CRAVO, M.

LEHMANN, J.

ZECH, W.

English

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.995w71998.00113Water and nutrient fluxes as1997 LV-199B.OO1l31111111111111111111111111111111111111111584) Single tree effects on soil organic matterJohannes Lehmann', Manoel Cravo" and Wolfgang Zechl'Institute of Soil Science, University of Bayreuth, Bayreuth, Germany; 2EMBRAPA-CPAA,Manaus, Amazonas, BrazilABSTRACTIncreasing the soil orgamc matter (SOM) contents is an important factor for improving soilfertility. Not only the amount of organic carbon in soil is ecologically important for SOMstability, but also its physical and chemical properties. The appropriate technique for theassessment of ecologically relevant soil pools can vary between soils, and information about thisis lacking for oxisols in the Amazon basin. This work should identify soil pools which aresensitive to land-use changes in terra firme oxisols and yield easy to handle techniques for theevaluation of SOM properties. For this purpose, we have studied the influence of 10 differenttrees and annual crops on SOM properties on an oxisol near Manaus. A combination of aggregateand density fractionation was found to be most suitable for physical SOM characterisation. Theparticulate organic matter (POM, density less than 1.6 g em") varied by one order of magnitudebetween sites and could be used as a sensitive indicator of land-use changes. The obtaineô \e'S.~are extremely important for the evaluation of the sustainability of the investigated land-usesystems and are the basis for soil ecological research on terra firme oxisols.1. INTRODUCTIONThe terra firme oxisols near Manaus are characterized by severe nutrient limitation (Schroth etaI., 1998). Increasing the soil organic matter (SOM) content is an important factor of improvingsoil fertility, first as a source of slowly releasing nutrients and secondly as means of enhancingthe soil cation exchange capacity in order to reduce nutrient leaching (Coleman et aI., 1989). Noton1y the amount of organic carbon in soil is ecologically important for SOM stability, but also itsphysical and chemical properties (Zech et aI., 1997). Using particle size, aggregate and densityfractionation methods (Tiessen and Stewart, 1983; Tisdall and Oades, 1982) together with theanalysis of chemical SOM compounds (Degens, 1997), profound statements can be made aboutSOM stability and dynamics. The appropriate technique for the assessment of ecologicallyrelevant soil pools can vary between soils, and information about this is lacking for oxisols of the59jAmazon basin. This work should identify soil pools whieh are sensitive to land-use ehanges onterra firme oxisols and yield easy to handle teehniques for the evaluation of SOM properties.2. MATERIALS AND METHODSThe soil samples were taken from 0-5 em depth in 50 em distanee from 8 different trees andtwo understorey vegetation in June 1997. The sites were soils under eupuaeu (Theobromagrandiflorum (Willd. (ex Spreng.) K. Schum.), pupunha (Bactris gasipaes Kunth.) managed forpalmito produetion, brazil nut (Berthollecia excelsa Humb.&Bonpl.), urueum (Rixa orellana L.)and pueraria (Pueraria phaseloides (Roxb.) Benth.) in an agroforestry system, and under pupunhain monoeulture. These sites were fertilized aeeording to the reeommendation of the EMBRAPA.Additionally, sites with spontaneous gramineous vegetation (grama), in seeondary regrowth ofVismia spp. (vismia) and in the primary forest under mata-mata (Eschweilera spp.) und baeaba(Oenocarpus bacaba) were ehosen. Two samples were taken from three individual trees of thesame species and eombined. The soils were air-dried and sieved to pass 2 mm.buIk sai!I fine sandI coarse sand Isilt+clayFigure 1: Particle size fraetionation proeedure.60I bulk soi! I2000-10001000-500500-250250-20I POM* I<20Figure 2: Aggregate and density fractionation procedures.The soils were physically fractionated according to their particle size, aggregate size anddensity. For the particle size separation (Figure 1), 30 g fine earth were ultrasonically dispersedwith an energy of 200 J ml' with a soil:water mixture of 1:5 and then sieved to yield the coarsesand fraction (2000-250 J,Lm). After a subsequent sonication with 300 J ml', the fine sand fraction250-20 J,Lm was separated. The sonication energy was adjusted after preliminary experiments toensure complete dispersion (after North, 1976) without redistribution of organic carbono Bothfractions were dried at 40°C for 48 hours. The silt and c1ay fractions « 20J,Lm) were not furtherseparated and freeze dried. The mean recovery amounted to 87 %. Two different methods ofaggregate fractionation were conducted in three parallels: dry and wet fractionation (changed afterAngers and Giroux, 1996), the latter being combined with a density fractionation (Figure 2).First, 30 g soil was put on a cascade of sieves with 1000, 500, 250 and 20 J,Lm sieve openings,vertically shaken for 50 times, turned 90° and again shaken. Tests verified that after thisprocedure only aggregates larger than the respective sieve opening size remained on the sieves,and the recovery laid between 97 and 99 %. The separates were weighed. For the combinedaggregate and density fractionation, aggregates from the dry fractionation were combined and61again put on the cascade of sieves but with the 20 JLm sieve removed. The cascade was installedin a Yoder apparatus in a water bath (Yoder, 1936). The aggregates were slowly wetted frombelow. This procedure could not completely exclude slaking and corresponded to the method wetsieving without prewetting of Gijsman (1996). They were separated using a 3 em verticaldisplacement and 31 rpm for 30· mino The sieves were removed from the water bath andtransfered into a pan with a sodiumpolytungstate (SPT) solution of 1.6 g em". The particulateorganic matter (POM) which was lighter than 1.6 g cm' was washed with the SPT solutionthrough a 250 JLm sieve to separate POM and SPT. All aggregate fractions and POM werewashed several times with deionized water to remove the SPT. The aggregates which passed the250 JLm sieve of the Yoder apparatus were poured through a 20 JLm sieve. The remaining fraction<20 JLm was flocculated with MgCl2 and allowed to settle before discarding the supernatant. Allfractions were dried at 40°C for 48 hours and weighed (recovery 95 %). The aggregates wereagain fractionated in primary particles, POM* and mineral matrix after sonication with therespective sieve and energy described above. Thus, the "true" amount of aggregates could bedetermined excluding the mineral particles which would belong to the aggregate class because oftheir size alone. The primary particles were separated with the sieve of the respective size class,dried, weighed and discarded. POM* was separated from the aggregates with the SPT solution asdescribed above. POM* and the remaining soil were thoroughly washed with deionized water,dried and weighed.The mean weighted diameter (MWD) was calculated from dry and wet aggregate separates inorder to get a parameter of aggregate size distribution from each sample (Kemper and Rosenau,1986):nMWD= L~iWii=1The MWD and the ratio of MWD from wet and dry aggregate fractionation(MWDwe/MWDdry)was taken as a measure of aggregate stability.Carbon and nitrogen were anaIysed by dry combustion with an automatic C/N-Analyser. Thelignin content and its stage of oxidative degradation was determined in the the bulk soil, theparticle size fractions and alI aggregate and POM fractions with combined parallels using alkalineCuO oxidation (ErteI and Hedges, 1984). 500 rng of soil or 50 rng of POM were oxidized withCuO and 2 M NaOH for 2 hours at 170°C under a N2 atmosphere. The oxidation products were62analysed as trimethylsilyl derivates by capillary gas chromatography (OV-1 column) with a FIDdetector. The amounts of lignin were calculated as the sum of the phenolic compounds (vanillyl,syringyl and cumaryl compounds, V+S+C). The acid to aldehyde ratio (ac/al) was taken as anindicator of the degree of microbial degradation of the lignin (Ertel and Hedges, 1984). Non-cellulosic (NCS) and cellulosic (CS) sugars were determined with the MBTH method (Beudert,1988) after sequential acid hydrolysis according to Miltner (1997). Statistical analyses wereperformed using linear regression (STATISTICA 5.0).3. RESULTS AND DISCUSSIONMost of the particles had clay and silt sizes, whereas only 21 and 11 % of the soil constituedof coarse and fine sand, respectively (Figure 3). When the aggregates were not disrupted but drysieved, however, more than 80 % of the soil was larger than 250 JLm (21 % in the coarse sandfraction) and 38 % in the 1-2 mm class (Figure 3). Even with wet sieving, the aggregates largerthan 250 JLm amounted to 70 %. The aggregate stability seemed to be very high in these soilscompared to e.g. a Neubois silty loam from Canada (Angers and Giroux, 1996). Higheraggregate stability was described from an oxisol from Colombia, which had only 10 % of totalsoil in the fraction <250 JLm (Gijsman, 1996).60 -,----------.----, 40eszJ dty frKllonat'onc::::::::J w.t frKtlon.1onI.I-r"~~ 50:;;sõ .0~c~ 30€~ 20.~GÜ 10l~ 35~ 30~ 25j 20:ê;; 15G~ 10~~ 52-0.25 0.25-0.02particle size Imm]<0.02 2-1 1-0.5 0.5-0.25 0.25-0.02 <0.02aggregate size [mmJFigure 3: Particle (left) and aggregate size distribution of dry and wet fractionation withoutremoval of primary particles (right) as percentages of the whole soil from means of all sites(n= 10); means and standard errors.The carbon and nitrogen distribution across particle, density and aggregate fractionsaccentuates this pattern (Figure 4): 70 % of the organic C was found in clay and silt sizeseparates, only 10 % in the coarse sand. When an aggregate fractionation was performed,63however, less than 2 % of the total soil carbon was found in the fraction < 20 Jlm, but more than80 % in aggregates from 0.02-2 mm. In the particulate organic matter (POM), 15 % of the soilorganic carbon was found.o 80~ 70Õ 60~c 50.2~ 40,g~ 30o 20i 10o80~ 70~ 60Õ 50~s 40~ 30~ 20z 10op a rt!c le s iz e densily - aggregale size fraclions [rn m JFigure 4: Organic carbon (above) and nitrogen (below) distribution in particle size, aggregateand density fractions as percentages of the whole soil from means of all sites (n= 10); means andstandard errors.The lignin distribution (Figure 5) revealed that the coarse sand and POM mainly consisted oflabile organic matter i.e. weakly decomposed and humified organic matter. High VSC contentsindicate low degradation and low Ac-to-Al ratios indicate low oxidation of intact lignin (Ertel andHedges, 1984). The coarse sand and POM were expected to be the fractions which react firstafter land-use changes which was indeed verified for the investigated soils. But also theaggregates 1-2 mm showed relatively high VSC contents compared to POMoThe POM and the coarse sand fraction generally varied more between sites than other soilfractions (Table 1). The differences between sites could be more sensitively assessed by POMstocks, which varied by one order of magnitude, than bulk soil carbon analyses. Although POMconstitued only of 15 % of the total soil organic carbon, it was significantly (p < 0.001) related tototal soil organic carbono This indicated not only that the effects of trees on SOM was sensitivelyreflected by POM carbon contents but also that SOM replenishment was related to POM in afunctional sense. However, there was no significant relation between the carbon in the coarsesand and bulk soil carbon (p<0.05). Thus, the POM fraction was a better indicator for SOMchanges than the coarse sand in terra firme oxisols at our site.6430~------------------------------,2-0.250.~.02 <0.02 POM 2-1 1-0.5 0.5-0.250.~.02 <0.022.5 ..,.-------------------------------,2.0--2' 1.5~ 1.00.52-0.25 0.25-0.02 <0.02particle sizePOM 2-1 1-0.5 0.5-0.250.25-0.02<0.02density aggregate fractions [mm)Figure 5: VSC-lignin (above) and Ae-to-AI ratios (below) in particle size, aggregate and densityfraetions (n= 10); means and standard errors.Table 1: Coeffieients of variation (CV [% standard deviation of mean]) of theearbon and nitrogen eontents in particle size, density and aggregate size separatesas indieators of the sensitivity of the respeetive soil pools towards land-useehanges (n= 10).Soil pools (fraetion ÚLm]) Carbon Nitrogenbulk soil 18 10particle size fractions:eoarse sand (2000-250) 67 49fine sand (250-20) 45 46silt+clay « 20) 17 16density fraction:partieulate organie matter 64 59aggregate size fractions:2000-1000 12 161000-500 2.0 14500-250 25 18250-20 16 9<20 21 20654. CONCLUSIONSThe POM fraction is an important indicator of the effects of different organic inputs on SOMproperties. It could be used for assessing the effects of land-use or single-tree effects on SOM.The investigation of different stages of humification and the chemical properties of POM shouldbe intensified. The relationship between aggregation and SOM is only poorly understood andshould be continued in future research, as it may give important information about SOMstabilization. The relation between soil organic matter and nutrient availability and the effects ofsingle-trees on the terra firme oxisols is not understood up to now. The soil nutrient availabilityin different soil pools should be looked at in the future.REFERENCESAngers, DA, and Giroux, M, 1996: Recently deposited orgamc matter m soil water-stableaggregates. Soil Sci. Soe, Am. J. 60, 1547-1551.Beudert, G, 1988: Mikromorphologische, naBchemische und !3C-NMR-spektroskopischeKennzeichnung der organischen Substanz von Waldhumusprofilen nach Dichtefraktionierung.Bayreuther Bodenkundliche Berichte 8, Bayreuth, Germany. 133p.Coleman, J, Oades, JM, and Uehara, G 1989: Dynamics of Soil Organic Matter in TropicalEcosystems. University of Hawaii Press, Hawaii. 249pp.Degens, BP, 1997: Macro-aggregation of soils by biological bonding and binding mechanismsand the factors affecting these: a review. Aust. J. Soil Res. 35, 431-459.Ertel, JR, and Hedges, 11, 1984: The lignin component of humic substances: distribution amongsoil sedimentary humic, fulvic and base-insoluble fractions. Geochim. Cosmochim. Acta 48,2065-2074.Gijsman, AJ, 1996: Soil aggregate stability and soil organic matter fractions under agropastoralsystems established in native savanna. Aust. J. Soil res. 34, 891-907.Kemper, WD, and Rosenau, RC, 1987: Aggregate stability and size distribution. In: Klute, A(ed), Methods of Soil Analysis, Part 1, Agronomy Monograph no. 9, 425-442. Madison, USAMiltner, A. 1997: Umsetzung organischer Bodensubstanz unter dem EinfluB von Mineralphasen.Bayreuther Bodenkundliche Berichte 52, Bayreuth, Germany. 1l0p.North, PF, 1976: An absolute measurement of soil structural stability using ultrasound. J. SoilSci. 27: 451-459.66Schroth, G, da Silva, LF, Seixas R, and Teixeira, WG, 1998: Significant subsoil accumulation ofmineral nitrogen under perennial crops in a ferralitic Amazonian upland soil: effects of land-use systems and single tree patterns. Agric. Ecosys. Environm.: in prep.Tiessen, H, and Stewart, JWB, 1983: Particle-size fractions and their use m studies of soilorganic matter. lI) CuItivation effects on organic matter composition in size fractions. Soil Sei.Soe. Am. J. 47, 509-513Tisdall, OM, and Oades, JM, 1982: Organic matter and water-stable aggregates in soils. J. SoilSei. 33, 141-163.Yoder, RE, 1936: A direct method of aggregate analysis of soils and a study of the physicalnature of soil erosion losses. J. Am. Soe. Agron. 28, 337-351.Zech, W, Senesi, N, Guggenberger, G, Kaiser, K, Lehmann, J, Miano, TM, Miltner, A. andSchroth, G, 1997: Factors controlling humification and mineralization of soil organic matter inthe tropics. Geoderma 79, 117-161.

Single tree effects on soil organic matter.

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Single tree effects on soil organic matter.

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