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Pedogenesis and pre colombian land use of terra preta anthrosols indian black earth of western amazonia 2

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  • 1. Pedogenesis and pre-Colombian land use of‘‘Terra Preta Anthrosols’’ (‘‘Indian black earth’’) ofWestern AmazoniaHedinaldo N. Lima a, Carlos E.R. Schaefer b,*, Jaime W.V. Mello b,Robert J. Gilkes c, Joa˜o C. Ker baUniversidade Federal do Amazonas, 69077-000 Manaus, Amazonas, BrazilbDepartamento de Solos, Universidade Federal de Vicßosa, 36571-000 Vicßosa-MG, BrazilcSoil Science and Plant Nutrition, University of Western Australia, Nedlands, AustraliaReceived 12 December 2001; received in revised form 25 March 2002; accepted 12 April 2002AbstractThe ‘‘Terra Preta de I´ndio’’ (Indian black earth) or Terra Preta of Western Amazonia is a thick,dark-coloured, anthropic epipedon, usually rich in nutrients. It occurs mostly at the fringes of theTerra Firme, along the Amazon river banks, overlying deep strongly weathered soils. We studiedselected chemical, physical and mineralogical properties of seven soils, ranging from the TertiaryPlateau down to the Amazon river floodplain in the Iranduba district, near Manaus, Amazonas,Brazil. Three Terra Preta soils were classified as anthropogenic (Anthropic Xanthic Kandiudult,Anthropic Xanthic Kandiudox and Anthropic Dystropepts). Chemical, mineralogical and micro-pedological attributes, such as high total and available P and mica flakes in pottery remains found inthe Terra Preta, indicate that the origin of soil materials of these anthrosols is closely associated withneighbouring floodplain (va´rzea) soils and sediments. Amazon floodplain soils were the source ofsoil material for pottery, since 2:1 clay minerals are not found in the Tertiary Plateau (Terra Firme)sediments. Total and available P contents of Terra Preta are associated with microfragments of boneapatite with high P and Ca values. In the anthrosols under cultivation, these values are less, withincreasing Al release suggesting acidification and losses of nutrients. Large amounts of Mn and Znoccur in the anthrosols and in high-fertility floodplain soils. It is unlikely that well-drained TertiaryPlateau (Terra Firme) area far away from lowland Amazon floodplain soils could develop high-fertility Terra Preta on the top of nutrient-poor Oxisols (Latosols). The suggested model of TerraPreta formation between the Tertiary Plateau and nutrient-rich Amazon floodplain does not extend toother nutrient-poor, smaller, floodplains draining the deep-weathered interfluves of the BrazilianUplands. This raises reservations about estimates of precolonial human population densities for the0016-7061/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.PII: S0016-7061(02)00141-6*Corresponding author. Fax: +55-31-899-2648.E-mail address: carlos.schaefer@solos.ufv.br (C.E.R. Schaefer).www.elsevier.com/locate/geodermaGeoderma 110 (2002) 1–17
  • 2. Amazon basin as a whole, assuming the widespread occurrence of such anthrosols farther inland.D 2002 Elsevier Science B.V. All rights reserved.Keywords: Amazonia; Terra Preta de I´ndio; Anthrosols; Human carrying capacity; Soil formation; Organicphosphorus1. IntroductionThe Terra Preta Anthrosols of Amazonia (Indian black earth) are mainly Oxisols,Ultisols and Inceptisols with an anthropic A horizon. They have been described by Katzer(1933), Gourou (1949), Sombroek (1966), Ranzani et al. (1970), Eden et al. (1984) andAndrade (1986), yet many aspects of their origin remain obscure. Detailed studies of theseprecolonial anthropogenic soils can help answer questions about population distribution,soil carrying capacity, settlement pattern and land uses of ancient Amazonian peoples.According to Roosevelt (1997), the history and ecology of Amazonian habitats are oftheoretical and practical relevance to the conservation of the vast tropical rainforest, andyet they are poorly documented. According to radiocarbon dating of Terra Preta sites(Hilbert, 1968), these pre-Colombian societies inhabited the Amazon valley and its maintributaries between 2400 F 75 and 1525 F 58 years BP.Fig. 1. The distribution of known anthrosols in the Amazon and the location of the present study.H.N. Lima et al. / Geoderma 110 (2002) 1–172
  • 3. Terra Preta Anthrosols in the lower Amazon valley and lower Tapajo´s have beenchemically studied by Kern (1988), Kern and Kampf (1989), Zech et al. (1990), Pabst(1991), Kern and Costa (1997) and Glaser (1999) among others, but little is known aboutthe Terra Preta sites of the middle Amazon valley. Near Manaus, there are manydiscontinuous patches of well-drained Tertiary Plateau, where Xanthic Oxisols (Latosols)are overlain by black earth deposits pedogenically transformed, containing archaeologicalartifacts such as pottery fragments, weathered bones and organic remains. The extent ofthese patches of high fertility epipedons is of local and regional importance, and has beenconsidered as an indication of former sustainable land use (Smith, 1980; Glaser, 1999).Even today, anthrosols are intensively cultivated by local population (the‘‘caboclos’’),highlighting its importance to the Amazonian social and ecological landscape (Fig. 1).The aim of this paper is to relate selected chemical, physical and mineralogicalattributes of Terra Preta Anthrosols (TPA) with aspects of precolonial land use and theorigin of TPA. Also, TPA attributes were compared with neighbouring nonanthropogenicsoils, ranging from the Tertiary Plateau to the floodplain of middle Amazon, emphasizingthe pedogeomorphological relationships of their occurrence and implications for precolo-nial human societies.2. Material and methodsWe studied seven soils along a toposequence ranging from the Tertiary Plateau(regionally called Terra Firme) down to the Amazon river floodplain (regionally calledva´rzea), in the Iranduba district, near Manaus, Amazonas, Brazil (Fig. 2). The soils wereclassified in the Brazilian System of Soil Classification (EMBRAPA, 1999) and SoilTaxonomy (USDA, 1999), respectively, as Anthropic Yellow Podzolic (Anthropic XanthicKandiudult) (P1), Anthropic Yellow Latosol (Anthropic Xanthic Kandiudox) (P2),Anthropic Cambisol (Anthropic Dystropepts) (P3), Yellow Latosol (Xanthic Kandiudox)Fig. 2. Location of the soils along of the transect Tertiary Plateau (terra firme)–floodplain (va´rzea) near Iranduba,Western Amazonia.H.N. Lima et al. / Geoderma 110 (2002) 1–17 3
  • 4. (P4), Plinthic Yellow Latosol (Typic Plinthudox) (P5), Low Humic Gley (Typic Fluva-quent) (P6) and Alluvial (Typic Udifluvent) (P7).The pits were dug to a depth of 1.50 m, and soil profiles described according toEMBRAPA (1999). Undisturbed samples were collected for micropedological analyses.The samples from genetic soil horizons were subjected to chemical, physical andmineralogical analyses.Particle size distribution was determined by wet sieving and ultrasonic dispersion,adapted from EMBRAPA (1997). The clay ( < 0.002 mm) and silt (0.002–0.053 mm)fractions were separated by sedimentation. Subsamples of clay for X-ray diffractionanalysis were flocculated in 5 M NaCl solution. The mineralogy of the clay fraction wasdetermined for all horizons by X-ray diffraction analysis (XRD) using monochromaticCuKa radiation on oriented clay samples. The diffractograms were interpreted followingBrindley and Brown (1980). Available P, pH, Ca, Mg, K, exchangeable Al3 +and H + Alwere measured by standard procedures (EMBRAPA, 1997). Soil humic substances werechemically fractioned as recommended by Swift (1996) and total organic carbon wasdetermined according to Yeomans and Bremner (1988). Total P was determined by themethod of Kuo (1996), and citric acid extractable P according to the procedures of USDA(1996).For each horizon, 1 g of fine earth was digested by 20 ml 1:1 H2SO4, and the relativeproportions of SiO2, Al2O3, Fe2O3 and TiO2 were measured by atomic absorptionspectrometry (EMBRAPA, 1997), to provide data required for the Brazilian System ofSoil Classification (EMBRAPA, 1999). The total microelements in 100 mg clay wereextracted by HF + HNO3 + HCl (USDA, 1996), and the elements were determined byatomic absorption spectrometry.Undisturbed soil samples were impregnated with crystic resin under vaccum, followingrecommendations of FitzPatrick (1993). The micromorphology of these selected horizonswas studied in thin sections at  30 or greater magnifications. Pottery fragments andpedological features such as structural units, porosity, pedofeatures (nodules, concrections)and clay coatings were described according to Bullock et al. (1985) and Fitzpatrick (1993).Selected thin sections were polished down to 1 Am with diamond paste, carbon-coated andsubmitted to SEM/EDS analyses, using a JEOL 6400 fitted with EDS, in order to identifyand analyse the P forms.3. Results and discussion3.1. Chemical and physical characteristicsThe five soils from the Tertiary Plateau and its border (P1 to P5; Fig. 2) are generallydystric in subsurface, and dominated by kaolinite in the clay fraction, similar to thosedeveloped from preweathered sediments on the Tertiary Plateau elsewhere in Brazil(Resende et al., 1995) (Tables 1 and 2). The two nonanthropogenic soils, P4 and P5, havehigh levels of Al3 +in the exchange complex. At the surface, the anthropogenic Ahorizons of P1, P2 and P3 show a distinct eutrophic character, with elevated basesaturation and very high ‘‘available’’ P content (Table 1). In the soil under continuousH.N. Lima et al. / Geoderma 110 (2002) 1–174
  • 5. Table 1Chemical and physical characteristics of the soilsHorizon/depth pH PaK Ca2 +Mg2 +Al3 +H + Al BSbC.S.cF.S.dSilt Clay(cm) (H2O) mg kg À 1cmolc kg À 1% g kg À 1Anthropic Xanthic KandiudultA1 0–23 5.2 173 12 3.85 0.63 0.19 6.32 41.6 360 240 70 320A2 23–42 5.2 136 6 2.38 0.28 0.45 6.16 30.3 300 180 50 460Bt1 42–73 5.2 257 5 1.39 0.18 0.19 4.47 26.1 200 140 20 630Bt2 73–130 5.1 145 4 1.31 0.12 0.13 3.36 30.0 180 100 20 700Anthropic Xanthic KandiudoxA1 0–30 6.2 1991 55 14.13 1.32 0.00 6.37 71.0 390 130 130 350A2 30–60 6.2 2935 49 13.98 0.53 0.00 6.21 70.2 310 140 190 360A3 60–100 6.4 3921 53 9.34 0.44 0.00 4.31 69.7 290 120 180 410AB 100–130 6.5 3537 44 6.69 0.36 0.00 3.83 65.2 280 120 140 460Bw 130–150 6.5 1567 27 4.37 0.30 0.00 3.36 58.5 280 110 120 490Anthropic DystropeptsA1 0–15 6.3 1332 70 6.59 1.04 0.00 7.00 52.7 450 110 140 300A2 15–40 6.4 2032 44 5.60 0.44 0.00 5.78 51.6 380 130 130 360A3 40–55 6.3 816 36 2.35 0.86 0.00 3.52 48.4 410 100 90 400AB 55–110 6.4 115 24 0.66 0.06 0.00 1.82 30.0 430 140 90 340Bi 110–180 6.0 92 18 0.55 0.08 0.00 1.50 31.1 480 110 90 320Xanthic KandiudoxA 0–18 4.6 1 15 0.01 0.03 1.06 6.84 1.1 340 220 50 390AB 18–40 4.3 1 4 0.01 0.01 1.34 5.10 0.6 290 220 30 460Bw1 40–64 4.4 1 3 0.01 0.01 0.86 3.52 0.8 270 180 50 500Bw2 64–90 4.4 1 2 0.01 0.01 0.77 2.88 0.9 200 180 40 580Bw3 90–150 4.4 1 2 0.01 0.01 0.77 2.72 0.9 390 220 60 330Typic PlinthudoxA 0–20 4.7 2 16 0.01 0.03 1.63 6.89 1.2 330 200 50 420AB 20–40 4.7 1 11 0.01 0.03 1.15 5.73 1.2 270 210 40 480Bw 40–95 4.7 1 2 0.01 0.01 0.99 3.52 0.7 230 190 50 520Bwf 95–150 4.8 1 2 0.01 0.01 0.67 3.36 0.7 180 120 40 560Typic FluvaquentAg 0–13 4.8 69 46 9.86 3.21 2.50 6.37 67.4 0 30 700 270ACg 13–35 5.8 34 39 12.45 4.99 0.48 3.44 83.6 0 60 650 290Cg 35–62 5.9 33 30 11.92 5.33 0.35 2.57 87.1 0 60 660 2802Cg 62–100 6.5 33 44 13.01 7.37 0.08 2.57 88.9 0 0 580 420Typic UdifluventA 0–5 5.4 25 79 10.62 2.52 0.51 5.53 70.7 0 480 370 1502C2 24–34 5.8 108 38 10.88 2.42 0.10 3.20 80.7 0 440 380 1805C5 50–150 5.6 45 44 11.17 3.44 0.42 3.20 82.1 0 140 590 270aP-Mehlich-3.bBase saturation.cCoarse sand.dFine sand.H.N. Lima et al. / Geoderma 110 (2002) 1–17 5
  • 6. Table 2Clay mineralogy, Fe contents extracted by dithionite-citrate (Fed) and ammonium-oxalate (Feo) and Feo/Fedratios of fine-earth ( < 2 mm) of the soilsHorizon Feo Fed Feo/Fed Clay mineralogyg kg À 1Anthropic Xanthic KandiudultA 2.6 35.7 0.073 Kt, Gt, Hm, AnaAB 2.2 47.1 0.047Bt1 1.4 48.4 0.029Bt2 1.0 51.6 0.019 Kt, Gt, Hm, AnaAnthropic Xandic KandiudoxA1 5.5 35.2 0.156 Kt, Gt, Hm, AnaA2 4.8 40.7 0.118A3 3.6 41.4 0.087AB 2.9 46.4 0.063Bw 1.7 52.8 0.032 Kt, Gt, Hm, AnaAnthropic DystropeptsA1 4.4 43.6 0.101 Kt, Gt, Hm, AnaA2 3.1 59.2 0.052A3 1.2 71.9 0.017AB 0.4 49.1 0.008Bi 0.1 34.2 0.003 Kt, Gt, Hm, AnaXanthic KandiudoxA 4.5 33.7 0.134 Kt, Gt, Hm, AnaAB 1.8 37.4 0.048BA 1.0 37.8 0.026Bw1 0.5 47.8 0.010Bw2 0.3 50.2 0.006 Kt, Gt, Hm, AnaTypic PlinthudoxA 2.7 35.9 0.075 Kt, Gt, Hm, AnaAB 3.3 39.0 0.085Bw 0.7 40.2 0.017 Kt, Gt, Hm, AnaBwc 0.4 63.0 0.006Typic FluvaquentA 15.0 25.7 0.584 Kt, Smec, Ill, Cl, Bi, Gt, HmACg 13.8 23.8 0.580Cg 12.2 24.2 0.5042Cg 10.5 23.9 0.439 Kt, Smec, Ill, Cl, Bi, Gt, HmTypic UdifluventA 11.8 23.6 0.500 Kt, Smec, Ill, Cl, Bi, Gt, Hm2C2 10.3 22.1 0.4665C5 12.5 25.2 0.496 Kt, Smec, Ill, Cl, Bi, Gt, HmKt—Kaolinite; Gt—Goethite; Hm—Hematite; An—Anatase; Ill—Illite; Smec—Smectite; Bi—Biotite; Cl—Clorite.H.N. Lima et al. / Geoderma 110 (2002) 1–176
  • 7. cultivation (P1), however, there is more extractable Al, similar in amount to the recentlyreported incipient Al release in eutrophic soils under shifting cultivation in Amazonia(Vale, 1999). The high silt content and considerable textural variation with depth of thefloodplain soils (P6 and P7) reflect the complex sedimentary history of this environment,and the very weak degree of pedogenesis compared with the Tertiary Plateau upslope(Schaefer et al., 2000). Clay in the floodplain soils is typically dominated by high activityand some horizons contain large amounts of exchangeable Al (Table 1) associated with 2:1expanding clays (Table 2). This suggests weathering of the smectite under presentconditions. The widespread occurrence of petroplinthite in the transition segment of theTertiary Plateau, as observed in P5, is attributable to lateral Fe-flux from upland sourcesand its precipitation along the escarpment edge above the floodplain (va´rzea).The distribution of Fe-dithionite between anthropogenic and nonanthropogenic profilesis not very different (Table 2), but there is more Fe-oxalate in surface horizons of theanthrosols, a feature possibly related to the greater organic matter content. The higher Fe-oxalate/Fe-dithionite ratios (0.47 and 0.58) in the soils of the floodplain (P6 and P7; Table2) indicate the dominance of less crystalline forms of Fe-oxides, and confirm the relativeimmaturity of the soils associated with aquic regimes, compared with those at higherlevels.In the Anthropic Xanthic Kandiudox, total P2O5 reached the exceptionally high valueof 13,870 mg kg À 1P2O5 in the anthropic epipedon, whereas the distribution of total P inthe Anthropic Xanthic Kandiudult is relatively uniform, probably because of repeatedcultivation and mixing of this site (Table 3). In noncultivated Terra Preta, the surfacevalues are much enhanced in relation to the underlying soil horizons.The high levels of P and their association with organic matter are being furtherinvestigated in a broader project currently underway. Scanning electron microscopical(SEM) observations suggest that most of the phosphorus in the anthropic epipedon is inamorphous/low crystalline forms, associated with bone apatite from fish middens (Lima,2001). Two examples are illustrated in Fig. 3, showing the chemical composition by EDS-microprobe.Table 3Total, Mehlich-1, citric acid P2O5 contents and phosphate maximum adsorption capacity (PMAC) of theanthropogenic soilsHorizon Total Mehlich-1 Citric acid PMACmg kg À 1Anthropic Xanthic KandiudultA1 3070 396 345 0.78Bt2 3460 332 323 0.77Anthropic Xanthic KandiudoxA1 13,870 4559 4548 0.50Bw 7180 3588 3172 0.17Anthropic DystropeptsA1 8800 3050 3066 0.31Bi 960 211 124 0.07H.N. Lima et al. / Geoderma 110 (2002) 1–17 7
  • 8. 3.2. Organic carbon and humic fractionsThe soils developed on Tertiary sediments (P4 and P5) generally contain moreorganic carbon than those on the floodplain (Table 4), because of the dystrophy, andtherefore less favorable conditions for mineralization. Also, the organic carbon values inthe anthrosols are greater than in the nonanthropogenic soils. The floodplain soilcontains the least organic carbon, suggesting rapid mineralization or burial by floods.All the anthrosols are dominated by strongly humified fractions (humins and humicFig. 3. SEM photomicrographs of some P forms in Terra Preta anthrosols: (A) bone-apatite microfragment withhigh P/Ca values on EDS chemical mapping (CaO 35.45%; P2O5 16.45%); (B) secondary P concentration along abiological channel, revealing a high Fe/Al composition (CaO 3.35%; P2O5 11.29%; Fe2O3 12.62%; Al2O320.23%).H.N. Lima et al. / Geoderma 110 (2002) 1–178
  • 9. acid—HAF) with smaller amounts of the more soluble and mobile fulvic acid fraction(FAF), as indicated by the high HAF/FAF ratios (Table 4). These results agree withthose Zech et al. (1990), who reported small amounts of mobile humic substances andrapid humification in Terra Preta of the lower Tapajo´s area. It is possible that mosthumins and humic acids are formed by progressive polymerization, with decreasingaliphaticity and slow turnover (Duchaufour, 1998), but a marked contribution ofrefractory, ‘‘inherited’’ humin as charcoal (black carbon), through burning and pedobio-logical mixing has been reported by Glaser et al. (2000) and Lima et al. (2001) in TerraPreta soils. These results highlight the importance of this sequestered organic carbonpool in Terra Preta Anthrosols due to long-term charcoal formation, enhancing thecontent of aromatic carbon, only partly oxidized.The greater humification of the Terra Preta soils compared with the others analysedmay also be related to the larger amounts of Ca in the exchange complex (Table 1), whichfavors earthworm activity and renders the organic matter less soluble by forming morestable aggregates. The Terra Preta A horizon would therefore be equivalent to the ‘‘Ca-richeutrophic mull’’ (Duchaufour, 1998). Its complex crumb structure consists of very stablemicroaggregates < 50 Am across with larger macroaggregates >250 Am (Guggenberger etal., 1995) possibly created by bioturbation (Schaefer, 2001), attributable to the earthwormand termite activity, indicated by abundant burrows and channels.Table 4Total organic carbon, soil humic substances and HAF/FAF ratios in superficial soil horizonsHorizon TOC FAFaHAFbHumcHAF/FAFg kg À 1Anthropic Xanthic KandiudultA 18.3 1.7 7.3 7.2 4.29Anthropic Xandic KandiudoxA 34.6 3.3 11.8 19.5 3.55Anthropic DystropeptsA 35.3 0.7 10.5 20.1 14.22Xanthic KandiudoxA 14.8 3.5 3.0 8.6 0.86Typic PlinthudoxA 13.6 4.1 1.5 7.4 0.36Typic FluvaquentA 8.3 1.5 0.4 5.6 0.28Typic UdifluventA 9.7 1.3 1.1 6.8 0.82aFulvic acid fraction.bHumic acid fraction.cHumin.H.N. Lima et al. / Geoderma 110 (2002) 1–17 9
  • 10. 3.3. MicroelementsThe clay fraction of the A horizons of the anthrosols contains more Mn and Zn thaneither the B horizons or the A and B horizons of nonanthropogenic oxisols (Table 5). Thefloodplain soils also have large amounts of Mn and Zn. On the other hand, there are littledifferences in Cu, Cd, Ni and Cr between Terra Preta and floodplain soils. High Mn andZn concentrations in the Terra Preta Anthrosols of the lower Amazon have also beenreported by Kern and Kampf (1989) and Kern and Costa (1997).3.4. Micromorphological featuresHuman activity in the Terra Preta soils is also indicated by the micromorphologicalfeatures of the Anthropic Xanthic Kandiudult (A horizon). The crumb structure (FitzPa-trick, 1993) is typical of soils with mollic epipedons (Fig. 4a), and indicates a progressivedownwards mixing of organic aggregates (Fig. 4a and c). The overall impression is ofefficient pedobiological mixing of the organic-rich A horizons with the underlying Bw/BtTable 5Microelements concentration in clay fraction of the soilsHorizon Fe Mn Cu Zn Cd Ni Crg kg À 1mg kg À 1Anthropic Xanthic KandiudultA 79.4 627 104 150 21 78 172Bt 90.2 71 62 45 22 84 146Anthropic Xandic KandiudoxA 57.4 387 90 245 20 97 17Bw 68.5 84 42 97 19 94 1Anthropic DystropeptsA 59.3 289 69 248 18 88 3Bi 54.2 97 49 73 21 97 0Xanthic KandiudoxA 53.7 84 81 41 21 86 92Bw 76.8 81 36 44 21 89 138Typic PlinthudoxA 67.2 87 123 41 21 91 98Bw 71.0 102 101 50 20 84 89Typic FluvaquentA 72.3 778 148 176 22 108 92C 57.7 379 106 168 18 99 80Typic UdifluventA 74.9 755 121 156 17 103 05C5 64.1 502 82 158 16 91 54H.N. Lima et al. / Geoderma 110 (2002) 1–1710
  • 11. Fig.4.PhotomicrographsofA(a,b),AB(c)andB(d)horizonsoftheAnthropicXanthicKandiudult.H.N. Lima et al. / Geoderma 110 (2002) 1–17 11
  • 12. horizons, as indicated by earthworm channels filled with black material in the B horizonsand with B horizons material in the A horizons. In the Bt horizon of the Anthropic XanthicKandiudult, there is a coalesced pattern of oxidic microaggregates, associated withfeatures of clay illuviation (argillans) (Fig. 4d) along burrows, channels and aggregates.There are also abundant black charcoal fragments documenting long-lasting humansettlement, followed by intense biological mixing.The pottery fragments contain many mica flakes (Fig. 4c) and XRD analysis of thepottery fragments indicated an abundance of illite, but no kaolinite; this suggests that thepottery clay came from the gley soils from the floodplain, as mica (illite) is absent from theTertiary sediments, uplands (Table 2).In the Bw horizon of the Anthropic Xanthic Kandiudox, the granular microstructure istypical of the Oxisols from elsewhere in Brazil (Schaefer, 2001), and fragments of lateriteor petroplinthite are common (Fig. 5b). The A horizon shows the typical crumb structureof mollic epipedon (Fig. 4a), reflecting intense earthworm activity.The floodplain soils contain features indicating weak pedogenesis only. Wetting anddrying (Fig. 5d) have resulted in incomplete, incipient development of peds, and redoxprocesses have resulted in Fe precipitation in voids/channels, or between sand and siltparticles (Fig. 5c).3.5. Terra Preta Anthrosols and human carrying capacityIn the Western Amazon, as elsewhere in Amazonia, the Terra Preta Anthrosols areclosely associated with flat-tops of escarpments of the well-drained Tertiary Plateau (TerraFirme), where they form patches resulting from ancient middens (waste deposits). It isunlikely that any well-drained Tertiary Plateau located far from the rich Amazon flood-plain environment could ever attain such high concentrations of P, Ca and K such as thosereported here, without an anthropic influence. Terra Firme soils, especially Oxisols andUltisols, are extremely poor in nutrients (Schaefer et al., 2000). It is also unlikely that theAmazon as whole could sustain a high population density, if one considers the occurrenceof ‘‘Terra Preta’’ as the basis for calculation. This is because the Amazon floodplain coversless than 5% of the Amazon basin, and represents the only chemically enriched enviro-ment, apart from the Terra Preta (Schaefer et al., 2000).The abundance of total and exchangeable nutrients is probably a result of the sustainedhigh primary productivity of the neighbour floodplain (va´rzea), the only area capable ofproviding high-nutrient middens (Schaefer et al., 2000). It seems that the only environmentin the Amazon lowlands suitable for prolonged cultivation is the floodplain, wherenutrients removed by leaching and harvesting are replaced annually by flooding. In thefloodplain, maize can be grown at lower levels, reaching maturity in 3 months, whereascassava requires at least 6 months, and would be better adapted to the higher transitionzone to the Tertiary Plateau, due to soil drainage requirements. At the time of Europeancontact in the Amazon, there were reports of up to six harvests of maize per year(Meggers, 1971) and widespread gathering of wild rice.Indian villages along the escarpment of the Tertiary Plateau were said to have as manyas a thousand inhabitants in 1542 (De Carvajal, 1894), but their presence dates back to2400–1500 years BP. Some cultural sophistication and hierarchical division can be furtherH.N. Lima et al. / Geoderma 110 (2002) 1–1712
  • 13. Fig.5.PhotomicrographsofA(a)andB(b)horizonsoftheXanthicKandiudoxandC(c,d)horizonoftheTypicFluvaquent.H.N. Lima et al. / Geoderma 110 (2002) 1–17 13
  • 14. Fig.6.Generalaspectsof‘‘TerraPreta’’sitesinprecolonialtimesalongthefringesofTerraFirmewithseasonalinputsoffish,bones,potteryandothermineral/organicmaterialsfromthefloodplain.Inthisscenario,thefloodplainwasunsuitableforpermanentsettlement.H.N. Lima et al. / Geoderma 110 (2002) 1–1714
  • 15. implied from the Terra Preta evidence of these sites (Meggers, 1995), based on pottery andartifact studies.According to our preliminary model (Fig. 6), the Terra Preta Anthrosols result fromdiversification in adaptative strategies to face natural adversities in the ‘‘human flood-plain’’ ecological adaptation. When people settled on the fringes of the Tertiary Plateau,there were considerable constraints to permanent settlement in the floodplain. A possiblescenario is a combination of seasonal high watertable level or flood intensity and limitingland for housing. Thus, prehistoric settlement along the Amazon developed towards aTertiary Plateau–floodplain complement, as recently postulated by Denevan (1996).Consequently, estimates of precolonial population density for the whole Amazon assum-ing similar soil qualities between the rich floodplain and the nutrient-poor well-drainedsoils can be erroneous.Seasonal fishing of the nutrient-rich Amazon and its associated system of lakes andchannels (Fig. 6) was the main source of protein for these pre-Colombian societies. Thefloodplain had the natural resources to support large permanent settlements and highersocial complexity than could be sustained in the nutrient-depleted Tertiary Plateau, but wasapparently unsuitable for permanent settlement. On this basis, it seems unlikely that therewas ever a substantial settlement on the active floodplain.4. ConclusionsThe investigated chemical, mineralogical and micropedological attributes, such as highavailable P and mica flakes in pottery remains of the Terra Preta, indicate that theallochthonous materials in the Terra Preta Anthrosols have their source in the neighbouringfloodplain soils and from the Amazon river. Amazon floodplain soils were the only source ofsoil material for pottery, since 2:1 minerals are not found in upslope Tertiary Plateau soils.The total and available P contents of noncultivated Terra Preta soils (Anthropic XanthicKandiudox) are greater than values reported in the literature. In the Terra Preta undercultivation (Anthropic Xanthic Kandiudult), these values are less, but still large. The levelsof Mn and Zn in the anthrosols are similar to the floodplain soils, corroborating afloodplain source.It is unlikely that any large area of interfluvial Tertiary Plateau, far away from lowlandAmazon va´rzea soils, could develop widespread Terra Preta Anthrosols on the top ofnutrient-poor Oxisols. The formation of large areas of Terra Preta Anthrosols containinghigh P contents in areas of rivers draining deep-weathered terrains of the Brazilian Plateauis unlikely. This raises reservations about estimates of precolonial population densities forthe Amazon basin as a whole, assuming a widespread occurrence of nutrient-richanthrosols away from the Amazon floodplain.AcknowledgementsThe authors are grateful for the careful reviews and comments by Prof. J. Catt andDr. G. Guggenberger on earlier version of this paper. This work has been partiallyH.N. Lima et al. / Geoderma 110 (2002) 1–17 15
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