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Scientific registration n° : 958Symposium n° : 7Presentation : poster

Stability of soil organic matter in Terra Preta soilsStabilité de la matière organique dans les sols de Terra

Preta

GLASER Bruno, HAUMAIER Ludwig, GUGGENBERGER Georg and ZECHWolfgang

Institute of Soil Science, University of Bayreuth, D-95440 Bayreuth, Germany. email:bruno.glaser@uni-bayreuth.de

1. Introduction

Soil degradation is one of the most severe problems of land use in the tropics (Zech,1997). It is expressed by a decline in soil organic matter (SOM) and a loss of plantnutrients. These phenomena explain the infertility of tropical soils after slash and burnwith subsequent agricultural use. In the Brazilian Amazon basin, there are small plots ofsustainable and very fertile anthropogenic black earth soils of Precolumbian origin (TerraPreta do Indio, TP) within the typical rather infertile Oxisol landscape. TP arecharacterized by a higher pH, a higher stock of soil organic matter (SOM), a highercation exchange capacity (CEC) and higher levels of phosphorus and nitrogen. Theexistence of TP proofs, that also very infertile Oxisols can be transformed to fertile andsustainable soils. The most important factor to achieve this seems to be the installation ofa stable SOM pool (Zech et al., 1990). Usually, deforestation in the humid tropics leadsto a rapid decline of SOM (Tiessen et al., 1994). TP in contrast, maintained a stable Ahorizon, concurrently with a high soil fertility, over hundreds of years.CPMAS-13C-NMR-spectroscopy and pyrolysis mass spectroscopy showed, that SOM ofTP contain more aromatic carbon than their adjacent Oxisol (Zech et al., 1990). Thestability of SOM in TP could derive from a selective enrichment of stable, aromaticbiogenic substances during humification (Zech et al., 1990; Kögel-Knabner, 1993).Haumaier and Zech (1995) stated, that residues of incomplete buring of organic material(pyrogenic carbon; black carbon, BC) could be a source of highly aromatic carbon insoils.The aim of our study was to verify or to reject the hypothesis, that stable BC is animportant constituent of SOM in TP. The approach includes a comperative investigationof carbon sources in Oxisols and TP which have been formed from the former. For thispurpose, we developed a wet-chemical method being specific for BC.

2. Materials and Methods

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2.1. Materials

We investigated pairs of sand- and clay-rich TP and Oxisols near Manaus and Santarémin the Amazon basin of Brazil. The parent materials of these soils were sediments of theAlter do Chao formation in the tertiary. Mean annual temperature is 26.5 °C and meanannual rainfall 2050 mm, of which more than 90 % precipitated in the rainy seasonbetween December and July. Natural vegetation was tropical rain forest. All sites havebeen cultivated continuously within the last 30 years. We identified the followingcultures: rubber gum (Hevea brasiliensis), papaya (Carica papaya), passiflora(Passiflora sp.), mango (Mangifera indica) and maniok (Manihot esculenta). All soilprofiles were on the Terra firme, an upland plateau, which never was flooded. Somegeneral soil characteristics are given in Table 1.

Table 1. Selected soil characteristics of A horizons (0 – 10 cm) of Terra Preta (TP) andtheir adjacent Oxisols (Oxi) near Manaus (TP1 to 4, Oxi1 to 4) and Santarém(TP5, Oxi5), Amazon basin, Brazil. cs coarse sand (2000 – 250 µm), fs finesand (250 – 20 µm), silt (20 – 2 µm), clay (<2 µm).

soil cs fs silt clay pH C N ECEC[g kg-1] [g kg-1] [g kg-1] [g kg-1] 1 m KCl [g kg-1] [g kg-1] [meq kg-1]

TP1 397 482 60 50 6,56 37.4 2.90 114Oxi1 316 534 13 104 3,58 13.6 0.88 14TP2 355 287 56 499 5,25 34,5 2,93 78Oxi2 293 311 232 121 3,95 13,3 0,99 19TP3 226 218 50 328 4,90 23,3 1,80 125Oxi3 293 311 232 121 3,95 13,3 0,99 19TP4 188 177 89 508 4,23 29,5 2,21 33Oxi4 190 170 73 566 4,10 26,0 1,56 26TP5 56 42 147 706 6,40 91,2 6.25 409Oxi5 14 13 17 891 3,77 40.4 2.81 154

2.2. Methods

Effective cation exchange capacity (ECEC) and pH were determined accordingSchlichting et al., (1995). Total organic carbon (TOC) and total nitrogen (Nt) weredetermined by dry combustion on a Carlo Erba ANA 1500 C/N analyser. Physicalfractionation of soil according to particle size supported by chemical analyses of thefractions has proven to be a useful tool in process oriented research (Christensen, 1992).Therefore, the bulk fine earth (< 2 mm) and the following particle size fractions wereanalysed separately: coarse sand (2000 - 250 µm), fine sand (250 - 20 µm), silt (20 - 2µm) and clay (< 2µm). The amount and stage of oxidative decomposition of lignin wereestimated using lignin parameters obtained from alkaline CuO oxidation (Hedges andErtel, 1982). Cellulosic and non cellulosic carbohydrates were analyzed after a two stephydrolysis with diluted and concentrated sulfuric acid with subsequent colorimetric

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determination of the degradation products (Johnson and Sieburth, 1977). To quantify theSOM pool being readily available for microorganisms, we carried out a labratoryincubation experiment according to Hassink (1994). The incubation was conducted at 30°C at field capacity. BC was determined by a two step digestion with concentrated HCland concentrated HNO3 followed by a determination of benzene polycarboxylic acids(BPCA) according to Glaser et al., (1997).

3. Results and discussion

3.1. Stocks of total carbon and nitrogen and their distribution between particlesize fractions

TPs have higher TOC and Nt contents in the topsoil as their corresponding Oxisols(Table 1). With increasing profile depth, these values become more and more similiar.The TOC and N stocks of TP are twice to three times higher (80 to 293 Mg TOC ha-1 m-

1 and 4 to 18 Mg Nt ha-1 m-1) compared to the Oxisols (36 to 93 Mg TOC ha-1 m-1 and 2to 6 Mg Nt ha-1 m-1).TOC in particle-size seperates from TPs exceeded alway TOC in the respective fractionsfrom the Oxisols. The ratios of TOC in TP to TOC in the corresponding Oxisol were forcoarse sand 1.2 ± 0.3, 6.3 ± 2.4 for fine sand, 25.3 ± 16.8 for silt and 3.2 ± 0.8 for clay.The highest TOC contents were always observed in the silt fraction, which is consideredto represent a stable SOM pool (Christensen, 1992). TOC declined in the order silt >clay > fine sand > coarse sand in both soil types (

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Table 2). Guggenberger et al. (1995) found the same tendency in colombian Oxisols.Christensen (1992) related the TOC content found in size seperates to that in the bulksoil and defined an enrichment factor (EC = C [g kg-1 size fraction / C [g kg-1 bulk soil]).This calculation eliminates different C contents of the soils, and a comparison of SOMpools in respect to carbon enrichment (EC>1) or depletion (EC<1) is possible. Coarsesand is depleted in carbon both in TP and in Oxisol (

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Table 2). TP contain more TOC in the fine sand fraction which is considered to representthe labile SOM. This SOM pool may play an important role in the supply of nutrients dueto mineralization. The enrichtment of C in the silt and clay fractions is strong texturedependent (not shown here). Christensen (1992) and Leinweber (1995) found anexponential decrease of EC in the clay and silt fractions with increasing clay content ofthe investigated soils.

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Table 2. Total carbon contents and carbon enrichment factors (EC) in particle sizefractions of A horizons of Terra Preta (TP1 to 5) and Oxisols (Oxi1 to 5). cscoarse sand, fs fine sand.

horizon cs fs silt clay clay cs fs silt clayC [g kg-1] [g kg-1] EC EC EC EC

TP1 3.54 15.06 177.83 224.13 50 0.09 0.40 4.75 5.99Oxi1 4.40 1.85 52.58 87.96 104 0.32 0.14 3.86 6.46TP2 3.09 6.78 133.26 74.68 499 0.09 0.20 3.87 2.17Oxi2 2.39 1.25 3.61 24.06 121 0.18 0.09 0.27 1.80TP3 3.55 10.28 143.97 65.93 328 0.15 0.44 6.18 2.83Oxi3 2.39 1.25 3.61 24.06 121 0.18 0.09 0.27 1.80TP4 3.36 11.48 81.31 34.44 508 0.11 0.39 2.76 1.17Oxi4 16.94 20.19 33.22 32.33 566 0.65 0.78 1.28 1.24TP5 23.03 109.88 195.37 61.45 706 0.28 1.33 2.36 0.74Oxi5 18.18 33.24 9.30 14.35 891 1.26 2.31 0.65 1.00

3.2. Biogenic pools of soil organic matter

The sum of vanillyl (V), syringyl (S), and cinnamyl (C) units (VSC) was adopted as anindicator of the amount of intact lignin moieties (Kögel, 1986). The mass ratio of acid toaldehyde of the vanillyl units [(ac/al)V] was taken to assign the degree of side chainoxidation of lignin within a sample (Haider, 1986). High VSC contents and low (ac/al)V

ratios indicate fresh litter input. VSC values in TPs and Oxisols are in the same rame asfound by Amelung (1997) in American Prairie topsoils (16 g VSC kg-1 C) andGuggenberger et al. (1995) in Columbian Oxisols (3.5 to 20 g VSC kg-1 C). The (ac/al)V

ratios of TP were higher than in the corresponding Oxisols, which indicates that lignindegradation was more advanced.Noncellulosic, cellulosic and total sugars were determined to estimate the pool of easylymineralizable SOM. The total sugar concentration decrease in the order sand-rich TP >clay-rich TP > sand-rich Oxisol > clay-rich Oxisol (Table 3). TP contain more poteniallymineralizable SOM. Amelung (1997) found in Prairie soils of North America 87 g C kg-1

C and Guggenberger et al. (1995) 76 g C kg-1 C in Colombian Oxisols.To verify the potential mineralizable SOM pool, we conducted an incubation experimentaccording to Hassink (1994). As can be seen in Figure 1, C mineralization in the sand-rich TP was higher than in the adjacent Oxisol. The clay rich-soils showed the samepattern (not shown). According to Hassink (1994), a higher soil fertility causes a higherSOM turnover. Within the whole incubation time TP mineralized about 30 % and thecorresponding Oxisol 20% of their total carbon content.

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Table 3. Amounts of CuO oxidation products in bulk soil of A horizons of sand- andclay-richt Terra Preta (TP1 and TP5, respectively) and Oxisol (Oxi1 and Oxi5,respectively) expressed as the sum of vanillyl (V), syringyl (S) and cinnamyl(C) units. Degree of lignin side chain oxidation indicated by the (ac/al)V.Amounts of sugars: non cellulosic sugars (non cell sugars), cellulosic sugars(cell sugars).

VSC (ac/al)V non cell sugars cell sugars total sugarssample [g C kg-1 C] [g C kg-1 C] [g C kg-1 C] [g C kg-1 C]

TP1 16.2 0.34 40 100 140Oxi1 9.3 0.25 40 140 180TP5 5.9 0.50 30 20 50Oxi5 4.3 0.25 20 20 40

0

50

100

150

200

250

300

350

400

450

500

0 20 40 60 80 100 120 140 160 180 200 220 240

Inkubationszeit [Tage

C m

in [m

g C

kg-

1 T

ag-1

]

TP

Oxisol

Figure 1. C-mineralization during laboratory incubation (30 °C, field capacity) in the fineearth of A horizons of sand rich Terra Preta (TP) and Oxisol (Oxi), n = 5.

3.3. Stable pool of soil organic matter

During soil sampling in 1996 we found always charcoal in black colored TP. The residueof incomplete combustion of organic materials is known as black carbon (BC, Goldberg,1985). We assumed that BC could be part of the SOM in TP. Table 4 shows that in theA horizon the SOM of the sand-rich TP (29%) contains three times more BC than the

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corresponding Oxisol (10%). The difference between the clay-rich TP (51%) and theadjacent Oxisol (11%) are even higher. These results indicate that BC is a majorconstituent of the stable SOM pool in TP. The highly aromatic core of BC is chemicallyresistant (Haumaier and Zech, 1995). We assume that BC is oxidized very slowlyforming carboxylic groups, which can act as cation exchange places. This could explainthe high CEC of TP compared to the Oxisols.

Table 4. Amounts of black carbon (BC) in Terra Preta und Oxisol. Terra Preta sand-rich(TP1), Oxisol sand-rich (Oxi1), Terra Preta clay-rich (TP5), Oxisol clay-rich (Oxi5), nd not determined.

soil horizon depth BC BC[cm] [ g kg-1 ] [ g C kg-1 C ]

TP1 Ah1 0 - 5 10.45 280Oxi1 Ah 0 - 13 1.35 100TP5 Ah1 0 - 10 42.54 510Oxi5 Ah 0 - 10 1.55 110

4. Conclusions

The stability of the SOM of TP is mainly due to BC. TP contain also a pool of labileSOM, which can be mineralized and release soil nutrients, important for nutrient supplyto plants.

5. Literature

AMELUNG W, 1997. Zum Klimaeinfluß auf die organische Substanz nordamerikanischerPrärieböden. Bayreuther Bodenkundliche Berichte, 53: 1-129.

CHRISTENSEN B T, 1992. Physical fractionation of soil and organic matter in primaryparticle size and density separates. Adv Soil Sci, 20: 1-90.

GLASER B, HAUMAIER L, GUGGENBERGER G AND ZECH W, 1997. Black carbon in soils:the use of benzene carboxylic acids as specific markers. Mitt Dtsch Bodenkundl Ges:85, 237 - 240.

GOLDBERG E D, 1985. Black Carbon in the Environment, John Wiley, New York.

GUGGENBERGER G, CHRISTENSEN B T AND ZECH W, 1994. Land-use effects on thecomposition of organic matter in particle-size separates of soil: I. Lignin andcarbohydrate signature. Eur J Soil Sci, 45: 449-458.

GUGGENBERGER G, ZECH W AND THOMAS R J, 1995. Lignin and carbohydrate alterationin particle-size separates of an Oxisol under tropical pastures following nativesavanna. Soil Biol Biochem, 27: 1629-1638.

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HAIDER K, 1986. Changes in substrate composition during the incubation of plantresidues in soil. In: Jensen V, Kjöller A and Sörensen L H (Editors), MicrobialCommunities in Soil, Elsevier, London, pp. 133-147.

HAUMAIER L AND ZECH W, 1995. Black carbon - possible source of highly aromaticcomponents of soil humic acids. Org Geochem, 23(3): 191-196.

HEDGES J I AND ERTEL J R, 1982. Characterization of lignin by gas capillarychromatography of cupric oxide oxidation products. Analytical Chemistry, 54: 174-178.

JOHNSON K M AND SIEBURTH J MCN, 1977. Dissolved carbohydrates in seawater. I.: Aprecise spectrophotometric analysis for monosaccharides. Marine Chem , 5(1): 1-13.

KÖGEL I, 1986. Estimation and decompostion pattern of the lignin component in foresthumus layers. Soil Biol Biochem, 18: 589-594.

KÖGEL-KNABNER I, 1993. Biodegradation and humification processes in forest soils. In:Bollag J M and Stotzky G (Editors), Soil Biogeochem, Vol. 8, Marcel Dekker, NewYork, pp. 101-137.

LEINWEBER P, 1995. Organische Substanzen in Partikelgrößenfraktionen:Zusammensetzung, Dynamik und Einfluß auf Bodeneigenschaften. In: VechtaerStudien zur Angewandten Geographie und Regionalwissenschaft (Editor), Vol. 15.

SCHLICHTING E, BLUME H P AND STAHR K, 1995. Bodenkundliches Praktikum, 2nd Ed.,Berlin.

SKJEMSTAD J O, CLARKE P, TAYLOR J A, OADES J M AND MCCLURE S G, 1996. Thechemistry and nature of protected carbon in soil. Aust J Soil Res, 34: 251-271.

TIESSEN H, STEWART J W B AND HUNT H W, 1984. Concepts of soil organic mattertransformations in relation to organo-mineral particle size fractions. Plant Soil, 76:287-295.

ZECH W, 1997. Tropen - Lebensraum der Zukunft. Geogr Rundsch, 49: 11-17.

ZECH W, HAUMAIER L AND HEMPFLING R, 1990. Ecological aspects of soil organicmatter in tropical land use. In: McCarthy P, Clapp C E, Malcolm R L and Bloom P R(Editors), Humic Substances in Soil and Crop Sciences. Selected Readings, AmericanSociety of Agronomy and Soil Science Society of America, pp. 187-202.

Keywords : soil organic matter, black carbon, stability, Terra Preta, anthropogenic soil,soil fertilityMots clés : matière organique du sol, carbone, charbon de bois, Terra Preta, solanthropisé, fertilité