INFLUENCE OF DIFFERENT GRAZING SYSTEMS ON PHYSICAL PROPERTIES AND AGGREGATION IN SAVANNAH SOILS 1

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1 INFLUENCE OF DIFFERENT GRAZING SYSTEMS ON PHYSICAL PROPERTIES AND AGGREGATION IN SAVANNAH SOILS 1 Silvno Alves Pereir 2, Gerldo Césr de Oliveir 3, Huberto José Kliemnn 2, Luiz Crlos Blbino 4, Aldi Fernndes de Souz Frnç 5, Edurdo Rodrigues de Crvlho 5 RESUMO INFLUÊNCIA DE DIFERENTES SISTEMAS DE PASTEJO SOBRE AS PROPRIEDADES FÍSICAS E AGREGAÇÃO EM SOLOS DE CERRADO Objetivou-se vlir lterções d estrutur do solo, que ocorrerm em um Ltossolo distrófico típico, cusds por diferentes sistems de pstejo. No no de 2004, mostrs de solo (0-2 cm, 2-7 cm e cm de profundidde) form colhids, em fzend pertencente à Escol de Veterinári d Universidde Federl de Goiás, com s seguintes especificções: I) solo sob pstgem de Pnicum mximum cv. Tnzâni, em sistem rotciondo de pstgem, posição touceir ( C ); II) solo sob pstgem de Pnicum mximum cv. Tnzâni, em sistem rotciondo de pstgem, posição entre touceirs ( AC ); III) solo sob pstgem de Cynodon dctilum cv. Tifton 85, em sistem contínuo de pstgem; e IV) solo sob vegetção de Cerrdo, como referênci. A implntção ds pstgens, com s grmínes Tifton 85 e cpim Tnzâni, modificrm s proprieddes físics do solo, por meio d redução de mcroporos (MAC) e umento n densidde do solo (SD). Solos sob pstgem com Tifton 85 e cpim Tnzâni (posição C ) presentrm mior diâmetro médio geométrico (MGD), se comprdos os solos sob Cerrdo e cpim Tnzâni (posição AC ). As lterções n estrutur do solo form mis relevntes ns cmds superficiis (0-2 cm e 2-7 cm de profundidde). As txs de lotção, prticds ns pstgens com s grmínes Tifton 85 e cpim Tnzâni, form excessivs, sendo principl cus d deteriorção ds proprieddes físics do solo. PALAVRAS-CHAVE: Cynodon dctilum; Pnicum mximum; pstejo contínuo; pstejo rotciondo; tx de lotção. ABSTRACT The objective of this study ws to evlute soil structure ltertions tht occurred in typicl Red distroferric Ltossol under different grzing systems. In 2004, soil smples from three depths (0-2 cm, 2-7 cm, nd cm) were collected in frm owned by the College of Veterinry of the Federl University of Goiás, Goiâni, Goiás Stte, Brzil, s it follows: I) soil under psture with Pnicum mximum cv. Tnzni, in rottionl grzing system, clump position ( C ); II) soil under psture with Pnicum mximum cv. Tnzni, in rottionl grzing system, mong clumps position ( AC ); III) soil under psture with Cynodon dctilum cv. Tifton 85, in continuous grzing system; nd IV) soil under Svnnh, s reference. The implnttion of pstures with Tifton 85 nd Tnzni grsses chnged physicl properties of the soil, through the reduction of mcropores (MAC) nd increse in soil density (SD). Soils under pstures with Tifton 85 nd Tnzni grsses ( C position) presented lrger men geometric dimeter (MGD), if compred to the soils under Svnnh nd psture with Tnzni grss ( AC position). Altertions in soil structure were more significnt in the topsoil lyer (0-2 cm nd 2-7 cm depth). Stocking rtes prcticed in the pstures with Tifton 85 nd Tnzni grsses were excessive nd lso the min reson for deteriorting physicl soil properties. KEY-WORDS: Cynodon dctilum; Pnicum mximum; continuous grzing system; rottionl grzing system; stocking rte. INTRODUCTION Grsslnds cover pproximtely 19.4 out of 34.1 million hectres of frming soil in the Goiás Stte (Cózer et l. 2001). The sum of diry nd beef herds, in Brzil, is estimted in 192 million heds, nd the Goiás Stte hs the third biggest herd nd is the second biggest milk producer of the whole country (Goiás 2003). 1. Trblho recebido em jn./2009 e ceito pr publicção em jul./2010 (n registro: PAT 5353/ DOI: /pt.v40i3.5353). 2. Universidde Federl de Goiás, Escol de Agronomi e Engenhri de Alimentos, Setor de Ciênci do Solo, Goiâni, GO, Brsil. E-mils: pereir@incr.gov.br, kliemnn@gro.ufg.br. 3. Universidde Federl de Lvrs, Escol de Agronomi, Deprtmento de Ciênci do Solo, Lvrs, MG, Brsil. E-mil: gerldooliveir@ufl.br. 4. Empres Brsileir de Pesquis Agropecuári, Embrp Trnsferênci de Tecnologi/Embrp Sede, Setor de Pesquis e Desenvolvimento, Brsíli, DF, Brsil. E-mil: luizcrlos.blbino@embrp.br. 5. Universidde Federl de Goiás, Escol de Veterinári, Deprtmento de Produção Animl, Goiâni, GO, Brsil. E-mils: ldi@vet.ufg.br, edurodriguesuk@yhoo.co.uk.

2 Influence of different grzing systems on physicl properties nd ggregtion in Svnnh soils 275 Pstures degrdtion hs reched round 8 million hectres in the Goiás Stte (Goiás 2003), bsiclly due to indequte mngement. The consequence of tht process hs been the reduction of both chemicl fertility nd physicl soil structure, hence those soils hve become vulnerble to erosion nd compction cused by grzing nimls (Blbino et l. 2003). Ltossols from Cerrdo (Brzilin Svnnh) still under ntive vegettion hve pproprite physicl structure (Crvlho Júnior et l. 1998). However, vilbility of nutrients in those soils is limiting for idel growth nd yield of cultivted plnts. Moreover, the doption of indequte grsslnd mngement implies in losses of physicl qulity of the soil, for medium nd long terms, hindering the growth of roots, ir circultion, nd wter infiltrtion (Crvlho Júnior et l. 1998, Bertol et l. 2000, Fregonezi et l. 2001, Müller et l. 2001). The pressure from grzing nimls cn rech up to kp (Betteridge et l. 1999) nd cuse chnges in density nd porosity of the soil, especilly in the first 3-6 cm depth (Grdwell 1960, Chncellor et l. 1962, Grdwell 1966, Bertol et l. 1998). Thus, one of the strtegies tht hve been used to recover degrded pstures is the rottionl grzing system, where fertiliztions re common prctice nd even irrigtion, during the dry seson, hs been recommended (when this fcility is vilble), lthough Lim et l. (2004) hve reported tht compction nd soil density, in rottionl grzing system psture under irrigtion, increses, if compred to the sme psture without irrigtion. The objective of this study ws to evlute structurl ltertions occurred in typicl Red Ltossol, fter four yers of the implnttion of two pstures: the first with Pnicum mximum cv. Tnzni, in rottionl grzing system, nd the second with Cynodon dctilum cv. Tifton 85, in continuous grzing system, with soil under Svnnh s reference. MATERIALS AND METHODS The experiment ws conducted in 2004 (dry nd rin sesons), in frm owned by the College of Veterinry of the Federl University of Goiás, Goiâni, Goiás Stte, Brzil (16º35 12 S, 49º21 14 W, nd ltitude of 730 m). According to the Köppen clssifiction, the re hs semi-humid tropicl climte (Aw). Averge nnul rinfll is 1,578 mm, being the months from November to Mrch the riniest, nd from June to August the driest ones. Averge nnul temperture is 23.2ºC, with minimum of 17.9 C nd mximum of 28.9 C (Mgnno et l. 1983). The soil is clssified s typicl Red distroferric Ltossol, lomy texture (Embrp 2006). Soil smples were: I) soil under psture with Pnicum mximum cv. Tnzni, in rottionl grzing system, clump position ( C ); II) soil under psture with Pnicum mximum cv. Tnzni, in rottionl grzing system, mong clumps position ( AC ); III) soil under psture with Cynodon dctilum cv. Tifton 85, in continuous grzing system; nd IV) soil under Svnnh, s reference. Previously to the implnttion of the current grsses, the pstures were plnted with Brchiri decumbens. Grsses in the rottionl nd continuous grzing systems were estblished in Jnury (2000). Soil texture nd soil chemicl ttributes re shown in Tbles 1 nd 2, respectively. Bse sturtion ws rised to 60%, ccording to results of soil chemicl nlyzes. For soil tillge, Tble 1. Soil texture of Red distroferric Ltossol (Goiâni, GO, Brzil, 2000). Depth (cm) / Horizon Grnulometry (g kg -1 ) Cly Silt Snd Bw* * Source: Lopes et l. (1997). Tble 2. Soil chemicl ttributes of Red distroferric Ltossol (0-20 cm) (Goiâni, GO, Brzil, 2000). C 2+ Mg 2+ Al 3+ H ph P(Mel) K CEC* V** OM*** cmol c dm -3 CCl 2 mg dm -3 cmol c dm -3 % g kg * Ction exchnge cpcity; ** Bse sturtion; ** Orgnic mtter.

3 276 S. A. Pereir et l. (2010) firstly, hevy disking ws crried out, in order to brek down the existing vegettion (Brchiri decumbens). Then, limestone ws brodcsted, followed by ploughing (30 cm depth), nd, finlly, two light diskings were pplied to level off the soil. In greement with Vilel et l. (1998), 90 kg of P 2 O 5 h -1 were pplied: 60 kg h -1 incorported into the soil nd the remining 30 kg h -1 mixed to the seeds, t the moment of sowing. Besides phosphorus fertiliztion, 30 kg h -1 of trce minerls (FTE BR 12) were lso mixed to the seeds t sowing. After estblishment of the grsses, nitrogen ws brodcsted in the form of ure (250 kg h -1, divided into three pplictions). The psture under the rottionl grzing system ws formed by thirty pens of 0.33 h ech. Stocking rte ws 6 LU/h (livestock units; 1 LU = 450 kg of live weight) nd grzing ws performed by Holstein-Friesin lctting cows. The psture under continuous grzing system consisted of totl re of 0.51 h (41 m x 125 m) nd the stocking rte, in this cse, ws 2 LU/h, where grzing ws performed by Holstein-Friesin dry cows. In the dry seson, the irrigtion system (sprinklers) ws turned on once week, providing precipittion of 4 mm, which llowed cows to sty in the pstures during the whole experimentl length. Tnzni grss nd Tifton 85 pstures were 100 m prt from ech other. Soil smples were collected to evlute porosity nd ggregtion, in 12 mini-trenches, in the 0-2 cm, 2-7 cm, nd cm depth lyers (totling 36 smples/ smpling site), under psture with Tnzni grss ( C position), tken s closely s possible to the clumps, while in the AC position, there ws stndrdiztion of 0,25 m below the clump, towrds the steepness of the lnd. C nd AC positions were smpled with the objective to determine possible differences in soil properties under the influence of the sme grss species (Tnzni grss). For evlution of porosity, cylinders of cm 3 were used for smpling the 0-2 cm lyer, while cylinders of cm 3 were used for the 2-7 cm nd cm lyers. After smpling, cylinders filled with soil were wrpped in plstic to mintin intct the soil structure. In the lbortory, smples were sturted nd tken to the tension tble, where they were submitted to 6 kp (Oliveir 1968), for determintion of microporosity (< 50 µm), ccording to Boum (1991), nd field cpcity (Oliveir et l. 2003). Soil density, totl volume of pores nd mcroporosity were determined ccording to Embrp (1997). For determintion of permnent wilting point, Richrds s chmber ws used, under 1,500 kp. In this methodology, smples were grounded in 2 mm screen nd, fter sturtion, they were plced in rubber rings nd lid out on cermic pltes, until equilibrium ws reched. Smples were then dried t C, for 48 hours, in forced ir oven, for determintion of grvimetric humidity. Following suggestions by Sntos (1997), totl vilble wter ws clculted by the expression TAW = θ 6 - θ 1500 (TAW = totl vilble wter (m 3 m -3 ); θ 6 = volumetric wter content equivlent to the tension of 6 m 3 m -3 ; nd θ 1500 = volumetric wter content equivlent to the tension of 1,500 m 3 m -3 ). For stbility of ggregtes nlyses, smples of pproximtely 350 cm 3 were tken from ech depth of the pprised sites. In the lbortory, smples were slightly crumbled mnully nd grounded in n 8 mm screen. Aggregtes with dimeter smller thn 8 mm (25 g) were pre-moistened by cpillrity, in try with humid snd covered by filter pper. Moistened ggregtes were submitted to gittion (Yoder 1936), following the method described by Kemper & Rosenu (1986). For tht, set of five sieves ws used, with the following clsses of dimeter: 2.0 mm, 1.0 mm, 0.5 mm, 0.25 mm, nd mm. Aggregtes retined in ech sieve were trnsferred to previously weighted cns, dried t C, for 48 hours, in forced ir ovens, nd then the ggregtes dry mss retined in ech sieve ws weighted. Aggregtes stbility ws represented by men geometric dimeter (MGD), ccording to the eqution MGD = 10 x, where d is the men dimeter of ny prticulr clss of ggregtes seprted by sieving, nd n the percentge of ggregtes from ech clss size represented s frction of the totl dry mss of the smple. Prticles density ws determined by following the volumetric blloon method with ethyl lcohol, described by Embrp (1997). Four composite depth smples were used in ech smpling site.,

4 Influence of different grzing systems on physicl properties nd ggregtion in Svnnh soils 277 Sttisticl dt nlyzes were ccomplished using SAS (2002) through PROC GLM. Mens were compred by using the Duncn test (p < 0.05). RESULTS AND DISCUSSION Mens of mcropores (MAC) in the soil under Svnnh were higher (p < 0.05) thn in the soils under pstures with Tifton 85 nd Tnzni grsses (for both C nd AC positions), for ll depths evluted (Tble 3). High MAC vlues re common in Svnnh Ltossols (Resende et l. 2002). The highest mens for totl volume of pores (TVP) were lso found in the soil under Svnnh, with vlues of 0.69 m 3 m -3, 0.63 m 3 m -3, nd 0.60 m 3 m -3, respectively for the 0-2 cm, 2-7 cm, nd cm depth lyers (p < 0.05). Regrding MAC = 0.10 m 3 m -3, s the lowest vlue for stisfctory growth of cultivted plnts (Blck 1975, Forsythe 1975, Kiehl 1979, Lipiec & Htno 2003), it cn be observed tht MAC ws restrictive in the soils under pstures with Tifton 85 nd Tnzni grsses ( AC position), for both the 0-2 cm nd 2-7 cm lyers (Tble 3). Although not mesured in the present study, the resonble explntion for the low MAC vlue in those soils is the lower quntity of roots, resulting in less ccumultion of orgnic crbon, which plys mjor role in the formtion nd stbiliztion of mcroggregtes, since it is known s n efficient cementing gent (Cmpos et l. 1995). Furthermore, low MAC in the soils under pstures with Tifton 85 nd Tnzni grsses ( AC position) hd n effect in the increse of SD in the 0-2 cm nd 2-7 cm lyers (Figure 1), which cn indicte soil structure degrdtion. Higher MAC vlues were expected in the soil under psture with Tifton 85 (when compred to the one under Tnzni grss, C nd AC positions), tking into ccount its lower stocking rte (2 LU/h), s well s its root system constituted by rhizomes (Vilel & Alvim 1998). However, there ws no difference in MAC between those two soils Tble 3. Mens of mcroporosity (MAC), microporosity (MIC), totl volume of pores (TVP), permnent wilting point (PWP), totl vilble wter (TAW), nd prticles density (PD), in three depths of Red distroferric Ltossol under two grzing systems nd Svnnh (Goiâni, GO, Brzil, 2004). Sites MAC MIC TVP PWP* TAW** PD m 3 m -3 kg dm -3 Depth: 0-2 cm Svnnh 0.43 *** 0.26 c c 0.12 b 2.52 Tifton c c Among clumps 0.09 c 0.39 b 0.49 c Clumps 0.17 b 0.38 b 0.55 b 0.24 b C.V. (%) Depth: 2-7 cm Svnnh b b 0.12 b 2.65 Tifton b b Among clumps 0.06 c c Clumps 0.08 bc c C.V. (%) Depth: cm Svnnh b Tifton b b Among clumps 0.16 c c Clumps 0.19 bc bc C.V. (%) * Wter kept t 1,500 kp; ** Wter vilble between 1,500 kp nd 6 kp (Field cpcity = MIC); *** Mens with different letters in the sme lyer differ ccording the Duncn test (p < 0.05).

5 278 S. A. Pereir et l. (2010) Depth (cm) Soil density (kg dm -3 ) C c B b SCN TC AC ET T C Figure 1. Soil density of Red distroferric Ltossol in the depths of 0-2 cm, 2-7 cm, nd cm, under S = Svnnh; T = psture under continuous grzing system (Tifton 85); AC = psture under rottionl grzing system (Tnzni grss; AC position); C = psture under rottionl grzing system (Tnzni grss; C position) (Goiâni, GO, Brzil, 2004). * Cpitl letters compre the effect of depth in ech site nd smll letters compre sites in the sme depth. (p > 0.05), suggesting tht the stocking rte in the soil under psture with Tifton 85 ws too high, especilly in the riny seson, when the soil is vulnerble to compction due to excess of moisture combined with nimls grzing (Kondo & Dis Júnior 1999, Oliveir et l. 2004). Tht sme explntion cn be extended to the soil under psture with Tnzni grss, in the AC position. Besides fcing n even higher niml pressure becuse of the stocking rte (6 LU/h), this soil did not hve the protection from the grss (clumps). Since cows wlk preferbly mong clumps, the pressure per unit re ws even higher in the AC position nd prt of tht pressure ws trnsmitted to the 2-7 cm lyer, in greement with the findings reported by Imhoff et l. (2000). MAC re the min route for roots growth, hence MAC reduction stimultes the development of side roots, which re less efficient in bsorption of wter, nutrients nd gseous exchnges (Cmrgo & Alleoni 1997). The reson for MAC reduction, in the soil under psture with Tnzni grss ( C position), in the 2-7 cm lyer (0.08 m 3 m -3 ; Tble 3), when compred to the 0-2 cm lyer (0.17 m 3 m -3 ; Tble 3), ws the continuous nd repetitive pressure mde by nimls mong clumps (s previously mentioned) nd its trnsmission to the 2-7 cm lyer, incresing SD in this lyer (Figure 1), fct lso observed by Bertol et l. (2000). These results demonstrted tht the A b B b B B A A B A A A stocking rte in the soils under psture with Tnzni grss ( C nd AC positions) were hrmful to the soil structure, minly through increse in SD, in the first two lyers (0-2 cm nd 2-7 cm depth). On the other hnd, SD in the soil under Svnnh incresed s smpling went deeper (Figure 1), bsiclly becuse there ws reduction in the ccumultion of orgnic mtter dded to the soil under Svnnh, lso in greement with Bertol et l. (2000). In the cm lyer, ll smpling sites presented higher MAC vlues thn the minimum ones (Tble 3). So, it cn be stted tht reductions in MAC nd increses in SD, cused by grzing nimls, hppen just in the superficil soil lyer, s reported by Crvlho Júnior et l. (1998), Fregonezi et l. (2001), Müller et l. (2001), nd Blbino et l. (2004). However, the soils under pstures with Tifton 85 nd Tnzni grsses ( C nd AC positions) presented higher TAW rte thn the soil under Svnnh, for both the 0-2 cm nd 2-7 cm lyers (Tble 3, p < 0.05), in greement with Resende et l. (2002). Sometimes, compction might be beneficil, in terms of wter retention in Ltossols, nevertheless it hs to be pointed out tht TAW should be lwys compred with porosity, becuse, in the present study, the increse in TAW ws directly relted to both reductions in MAC nd increses in SD, in the soils under pstures with Tifton 85 nd Tnzni grsses ( C nd AC positions). In Figure 1, soil density (SD) ws lower in the soil under Svnnh, if compred to the soils under pstures with Tifton 85 nd Tnzni grsses ( C nd AC positions), in ll depths evluted (p < 0.05). There ws no difference in SD between soils under pstures with Tifton 85 nd Tnzni grsses ( AC position), in the 0-2 cm nd 2-7 cm depth lyers (p > 0.05), wht should be relted to the trnsmission of the pressure mde by cows to deeper lyers of the soil (Imhoff et l. 2000). However, in the 0-2 cm lyer, SD ws lower (p < 0.05) in the soil under psture with Tnzni grss ( C position), in reltion to the soils under pstures with Tifton 85 nd Tnzni grsses ( AC position), possibly due to the effect of the protection offered by the grss in the C position. SD incresed from the 0-2 cm to 2-7 cm lyer (p < 0.05), in the psture with Tnzni grss ( C position), in greement with Blbino et l. (2004). The reltion between MAC nd SD is presented in Figure 2. When SD ws higher thn 1.36 kg dm -3, MAC ws lower thn 0.10 m 3 m -3. According to

6 Influence of different grzing systems on physicl properties nd ggregtion in Svnnh soils 279 Mcroporosity (m 3 m -3 ) Volume of Mcropores = (0.563 x SD) R 2 = MGD (mm) c S (1) b T bc AC C S T AC C S T Depth (cm) b b AC C Soil density (kg/dm 3 ) Figure 2. Mcroporosity, in reltion to soil density (SD), in the 0-2 cm, 2-7 cm, nd cm depths of Red distroferric Ltossol under two grzing systems nd Svnnh (Goiâni, GO, Brzil, 2004). Figure 3. Men geometric dimeter (MGD) of ggregtes in three depths of Red distroferric Ltossol under S = Svnnh; T = psture under continuous grzing system (Tifton 85); AC = psture under rottionl grzing system (Tnzni grss; AC position); C = psture under rottionl grzing system (Tnzni grss; C position) (Goiâni, GO, Brzil, 2004). 1 Mens with different letters in the sme lyer differ by the Duncn test (p < 0.05). Blck (1975), Forsythe (1975), Kiehl (1979), nd Lipiec & Htno (2003), when MAC is equl or below 0.10 m 3 m -3, there is restriction in root growth. Besides, when considering SD = 1.40 kg dm -3 s bottom line to plnts cultivted in lomy soils (Arshd et l. 1996), the corresponding MAC vlue would be 0.08 m 3 m -3, hence below the minimum proposed by Blck (1975), Forsythe (1975), Kiehl (1979), nd Lipiec & Htno (2003). The liner eqution in Figure 2 shows tht when SD reched 1.54 kg dm -3, mcropores were no longer detected in the verticl xis of Figure 2. In this study, few smples presented SD = 1.54 kg dm -3, minly in reltion to the soil under psture with Tnzni grss ( AC position), in the 0-2 cm nd 2-7 cm depth lyers. SD presented positive correltion with micropores (r = 0.920, in the 0-2 cm nd 2-7 cm lyers, nd r = 0.631, in the cm lyer) nd correltion vlues helped to support the explntion tht only from certin degree of compction or SD there is corresponding increse in micropores (MIC). In this study, the increse in MIC resulted in increse in TAW (p < 0.05), in the soils under pstures with Tifton 85 nd Tnzni grsses ( C nd AC positions), in the 0-2 cm nd 2-7 cm lyers, when compred to the soil under Svnnh (Tble 3). In Figure 3, the men geometric dimeter (MGD) of ggregtes, in the 0-2 cm lyer of the soils under pstures with Tnzni grss ( C position) nd Tifton 85, ws higher thn in the soils under Svnnh nd psture with Tnzni grss, in the AC position (p < 0.05), in greement with Pereir (1997), who used ggregtes smller thn 8 mm in his methodology nd reported higher MGD in soils under psture with Brchiri decumbens, when compred to soil under Svnnh. Moreover, the soil under psture with Tnzni grss ( C nd AC positions) presented higher MGD thn soils under Svnnh nd Tifton 85 (p < 0.05). Overll, MGD rnged from 2.45 mm to 2.93 mm (0-2 cm depth), 2.14 mm to 2.39 mm (2-7 cm depth), nd 1.49 mm to 2.33 mm (30-35 cm depth). MGD presented positive correltion with ggregtes between 8-2 mm (r = 0.946; nd 0.973, for the 0-2 cm, 2-7 cm, nd cm depth lyers, respectively). The gret mount of ggregtes retined in the sieves with lrger meshes (minly 2 mm) cn explin the positive correltion between MGD nd ggregtes vrying from 8 mm to 2 mm. The correltion coefficients for MGD with ggregtes of 8-2 mm, in this reserch, were close to the ones reported by Angulo et l. (1984). The higher MGD vlues found in the soils under pstures with Tnzni grss ( C position) nd Tifton 85, in the 0-2 cm lyer (Figure 3), hve occurred due to the fct tht the Tnzni s root system contins 76% of live roots nd 82% of ded roots, in the first 20 cm depth (Pgotto 2001), while Tifton 85 hs root system formed by rhizomes. During smpling, it ws noticed, in the soil under psture with Tnzni grss ( AC position), tht the ggregtes presented solid spect, indicting tht they might hve been formed by the ction of compression forces nd then cemented by orgnic mtter, thus originting big ggregtes, but with poor

7 280 S. A. Pereir et l. (2010) porosity. According to Crpenedo & Mielniczuk (1990), compression forces cn originte dense ggregtes, whith predominnce of micropores. In the cm lyer, DMG in the soils under pstures with Tnzni grss ( C nd AC positions) ws higher thn in the soils under psture with Tifton 85 nd Svnnh (Figure 3). Tht difference cn be ttributed to the Tnzni s root system, which contins 24% of live roots nd 18% of ded roots, in the cm depth (Pgotto 2001). The percentges of stble ggregtes in wter re shown in Tble 4. There were higher contents of stble ggregtes in wter t 8-2 mm, in the soil under psture with Tnzni grss ( C position), in the 0-2 cm (77.1%), 2-7 cm (68.7%), nd cm (65.5%) lyers, when compred to the soil under Svnnh (p < 0.05). On the other hnd, in the clsses of ggregtes between 2 mm nd 0.25 mm, the soil under Svnnh presented higher contents of stble ggregtes in wter, if compred to the other smpling sites (0-2 cm nd 2-7 cm lyers). CONCLUSIONS 1. The implnttion of pstures with Tifton 85, under continuous grzing system, nd Tnzni grss, under rottionl grzing system, hs chnged the physicl soil properties, in comprison with the soil under Svnnh, especilly in the superficil lyers (0-2 cm nd 2-7 cm depth). 2. The depletion of physicl properties in the soils under those pstures cn be ttributed to the stocking rtes, which were excessive nd determinnt to reduce the volume of mcropores nd increse soil density. REFERENCES ANGULO, R. J.; ROLOFF, G.; SOUZA, M. L. P. Correlção entre diferentes forms de determinção e representção d estbilidde e resistênci de gregdos do solo. Revist Brsileir de Ciênci do Solo, Viços, v. 8, n. 1, p. 7-12, Tble 4. Distribution of ggregtes stble in wter in three depths of Red distroferric Ltossol under two grzing systems nd Svnnh (Goiâni, GO, Brzil, 2004). Clsses of ggregtes (mm) Sites < % Depth: 0-2 cm Svnnh 63.6 b* b 2.1 Tifton b 6.5 b 6.6 bc 4.0 b 1.9 Among clumps 69.8 b 6.8 b 6.4 b 7.4 b Clumps b 5.2 b 4.5 c 3.4 b 2.9 C.V. (%) Depth: 2-7 cm Svnnh 58.6 b b Tifton b 9.7 b 8.6 b 8.9 b b Among clumps 63.3 b 8.1 c 7.8 b 8.5 b Clumps c 7.5 b 7.7 b b C.V. (%) Depth: cm Svnnh 53.5 b 9.1 b b Tifton b Among clumps b 10.2 b 9.7 b 7.0 b 2.1 bc Clumps b 9.2 b 8.2 b 6.1 b 1.5 c C.V. (%) * Mens with different letters in the sme lyer nd ggregtes clss differ by the Duncn test (p < 0.05).

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