Distribution of water-stable aggregates and aggregating agents in Cerrado Oxisols

The effects of structural changes on Cerrado Oxisols in Brazil after land-u se change are still insufficiently studied. Therefore, topsoil samples of l oamy and clayey Cerrado Oxisols under crop, pasture and reforestation were fractionated using a wet-sieving procedure to obtain the distribution of wa ter-stable aggregates and compared to samples from natural savanna. The res ults were correlated to organic and inorganic soil compounds to identify th e main aggregating agents and discussed in relation to changes in the pore- size distribution of the sails. Clayey soils showed a significantly higher macro-aggregation than loamy soi ls. Compared to natural savanna, macro-aggregation was clearly reduced at t he 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 positivel y and negatively charged soil compounds, whereas in the loamy soils, bindin g of macro-aggregates by roots was significant. The introduction of crop-pa sture rotations was therefore proposed to take advantage of the strong root ing and polysaccharide production of pastures. Management-induced disaggregation strongly affected the pore-size distribut ion by compacting the soils. Thereby, macro-porosity was reduced and the am ount of meso-pores was increased, while micro-porosity was unaffected from management and only differed between the two substrates. Considering the lo w pore space at plant-available matrix potentials typical of Cerrado Oxisol s, the increase might be important for annual crops daring the frequent dry spells in the rainy season. (C) 1999 Elsevier Science B.V. All rights rese rved.