The aim of this study is to perform a detailed analysis of the porosity in a low productivity cultivated pasture and to compare the results with the porosity of a similar soil still under native vegetation (Cerradão, i.e. Scierophylous forest)

BALBINO, L. C.

BROSSARD, M.

BRUAND, A.

GUIMARÃES, M. F.

QUÉTIN, P.

Portuguese

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CONSEQUENCES OF TRANSFORMATION OF NATIVE CERRADO IN PASTURE LANDON THE POROSITY OF A CLAYEY-SAND RED LATOSOL.L.C. Balbino1, 2, A. Bruand1, 3, M. Brossard4, M. F. Guimarães5 and P. Quétin11: Science du Sol, INRA, BP 20619, 45166 Olivet cedex, France (Luiz.Balbino@orleans.inra.fr)2: Embrapa CNPAF, BP 179, 74001-970 Goiânia, GO Brazil3: ISTO, Université d'Orléans, Géosciences, BP 6759, 45067 Orléans cedex 2, France4: IRD/Embrapa CPAC , BP 7091, 71619-970 Brasilia, DF Brazil5: Universidade Estadual de Londrina, Dept. de Agrônomia, BP 6001, 86051-990 Londrina, PR BrazilKey-words: Brachiaria decumbens, structure, bulk density, mercury porosimetry, scanning electronmicroscopyIn the Brazilian savannah (Cerrados area), about 60 % of the surface area is used as pasture landand 30 % of the latter is cultivated pasture. Many studies have reported that pastures degrade quicklyand degradation would be more intense when the soil was tilled prior pasture sowing. Pasturedegradation (decrease in the pasture quality and yield, weed invasion) would be partly a consequenceof the soil structure evolution (Balbino & Di Stefano, 1999). Origin of pasture degradation is stillunder discussion and analysis of the literature showed that there is a lack of quantitative data. The aimof this study is to perform a detailed analysis of the porosity in a low productivity cultivated pastureand to compare the results with the porosity of a similar soil still under native vegetation (Cerradão,i.e. Scierophylous forest).Material and methodsThe study site is located in the Mato Grosso do Sul state, Brasilândia county (21º 22,53' S52º 11,73' E). Mean annual rainfall is 1600 mm. The soil is a sandy-clay latosol (Latossolo vermelho)developed on deeply weathered cretaceous materials (Bauru sandstones). Clay fraction is kaolinitemainly with iron oxi-hydroxides in small proportion. Two sites with similar soils were selected: one innative forest (cerrado) and another one in a low productivity pasture of brachiara (Brachiariadecumbens) 20 years old. The soil was described down to 1 m depth with particular attention tostructure. Bulk density measurements were performed using cylinder 98 cm3 in volume. In thelaboratory we investigated the pore size distribution from 360 down to 0.006 µm using mercuryporosimetry (Fig. 1). Thin sections of undisturbed samples were examined in scanning electronmicroscopy using back-scattered electron emission (Bruand et al., 1996).Results and discussionUnder pasture, the upper horizon (0–3 cm) was massive with horizontal roots. Then from 3 to15 cm depth, well developed and small to medium polyhedral aggregates associated with a few micro-aggregates were observed. Roots are mainly located between the polyhedral aggregates. Under forest,small to medium polyhedral aggregates with lots of micro-aggregates were encountered from 0 to15 cm depth. Roots were located between and within the polyhedral aggregates. From 15 to 100 cmdepth, aggregates were micro-aggregates mainly in a closer packing under pasture than under forest. Afew polyhedral aggregates were present under pasture but their number and size decreased with depth.Biological activity (termites, ants and earthworms) was smaller under pasture than under forest andlocated close to the surface mainly.Under forest, the bulk density ranged from 0.80 in the first top centimetres to 1.03 at 15 cm depthwhen under pasture it ranged from 1.08 to 1.22 from the surface to 13 cm depth (Table). These bulkdensity values showed a much smaller pore volume in the topsoil under pasture. From 18 to 100 cmdepth, bulk density showed also smaller porosity under pasture.Analysis of the structure using microscopy showed the presence of polyhedral aggregates a fewmillimetres to one centimetre in size and micro-aggregates 50–100 µm in size (Fig. 2). These twotypes of aggregate were present in different proportions according to the site and horizon. Analysis ofthe porosity showed that pore were (Fig. 1): (i) pores larger than several hundreds of micrometers insize which resulted from biological activity (root, ants and termites channels) and polyhedralaggregate packing, (ii) pores a few micrometers to one hundred micrometers in size which resultedfrom micro-aggregate packing and (iii) pores which resulted from the packing of clay particles withinpolyhedral aggregates and micro-aggregates and which were not visible in scanning electronmicroscopy because they are too small. Comparison of porosity within every profile showed adecreased in the large pores with depth and an increase in the pores corresponding to micro-aggregatespacking because they are more numerous with depth.The mercury intrusion curve enabled the identification of several classes of pores (Bruand et al.,1993; Curmi et al., 1994). Results showed the presence of two classes of pores A and B whichcorresponded to pores with equivalent pore diameters ranging from 0.022 to 0.025 µm and from 18 to58 µm in equivalent diameter, respectively (Fig. 1). Class B corresponded to pores resulting from thepacking of micro-aggregates. Large pores resulting from polyhedral aggregates packing and biologicalactivity were not measured or just partially because they were too large to be studied using mercuryporosimetry. Results showed that the volume of pores B was greater in the 0–3 cm layer and muchsmaller (from 20 to 40 % smaller) in the 3–17 cm layer under pasture than under forest (Table). PoresB were also smaller under pasture than under forest as shown by the smaller Dm,B. Under pasture, thegreater Vp,B in the 0–3 cm layer would result from a great under-estimation of Vp,B in the 0–3 cm layerunder forest because the micro-aggregates were in a very loose packing leading to too large pores tobe measured using mercury porosimetry. From 17 to 100 cm depth, Vp,B was similar or smaller (from5 to 25 % smaller) under pasture. This is consistent with field observation which showed a lessdeveloped structure under pasture. This smaller Vp,B might result also from a smaller amount of micro-aggregates because of a smaller clay content (Table). The volume of pores belonging to class A rangedfrom 0.046 to 0.112 cm3 g-1 (Table). These pores resulted from the packing of clay particles within thepolyhedral and micro-aggregates. The no or very little variation of Dm,A showed no or very littlevariation of both the size and packing of the elementary kaolinite particles. Such a result is consistentwith the close relationship between Vp,A and the clay content (r2 = 0.95).ConclusionOur results showed a strong evolution of the structure which led to a decrease in the porosity,particularly in the topsoil layers. Except the very small pores which result from the packing of thekaolinite particles within aggregates, all the pores were less developed under pasture. Fieldobservations showed also a structure less developed from 15 to 100 cm depth under pasture andmercury porosimetry showed that micro-aggregates would be in a closer packing under pasture thanunder forest. However, this will need further analysis to be confirmed.Balbino, L.C., Di Stefano J.G., 1999. Projet PROCITROPICOS: Intégration de l'agriculture et del'élevége par les systèmes de semis direct, pp. 409-417, In: Rasolo, F. and M. Raunet (eds), Gestionagrobiologique des sols et des systèmes de culture. Actes de l'atelier international, Antsirabe,Madagascar, 23-28 mars 1998, Cirad, Collection Colloques, 658p.Bruand, A., D'Acqui, L.P., Nyamugafata, A P., Darthout, R., Ristori, G.G., 1993. Analysis of porosityin a tilled "crusting soil" in Zimbabwe. Geoderma, 59, 235-248.Bruand, A., Cousin, I., Nicoullaud, B., Duval, O., Bégon J.C., 1996. Backscattered electron scanningimages of soil porosity for analysing soil compaction around roots. Soil Science Society of AmericaJournal, 60, 895-901.Curmi, P., Kertzman, F.F., Quieroz Neto, J.P., 1994. Degradation of structure and hydraulic propertiesin an Oxisol under cultivation (Brazil). In: Soil Micromorphology: Studies in Management andGenesis. Developments in Soil Science 22, Elsevier, Amsterdam, 569-579.00.050.100.150.200.250.300.350.400.001 0.01 0.1 1 10 100 1000Equivalent diameter  De  / µm∆ V / ∆ (log D) A BFigure 1: Pore volume distribution for the 18–30 cm layer under forest.Table: Physical and chemical data for the soils studied.a___________________________________________________________________________________________________________________________________________________________________________________Horizon PSD OC pHw CEC Db Mercury porosimetry<2 2– 50– 200– Pore class A Pore class B50 200 2000 Vp,A Dm,A Vp,B Dm,Bcm _________________  g kg-1  __________________ cmolc kg-1 cm3 g-1 µm cm3 g-1 µm___________________________________________________________________________________________________________________________________________________________________________________Forest 0–3 32.1  8.6 14.2 45.1 20.1 3.9 3.14 0.80 0.090 0.023 0.189 583–6 35.3  7.2 30.2 27.4 14.2 3.9 2.63 1.03 0.101 0.023 0.196 516–15 29.6  6.8 32.6 31.0 - - - 1.03 0.083 0.023 0.256 2918–30 30.3  8.1 31.1 27.5  7.5 4.0 2.42 1.02 0.092 0.023 0.252 5030–40 31.0  4.7 35.2 29.1 - - -   - 0.089 0.023 0.165 4860–80 36.5  4.4 32.7 26.4  4.1 4.2 2.49 0.94 0.105 0.023 0.290 5090–100 44.4  3.7 30.4 21.5 - - - 0.92 0.112 0.023 0.255 48Pasture 0–3 18.6  4.0 36.2 41.3 14.2 5.8 3.33 1.08 0.046 0.025 0.209 273–6 21.2  4.5 30.0 44.2 11.7 5.3 2.62 1.17 0.055 0.022 0.162 1810–13 21.5  4.2 30.5 43.8  9.2 5.2 2.22 1.22 0.058 0.023 0.154 1814–17 22.1  6.4 30.6 40.9  7.9 5.3 2.22   - 0.065 0.023 0.182 3518–30 23.0  9.2 29.7 38.1  6.2 5.2 2.01 1.21 0.068 0.023 0.183 5530–40 26.4  4.7 30.2 38.7 - - - 1.22 0.075 0.025 0.164 4260–80 30.6  6.0 32.1 31.4  3.6 5.0 1.83 1.09 0.084 0.023 0.243 5690–100 32.8  4.4 29.8 33.0 - - - 1.13 0.088 0.023 0.194 27___________________________________________________________________________________________________________________________________________________________________________________aPSD is particle size distribution; OC is organic carbon content; CEC is cation-exchange capacity; Dbis bulk density; Vp,A and Dm,A are the volume of pores A and their modal equivalent diameter,respectively; Vp,B and Dm,B are the volume of pores B and their modal equivalent diameter,respectively.Figure 2: Back-scattered electron scanning image (x60, WD = 18 mm, 15 kV) of the 90–100 cm layerunder forest (left) and of the 0–3 cm layer under pasture (right). Voids occupied by resin are black,homogeneous grey areas are quartz and heterogeneous dark grey areas are porous clayey phase.1 mm1 mm

Consequences of transformation of native cerrado in pasture land on the porosity of a clayey-sand red latosol.

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Consequences of transformation of native cerrado in pasture land on the porosity of a clayey-sand red latosol.

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