02389naa a2200229 a 450000100080000000500110000800800410001910000170006024500870007726000090016452017910017365000150196465000130197965000190199265000120201165000220202365000170204570000180206270000200208070000130210077300460211315550562000-02-16       1999    bl ---       0--  u    #d1 aNEUFELDT, H.  aDistribution of water-stable aggregates and aggregating agents in Cerrado Oxisols.  c1999  aThe effects of structural changes on Cerrado Oxisols in Brazil after land-use change are still insufficiently studied. Therefore, topsoil samples of loamy and clayey Cerrado Oxisols under crop pasture and reforestation were fractionated using a wet-sieving procedure to obtain the distribution of water-stable aggregates and compared to samples from natural savanna. The results were correlated to organic and inorganic soil compounds to identify the main aggregating agents and discussed in relation to changes in the pore-size distribution of the soils. Clayey soils showed a significantly higher macro-aggregation than loamy soils. Compared to natural savanna, macro-aggregation was clearly reduced at the crop sites, whereas aggregation of soils under pasture and reforestation was only slightly affected. In both the clayey and the loamy soils, bonding of polysaccharides was the main aggregating agent. In the clayey soils, liming was also very important for disaggregation by weakening the electrostatic forces between positively and negativeley charged soil compounds, whereas in the loamy soils, binding of macro-aggregates by roots was significant. The introduction of crop-pasture rotations was therefore proposed to take advantage of the strong rooting and polysaccharide production of pastures. Management-induced disaggregation strongly affected the pore-size distribution by compacting the soils. Thereby, macro-porosity was reduced and the ampunt of meso-pores was increased, while micro-porosity was unaffected from management and only differed between the two substrates. Considering the low pore space at plant-available matrix potentials typical of Cerrado Oxisols, the increase might be important for annual crops during the frequent dry spells in the rainy season.  aFerralsols  aland use  asoil structure  aCerrado  aEstrutura do Solo  aUso da Terra1 aAYARZA, M. A.1 aRESCK, D. V. S.1 aZECH, W.  tGeoderma, Amsterdamgv.93, p.85-99, 1999.