand our understanding of thesustainability of short-fallowed shifting cultiva-tion in hilly areas on micaceous shale under hotand alternating dry/wet climatic conditions, astudy of the soil composition and clay mineral-ogy was combined with determinations of lossesof soil, water, and nutrients by runoff in a 1-hacatchment with steep slopes in the CHT. The de-terminations were carried out before and afterburning and compared with measurements in aneighboring non-burnt catchment. Special atten-tion was given to the role of parent materialweatherabilitv in relation to Jhum sustainability.

MATERIALS AND METHODS

Site anid Soil DescriptioniThe study was performed at the Soil Conser-

vation and Watershed Management Center, Ban-derban, Bangladesh (22°10' N and 92°11' E).The area has a typical monsoon climate with avery wet period from May to October and a dryperiod with very litde rain from November toApril. The test site is subject to an average tem-perature of 26 °C. a mean annual precipitation of2680 mm, and a potential evapotranspiration of1350 mm, leaving 1330 mm for runoff and deepinfiltration (Gafur et al., 2003). The parent rockof the soils is fine-grained, soft micaceous shale ofMio-Pliocene to Early-Pleistocene age. The soils,locally belonging to the Kuhalong Series, areclassified as Typic Haplohumults (Gafur et al.,2000; Soil Survey Staff, 1999).

The investigation was carried out at twoneighboring watersheds of approx. I ha eachwith slope gradients ranging from 5 to 200%(3-63°). Both catchments were underJhum prac-tice until 1993 and fallowed until spring 1999when one catchment (CJ) was slashed andburned and cultivated according to currentJhumtradition (Borggaard et al., 2003) followed by fal-

664 SOIL SCIENCE

WEATHERING CONTROLLED SUSTAINABILIFY OF ULTISOLS

lowing in 2000. The other catchment was left fal-lowed (FJ) as a control.Before the move to slash-and-burn, the CJ and FJ catchments had the samevegetation, consisting of different but not species-identified trees, bushes, and herbs. Rainfall events(318 in 1999,328 in 2000) were recorded by raingauges installed at the summit and at midpointbetween CJ and FJ watersheds. Iron-armed con-crete runoff gauging stations with stilling tankshaving a V-notch weir were constructed at theoutlets from the two catchments (Gafur et al.,2000). After cessation of runoff, the amounts ofsediment in the natural depositional areas up-stream (before) the gauging stations with stillingtanks (SBT), as well as the sediment deposition inthe stilling tanks (SIT), were measured. Theamounts of suspended material (SSM) were deter-mined in runoff samples from the overflow at theV-notch weir during flood periods with over-flow and high concentrations of suspended sedi-ment. More details about these measurementsmay be found elsewhere (Gafur et al., 2003).

Soil Samiipling and Chiemical Analyses

On the upper, middle, and lower parts of aslope in the CJ watershed, pits were excavated,profiles described, and soil samples were collectedfrom the various horizons. Profile descriptionswere made according to FAO (1990) guidelinesand are presented in Gafur et al. (2000). For com-parison, micaceous gravel from C horizons re-sembling the surface layers of the solid but softbedrock was included and referred to as R sam-ples. Regularly spaced (5 X 20 m) compositesamples of topsoil (0-10 cm) were collected fromboth watersheds (31 from CJ and 28 from FJ) inMarch 1999 and again, after burning, in April1999 from the CJ watershed. Soil and sedimentsamples were air-dried and sieved (2 mm).

Profile samples were analyzed for texture, pH,total C and N, exchangeable bases, and CEC atpH 7 (CECPH7). Topsoil (0-10 cm taken beforeand after burning) and runoff samples were ana-lyzed for pH, total C and N, Ca, Mg, and K andextractable P, S, Cu, Fe, Mn, and Zn. Soil texturewas determined by sieving and hydrometer mea-surement after dispersion in sodium pyrophos-phate at pH 10. Soil pH was determined in a 1:2.5soil in water or 0.01 M CaCl2 suspension using aMetrohm pH meter with a combination glasselectrode. Total C was determined by a LECO C-200 Carbon analyser (Tabatabai and Bremmer,1970). Exchangeable bases were extracted with 1M ammonium acetate (NH4 Ac) at pH 7. In theclear extract, K and Na were determined by flame

emission spectroscopy (FES), whereas Ca and Mgwere determined by atomic absorption spec-troscopy (AAS) after adding LaCI3 solution to theextract. For CEC ,H7 measurement, the sampleswere percolated with a solution containing 0.9 Msodium acetate and 0.1 M sodium chloride ad-justed to pH 7.0; excess salt was removed by wash-ing with 96% ethanol and the adsorbed sodiumwas subsequently exchanged by percolating with1 M NH4Ac (Van Reeuwijk, 1995). In the perco-late, exchanged sodium was quantified by FES.Total N was determined by Kjeldahl digestion,distillation, and titration with 0.05 M NaOH. Ex-tractable P was determined by the molybdenumblue method after extraction with 0.5 M sodiumbicarbonate (Olsen and Dean, 1965). Available S(sulphate) was extracted by calcium phosphate(Ca(H,PO4)2 ) and measured by turbidimetry inthe clear extract (Johnson and Fixen, 1990). Con-centrations of Cu, Zn, Mn, and Fe were analyzedby AAS after extracting the soil samples withDTPA (Martens and Lindsay, 1990). All analyseswere done in duplicate.

Clay }lIineral Atnalyses

Selected samples from profiles on upper, mid-dle, and lower parts of the hill slope in the CJcatchment were treated with 30% H,O2 at pH 5until the dark color caused by organic matter van-ished. The samples were washed with water andethanol to remove the soluble salts and dispersedby ultra sonic treatment. The suspensions wereshaken vigorously and particles smaller than 20L.m were collected using gravity sedimentation.The clay fraction (<2 p.m) was separated fromthe silt fraction (2-20 ,um) using a particle sizecentrifuge (Slater and Cohen, 1962). The clay sus-pensions were flocculated by addition of calciumchloride, and surplus Ca2 + ions were removed bycentrifugation and resuspension in water.

Free Fe and Al oxides were removed from theclay by DCB treatment (Mehra and Jackson,1960). The treatment was repeated three times, andthe contents of DCB-extractable Fe and Al (Fed,Ald) were determined by AAS. CEC H7 of the clayfraction was determined on DCB-treated, Ca-saturated clay at pH 7. Following saturation and re-moval of the Ca in the solution, the exchangeableCa was exchanged by KCI, and the content of Cain the extract was determined by AAS. The totalelemental analyses of the clay fractions were doneon DCB-treated, NH4-saturated samples. Thesamples were dissolved by concentrated hydroflu-oric acid and aqua regia in teflon bombs (Bernas,1968). Element concentrations of Si, Al, Fe, Ca,

665VOL. 169 -NO. 9

GAFUR, KOCH, AND BORGGAARD

Mg, K, and Na were measured byAAS or FES, andresults were calculated as wt% of elemental oxide.Loss on ignition (LOI) was determined on NH+-saturated clays following heating at 110 and950 'C. X-ray diffraction analysis was performedby means of Co-KoL radiation and a Siemens D500diffractometer equipped with a graphite crystalmonochromator. Unoriented clay samples werebackpacked into sample containers, and orientedsamples were prepared from suspensions of DCB-treated clay, deposited on glass slides, and dried inair. Ca2+ was used as the saturating cation, and theidentification treatments included air dry, ethyleneglycolation, and heating at 350 and 550 'C for 2 h.Infrared spectra were measured in transmissiongeometry using a Perkin Elmer FT-IR 2000 sys-tem and the KBr pressed pellet technique (0.3 or1 mg in 300 mg KBr). Spectra were measured be-fore and after overnight heating (110,350 and 550'C) of the samples. Mossbauer spectra were mea-sured using a conventional constant acceleratonspectrometer and 5 7 Co in Rh as a source. Thespectrometer was calibrated using a 12,5 ,am foilof natural iron at room temperature. The energyscale was calibrated using the center of the spec-tr-um of this sample. Samples were measured attemperatures between room temperature and 20 Kusing liquid nitrogen bath cryostats and dosed cv-cle helium cryostats. Spectra were fitted using sev-eral constraints: doublets were constrained to haveidentical lines and sextets were constrained to havepair wise similar lines and a relative area of 3:2:1.The untreated (non-DCB-treated) samples wereall measured at room temperature. In addition, allof the untreated samples from the Upper profilewere also measured at 80 and 20 K. Followingthe DCB treatnent the samples from the Upperprofile were measured at room temperature and80 K, and one of these samples was also measuredat 20 K.

Year

199'

2001

RESULTS AND DISCUSSION

Soil and Water Losses

During the study years, rainfall was higherthan normal, 3045 mm in 1999 and 3123 mm in2000, but the annual distribution resembled thenormal pattern with more than 50% of annualprecipitation and erosivity occurring in May andJune (Gafur et al., 2003). Accordingly,-soil lossfrom the CJ watershed was high after burning andcultivation in 1999, especially in June when ero-sivity is highest and the soil sparsely covered byweeds and seedlings (Table 1). However, waterrunoff (17-37% ofprecipitation) showed no rela-tion to slash-and-burn as even more water wasseen to be lost from the control catchment (FJ). In1999. nearly 4 times more upland soil was lostfrom CJ than from FJ, i.e.. 41.1 Mg/ha from CJand 11.5 Mg/ha from FJ, resulting in a net loss ofabout 30 Mg/ha due to slash-and-burn. However,43% (17.7 X 100/41.1) of the soil material re-mained in the depositional area in lower parts ofthe catchment before the stilling tank (SBT)-

Therefore, the total loss of sediment (SIT + SSM)from the watershed as a result of Jhum was 15.5Mg ha"". This figure corresponds to the amountof sediment in the tank (SIT) and in runoff water(SsM) from the CJ catchment less the amountfrom the FJ catchment: ((SIT(CJ) - SIT(FJ))

((SSM(CJ) - SSM(FJ)) = (2.73-0.17) + (20.6-7.7). This loss exceeds the recommended criticalupper limit of 5 Mg ha-l' yr-I (FAO, 1965; Re-nard et al., 1997). Considered separately, such aloss demonstrates the unsustainability of theJhumpractice, particularly if continued over more years.On the other hand, when the fast growing vege-tation has formed a dense cover over the soil aftera period of 2 to 3 months, erosion decreases, andafter I year (in 2000), water and soil losses weresimilar in the two watersheds. Furthermore, a sub-

TABLE IWater and soil losses from the CJ (current Jhum, burnt site) and FJ (fallowJhum, non-burnt site) watersheds in 1999

and 2000. The upland soil loss (UL) is deposited in the watershed before the tank (SBT) and in the tank (SIT)and lost as suspended material in runoffwater (SSM), i.e., UL = SBT + SIT + SSM

Runoff Soil loss (Mg ha"1 yr-1 )rWatershed

(nim) Ratiot UL SBT SIT SSM

9 CJ 776 0.25 41.1 17.7 2.73 20.6FJ 1139 0.37 11.5 3.7 0.17 7.70 CJ 533 0.17 NDt ND* 0.02 3.0FJ 748 0.24 NDt ND* 0.02 2.9

VRatio between runoff and precipitation.

:Not determined as weed growth prevented measurement of sediment in depositional area in 2000.

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VOL. 169 - No. 9 WEATHERING CONTROLLED SUSTAINABILITY OF ULTISOLS

stantial soil depth (>55 cm) remained even on thesteepest slopes (Gafur et al., 2000). These observa-tions indicate association ofJhum in CHT with ahigh degree of hydraulic resilience. Since re-silience is an irmportant component of sustainabil-ity, current Jhum in CHT does not seem to be asdestructive as indicated initially. Thus, if the ob-servedJhum-induced soil loss of 15.5 Mg ha-l isaveraged over 3 years, the annual loss is very closeto the critical upper limit of 5 Mg ha-l, indicat-ing that Jhum with fallow periods longer than 3years may be sustainable. In addition to dense veg-etation, which has been shown as an importantrunoff-lirmiting factor in sinilar slash-and-burnstudies (Rodenburg et al., 2003), the rather deepcracks observed during the dry period (Gafur etal., 2000) may also assist infiltration and reducerunoff. Nevertheless, although farmers considerimproved soil structure to be an important effectof burning (Ketterings et al., 1999), macroporos-ity and other physical aspects of the CHT soilshave not been investigated.

Effect on NIutienzt Status

From a soil chemical point of view,Jhura inCHT seems to be associated with good resilience(Table 2). The top 10-cm soils are moderatelyacidic, but pH is greater than 5, indicating limitedAl-toxicity problems (Borggaard and Elberling,2004; Sanchez et al., 2003), and pH increased af-ter burning because of the alkalinity of the ash(Espejo et al., 2003; Rodenburg et al., 2003). Incontrast, total C and N decreased because of theformation of volatile C and N compounds suchas CO 2, NO 2, and NO, as shown in numerousstudies (e.g.,iBrand and Pfiind, 1998).Despite thisloss, the soils have a rather high organic C con-tent (Table 3). Burning increased exchangeableCa, Mg, and K substantially, as shown by the dif-ferences between the contents after burning (AB)and before burning (BB);234 kg ha-1 is indicatedfor Ca, 33 kg ha- 1 for Mg, and 36 kg ha- I for K

(Table 2). As shown in Table 2, these additionsmore than compensate for base cation removal inrunoff sediments. Compared with plant needs,available phosphate is very low, and the contentdecreases by 0.4 kg ha- I because of burning. Fur-thermore, about 10% was removed by runoff, in-dicating strong P deficiency on these soils. Thiscan also be seen in many other tropical soils be-cause of low nutrient content in parent material(Sanchez et al., 2003). However, plants grew wellwithout signs of P deficiency (Gafur et al., 2000),maybe because the HCO3 -test used failed toshow the available P content in these soils cor-rectly. In contrast to the decrease found in thisstudy, Rodenburg et al. (2003) found an increasein extractable P (Bray method) after burning.Whether this difference is caused by the differentextraction methods (Bray and Olsen) or otherreasons is unknown.

Jhum increased available S (sulphate) by 1.8kg ha- , whereas runoff removed only 0.24 kgha-1. Copper registers low and at the critical levelfor sufficient plant supply (Borggaard, 1976), andthe content is decreasing both because of burn-ing and because of runoff (Table 2). Available Fe,Mn, and Zn exhibited increases after burningwhich -were considerably higher than the runofflosses (Table 2). The contents of Fe, Mn, and Znare considered sufficient, although Mn and Znare close to deficiency limits (Borggaard, 1976).

Soil Characteristics

Apart from minor differences in horizondepths, the three soil profiles have very similarmorphology and characteristics (Table 3; (Gafuret al., 2000)). Compared with other well-drainedtropical soils (Borggaard and Elberling, 2004), theorganic carbon content in the CHT soils is high,especially in the subsoil horizons. However, highsubsoil contents of soil organic matter (SOM) areoften observed in these hill soils (Brarnmer,1996). This is probably the result of water trans-

TABLE 2

The pHC,cn, together with total C and N, exchangeable Ca, Mg and K, anid plant available P, S, Cu, Fe, Mn, and Zn,all in kg ha- , in the upper 10 cm before burning (BB) and after burning (AB) and in runoff sediument (RS).

The difference AB - BB shows nutrient gain or loss by burning. The bulk densitv of the soil was 1.15 g cm- 3

Sample t pH C-r0 ,, NTOuI Ca Mg K P S Cu Fe Mn Zn

BB 5516 26500 2060 757 252 153 L3 7.5 1.24 89 43 1.31AB 5.31 25200 1890 991 285 189 0.9 9.3 1.17 107 56 1.43AB-BB 0.15 -1300 -170 234 33 36 -0.4 1.8 -0.07 18 13 0.12RSt 6.26 nd nd 47 10 10 0.11 0.15 0.04 5.1 4.6 0.05

tBB:clay: Before burning; AB: After burning; RS: Runoff sedimentt Basedon excess upland soil (UL) removed from CJ, i.e.,29.6Mgha-' (41.1 - 11.5, Table 1).

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GAFUR, KOCH, AND BORGGAARD

TABLE 3

Selected characteristics of the studied soils from upper, middle, and lower parts of the hill slopeof the CJ watershied in CHT

Horizon Clay Organic C PHH2O cm()g7

(cm) (M) (M) (cmol(+) kg-1)

Upperpart of hill slopeA (0-3) 39 3.3 6.0 17.9B (3-15) 36 1.5 5.6 13.5Bt (15-45) 50 1.0 5.4 12.2C (45-75) 42 0.6 5.3 12.9R (75+) 41 0.5 5.3 11.8

Middle part of hill slopeA (0-4) 36 2.7 5.7 16.1

E (4-16) 38 1.8 5.3 13.1Bt (16-47) 51 1.1 5.2 12.7Cl (47-75) 49 0.8 5.2 12.5C2 (75-100) 41 0.6 5.2 12.9

Loiver part of hill slopeA (0-4) 37 3.3 5.8 16.6E (4-15) 31 1.5 5.4 13.1Bt (15-33) 50 1.0 5.3 14.5C (33-55) 49 0.9 5.3 15.8R (55+) 48 0.5 5.3 14.1

porting organic matter or charcoal from theburnt vegetation into the deep cracks of the soil,although occurrence of charcoal was not re-ported (Gafisr et al., 2000). Cracks form in theseclay-rich soils during the dry period. Anotherpossible source of organic carbon in the subsoil isplant residues.

Although slash-and-burn adds alkalinity tothe soil (Espejo et al., 2003; Rodenburg et al.,2003), the soils are moderately acidic throughoutthe profile (Table 3). The acidification is the re-sult of a rather acidic parent material (micaceousshale), assisted by strong biological activity andleaching, in accordance with the vigorous plantgrowth and substantial surplus precipitation inCHT (Gafiir et al., 2000, 2003). The pH of theCJ soils resembles the pH in some burnt Incepti-sols on Sumatra (Rodenburg et al., 2003). How-ever, pH in the CHT soils is almost 1 pH unithigher than that in the Ultisols from Kalimantanthat were also developed in a very hot and humidclimate (Bender Koch et al., 1992); but while theKalimantan soils have formed on resistant sand-stone, the CHT soils develop on easily weather-able micaceous shale,

The percentage of clay in the profiles variesfrom 31 to 50%, with a marked maximum in theBt horizons (Table 1). The Bt horizons are re-ported to contain clay cutans, indicating clay mi-gration (Gafur et al., 2000). Clay migration mnight

be expected to increase the CEC of the illuvialhorizon since clay migration involves primarilytransport of the finest clay particles, especiallysmectite, with a high CEC (Borggaard and El-berling, 2004; Buol et al., 2003). However, CECsof the clay fractions of the Bt horizons resemblethose of the other horizons (Table 4). The ob-served increase in clay content from topsoil lay-ers to Bt horizon may also be the result of selec-tive removal of clay, silt, and fine-sand particlesfrom the surface layers by runoff inasmuch assubstantial amounts of such fine soil materialscould be collected at the outlet of the small wa-tershed (Table 1). This explanation agrees withthe results of Espejo et al. (2003) showing claymigration and erosion of fines after burning, at-tributed to disaggregation in surface layers, Onthe other hand, severe burning (>600 °C) maylead to increased particle size according to Ket-terings et al. (2000).

Clay Mlinerals

Dithionite-citrate-bicarbonate (DCB)-ex-tractable aluminium and iron, as well as theCECp87 , loss on ignition, and element composi-tion of the DCB-treated clay fractions, are re-markably uniform both within each profile andamong the profiles (Table 4). If all DCB-ex-tractable Al originates from Al for Fe-substitutediron oxides,Al should have replaced about 40% of

668 SOIL SCIENCE

WEATHERING CONTROLLED SUSTAINABILITY OF ULTISOLS

the Fe atoms, which is a substitution rate exceed-ing even the upper limit (33%) in goethite, whichis the soil iron oxide able to accommodate mostAl (Borggaard, 1990). However, although DCBseems to be a selective extractant of free (nonsil-icate-bound), Fe, i.e., iron oxides, DCB-extractable Al can also come from other sourcessuch as gibbsite and HIV/HIS present in thesesoils (see below). The feldspar content in the clayfractions is negligible, as indicated by the very lowCaO and Na2O contents (Table 4) as well as bythe absence of feldspar peaks in unoriented X-ray diffractograms (not shown). Accordingly, thesoil pool of calcium is limited and is concen-trated, as are the other exchangeable base cations,within the uppermost soil layer as a result of thepumping effect of the vegetation and the slash-and-burn action (Table 2; Bender Koch et al.,1992; Espejo et al., 2003; Gafur et al., 2000).Rather than being part of K-feldspar, the highpotassium content corresponding to about 3%K2O is indicative of an appreciable content of il-lite in all samples, as also shown by the high in-tensity of the X-ray peaks of this mineral (see Fig.2 later). Thus, if the illite contains 8% KO (Fan-ning et al., 1989), the clay fractions contain nearly40% illte, which may account for roughly half

the observed CECpH7 of 30 cmol(+) kg- I clay, ifan illite CEC in the range 20-50 cmol(+) kg-is anticipated (Borggaard and Elberling, 2004).

The infrared spectra of the various clay frac-tions are very similar (Fig. 1). Dominant in thespectra are bands caused by kaolinite (at 3695,3620, 1032, 1010, 913, 535, and 471 cm-1 andminor bands at 799, 753, and 696 cm1) andgibbsite (bands at 3620, 3525, and 3445 cm-l).These spectra corroborate further the similarityof the clay fractions within all horizons. The rel-ative intensity of the 3620 cm' 1 peak comparedwith the other peaks of kaolinite and gibbsite in-dicates that other phase(s) may contribute to thisband. One additional contributor to this band isillite, as evidenced clearly by the XRD investiga-tion (see below), and another possible contribu-tor is halloysite, also indicated by XRD. The pres-ence of halloysite is supported by the shoulderrather than a peak at approximately 1100 cm-(Russell and Fraser, 1994).

The X-ray diffxactograms of unoriented claysamples (not shown) revealed the dominance ofdioctahedral layer silicate types, as indicated bythe (060) peak at 0.1 5 nm. In addition to layer sil-icates, small amounts of quartz and gibbsite werealso detected. The presence of goethite was indi-

TABLE 4

Aluminum and iron extracted by dithionite-citrate-bicarbonate (DCB) from Ca-saturated clay fractions togetherwith CECpH7 and total element analysis of DCB-treated, NH4 -saturated clays from the studied soils

fiom upper, middle, and lower parts of the hill slope of the CJ watershed in CHT

Horizon/depth DCB-extract. (%) CEC,H, (cmol(+) kg-l) and total element analysis of DCB-treated clay (%o)

(cm) Al Fe CECPM7 SiO2 Al203 Fe203 CaO MgO K2O Na2O LOI* Sum

Uppei part of lill slopeA (0-3) 0.9 3.0 31.4 51.1 27.9 4.23 0.04 1.05 3.01 0.07 12.5 99.9* (3-15) 0.9 3.0 31.8 51.9 28.2 4.22 0.04 0.91 2.89 0.12 11.6 99.9

Bt (15-45) 1.0 3.2 28.1 51.7 28.7 4.03 0.05 0.87 2.92 0.16 11.8 100.2

C (45-75) 1.1 3.2 29.3 51.3 29.1 3.92 0.03 0.84 2.80 0.15 12.0 100.1R (75+) 1.0 2.9 29.8 51.2 29.0 4.04 0.07 0.89 2.78 0.21 12.2 100.4

Middle part of hill slopeA (0-4) 0.9 2.9 31.5 52.0 28.0 4.19 0.06 1,01 3.17 0.12 12.2 100.8

E (4-16) 0.8 2.9 30.5 51.3 28.7 3.97 0.08 0.94 3.16 0.14 11.3 99.6Bt (16-47) 0.9 2.9 26.7 53.0 27.6 3.85 0.03 0.90 2.56 0.13 11.1 99.2

Cl (47-75) 1.0 3.2 29.4 51.0 29.6 3.95 0,05 0.87 2.98 0.15 11.6 100.2

C2 (75-100) 0.9 2.8 27.4 52.1 27.6 4.41 0.08 1.09 2.92 0.12 11.8 100.1

Lower part of lill slopeA (0-4) 0.9 3.0 28.7 52.4 27.1 4.18 0.10 1.01 3.02 0.08 12.5 100.4E (4-15) 0.8 2.6 30.3 54.6 26.5 3.98 0.09 1.17 3.19 0.11 10.5 100.1

Bt (15-33) 1.5 3.8 28.3 52.0 28.1 4.13 0.06 0.82 3,15 0.15 11.8 100.2

C (33-55) 1.1 3.1 32.3 51.4 28.2 4.12 0.06 0.83 3.06 0.14 12.2 100.0

R (55+) 1.0 3.0 28.2 51.3 28.6 3.94 0.08 0.84 3.01 0.19 12.2 100.2

*Loss on ignition.

669VOL. 169 -No. 9

GAFUR, KOCH, AND BORGGAARD

4000 3600 3200 2800 2400 2000 1600 1200 S00 400Wavenumber cm- I

Fig. 1. Infrared spectrum of the Ca2+-saturated clay fraction from the Bt horizon of the upper profile from the CHT.G, H, 1, and K indicate gibbsite, halloysite, illite, and kaolinite, respectively.

cated by a weak peak at approximately 0.42 unand by investigations by Mossbauer spectroscopyas indicated below. The X-ray diffractograms ofair-dried, oriented clay fractions are very similar,and further treatments were only used for sam-ples from the upper profile as exemplified by thediffractograms of the A horizon in Fig. 2. The air-dried samples exhibit both very sharp as well asvery broad peaks. The relatively sharp peaks at

0.99 nm and 0.71 nm indicate the presence ofwell crystallized illite and kaolinite. The disap-pearance of the 0.71-nm peak at temperaturesbetween 350 and 550 °C corroborates the iden-tification of kaolinite. The collapse of the 1.4-nmpeak to 1.0 nm following heating at 350 °C andits partial resistance to swelling with ethylene gly-col indicate that both vermiculite and smectitecomponents are present. The high intensity of

12 15 18 21 24 27 30

Degrees 26 (C. Km)

Fig. 2. X-ray diffractograms of oriented Ca2+-saturated clay from the A horizon of the upper profile from the CHTfollowing (A) drying in air, (B) heating at 350 'C, (C) heating at 550 'C, and (D) ethylene glycolation.

670 SOIL SCIENCE

WEATHERING CONTROLLED SUSTAINABILITY OF ULTISOLS

the diffraction pattern between the 1.4 and 0.99nm peaks in the air-dried sample also indicatesthe presence of transitional phases. Some of theseare presumably interlayered by Al-hydroxy com-plexes as the broad peak remains after heating. Al-hydroxy interlayered vermiculite and smectite arefound commonly in Ultisols (Barnhisel andBertsch, 1989; Bender Koch et al., 1992). In thediffraction patterns of the air-dried sample heatedat 350 °C and the ethylene glycolated samples, abroad peak with a maximum of about 0.75 nmcan be distinguished. The peak disappears whenthe sample is heated to 550 °C, indicating amor-phization, which is typical for a kaolinite groupmineral. This broad peak is thought to be causedby a mixture of various halloysite minerals (1.0 to0.7 nm-halloysite). From the resistance to col-lapse at 350 °C, we infer partial hydroxy interlay-ering. Islam and Lotse (1986) also observed abroad contribution to the diffxaction curve, but

4 ~~~~~A

10-

12

14~

0

0.-0

10-

(11 's C

sa) r

they attributed it to randomly interstratifiedmica-chlorite. Halloysite is typically formed byweathering of volcanic ash materials, but it hasalso been reported in nonvolcanic environments(Jeong, 2000).

Taken together, the diffraction and the FT-IR data suggest that clay fractions of all soil sam-ples contain some well crystallized clay types (il-lite and kaohnite) that probably originated fromthe soft parent rock as well as types that can beconsidered as decomposition products of the illiteby preservation of the 2:1 layer structure (vermi-culite/smectite and HIV/HIS) or by recrystal-lization (halloysite and gibbsite). The major partof the kaolinite is thought to be inherited fromthe parent rock. However, current formation ofkaolinite cannot be precluded. Under acidic con-ditions (and low divalent base cation concentra-tion), a high silicic acid concentration favorskaolinite formation, whereas a low concentration

-4 0 4 8 12 -12 -8 .4 0 4

Velocity (mm/s) Velocity (mm/s)

Fig. 3. M6ssbauer spectra measured at room temperature (left) and 20 K (right) of clay samples from A and C hori-zons of the upper profile from the CHT. Spectra of DCB-treated clay from the A horizon are included.

VOL. 1 69 -No. 9 671

GAFUR, KOCH, AND BORGGAARD

makes gibbsite the most stable Al-containingmineral (Borggaard and Elberling, 2004). Duringthe hot and alternating very wet and very dry cli-mate in CHT, low as well as high silicic acid con-centrations undoubtedly occur favoring forma-tion of both gibbsite and kaolinite and probablyhalloysite as well.

Representative Mossbauer spectra of un-treated clay fractions and DCB-treated clay spec-tra are sh6wn in Fig. 3. The spectral componentsare very similar for all samples. Three spectralcomponents dominate: Paramagnetic Fe(III) andFe(II) in the layer silicate structures are the onlycomponents in the spectra following DCB treat-ments (isomer shifts of 0.32 and 1.2 mm s-I andquadrupole splittings of 0.77 and 2.33 mm s-Iand relative areas of 84% and 16% at room tem-perature, respectively). In the untreated samples,the Fe(III) component is overlapped with a dou-blet from superparamagnetic Fe-containing ox-ide minerals at room temperature. Cooling of theabsorbers to 20 K causes the appearance of amagnetically split sextet with a magnetic hyper-fine field of 47.4 T, an isomer shift of 0.47 mms- , a quadrupole shift of -0.12 mm s-1, and arelative area of approx. 65%, whereas the twocomponents created by the layer silicates remainparamagnetic. The magnetic hyperfine field issubstantially reduced compared with pure goe-thite (Morup et al., 1983), probably the result ofAl substitution in the lattice (Murad and Schwert-mann, 1983) as indicated by the high Al/Fe ratioin the DCB extracts (Table 4). In addition tothese three components, an additional weak sex-tet with broad lines can be seen as deviation fromthe background in the room temperature spectraand as an extra inflection of the outer flanks ofthe lines of the sextets in the spectra measured at20 K. This component is a minor contributionassigned to hematite/maghemite formed by theheating of goethite during the burning of thebiomass. Overall, the M6ssbauer results indicateuniform coordination and redox status in theprofile in agreement -with the results of the otherinvestigations.

The effect of burning on the compositionand properties of the minerals of the CHT soilsseems limited compared with that which can beobserved in severely burned areas (Ketterings etal., 2000; Murad and Wagner, 1998; Ulery et al.,1996). However, detailed studies with Mossbauerspectroscopy and other techniques have revealedonly small effects of burning on both iron andaluminium oxides and layer silicates of CHT soils(Bender Koch et al., unpublished data).

Resilience and Weatlieritng

The favorable hydraulic and chemical re-silience described above is closely related to thefast and profound weathering occurring in CHT.The high weathering rate is the result of a com-bination of easily weatherable parent material(micaceous shale) and an aggressive climate (hightemperature and precipitation) and vigorous veg-etation. Intensive weathering has affected the en-tire profile. In fact, all analyses showed composi-tion and properties of the clay fractions to beremarkably uniform with depth and among theprofiles. Even the <2-,um fractions from the Rsamples reflecting the composition of the parentrock fit into this pattern, indicating that the softrock material is chemically preweathered beforeit disintegrates under the formation of loose soilmaterial. Good mixing of the soils, also indicatedby the rather uniform vertical organic matter dis-tribution (Table 3), may explain that all clay frac-tions are almost identical despite development ofA-E-Bt horizontation (Gafur et al., 2000). Mix-ing of the soil is the result of pedoturbation (Buolet al., 2003). This is caused by formation of thehigh content of partly expandable clay minerals,which leads to development during dly periodsof deep cracks that close when the soil becomeswet. The deep cracks also improve infiltration,which will reduce surface runoff of water andsediments but will favor clay eluviation/illuvia-tion (Tables 1 and 3).

Under the prevailing climatic and biologicalconditions, chemical weathering must be intenseand leads to the alteration of the parent material,consisting of illite and kaolinite, to a mixture ofgibbsite, goethite, halloysite, HIV/HIS, and ver-miculite/smectite. During the weathering of il-lite, essential plant nutrients-potassium in par-ticular but undoubtedly other base cations andmicronutrients as well-are released in plant-available form. Fast weathering with release ofsufficient plant nutrients is a prerequisite for thesustainability of crop production systems relyingon zero-fertilizer input (Borggaard and Elberling,2004; Lal, 1997). According to Table 2, substan-tial amounts of nutrients are made available to theplants and returned to the soil after burning, in-dicating the strong resilience of the soil. Only thelow available phosphate and copper seem of con-cern (Gafur et al., 2000).

In contrast to the above-mentioned ratherlimited on-site effects,Jhum has been reportedto be associated with serious off-site effects(Borggaard et al., 2003). Important off-site ef-

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VOL. 169 No. 9 WEATHERING CONTROLLED SUSTAINABILITY OF ULTISOLS

fects include spreading of the fire to areas out-side the Jhum field, e.g., horticultural gardensand village houses, and siltation of waterways,which can lead to downstream flooding andtrafficability problems. Additional negative as-pects of slash-and-burn have been reported byothers (Ketterings et al., 1999). However, al-though they are very important, off-site effectsof slash-and-burn cultivation fall outside thescope of this paper, which focuses on the directeffects of Jhum on the soil as a medium for cropproduction.

CONCLUSIONS

As a result of burning, about 30 Mg ha- I ofupland soil was lost by runoff together withsubstantial amounts of plant nutrients from asmall catchment area in CHT. The soil loss is 6times the upper critical sustainability limit, in-dicating that with fallow periods of 3-5 years,the traditional slash-and-burn agriculture(Jhum) is not sustainable. However, almost halfthe sediments were deposited within the catch-ment, reducing the net catchment loss to 15.5Mg has1. Furthermore, the loss was largelyconfined to the 2 to 3 months immediately fol-lowing the burning, whereupon the field wascovered by vigorous vegetation and the losses ofsoil and nutrients were reduced to the samelevel as for a neighboring non-burnt catch-ment. Moreover, the amounts of nutrients sup-plied by the ash from the burnt vegetationmore than compensated for the runoff losses ofmost essential elements.

The clay fractions in three profiles from theupper, middle, and lower parts of a slope werefound to have almost identical composition andproperties irrespective of soil depth and site posi-tion. The mineral assemblage consists of gibbsite,goethite, halloysite, HIV/HIS, ilite, kaolinite, andvermiculite/smectite. Illite and probably mostkaolinite are inherited from the parent rock (mi-caceous shale), whereas gibbsite, goethite, hal-loysite, HIV/HIS, and vermiculite/smectite areweathering products. In addition to the parentmaterial, the hot alternating very wet and verydry climate in the CHT is considered to be re-sponsible for the uniformity and mineralogy ofthe clay fractions.

Therefore, it may be concluded that Jhum inthe CHT is associated with a high degree of re-silience. However, more studies over longer peri-ods of time are needed to show whether currentJhum with fallow periods of 3-5 years is sustain-able in the long run.

ACKNOWLEDGMENT

The authors express their sincere apprecia-tion to the DANIDA, Denmark, for financialsupport of the project. Thanks also to the Direc-tor, SRDI, Bangladesh, for permission to use theSoil Conservation and Watershed ManagementCenter, Banderban, for the study. The help of thetechnical staff at the Department of Natural Sci-ences was highly appreciated.

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TITLE: Weathering intensity controlling sustainability of Ultisolsunder shifting cultivation in the Chittagong Hill Tracts ofBangladesh

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