02966nam a2200193 a 450000100080000000500110000800800410001910000160006024500950007626000160017130000120018750000570019952024250025665000120268165000160269365000330270965000210274265000090276315601822008-11-14       2008    bl uuuu  m   00u1 u    #d1 aREIN, T. A.  aSurface chemical properties and nitrate adsorption of oxisols from the Brazilian savannas.  a2008.c2008  a509 p.   aTese (Doutorado) - Cornell University, Cornell, USA.  aOxisols virtually devoid of base cations and extractable anions predominate in the "cerrados" (savannas) of central Brazil. Chemical properties were studied for twenty pristine profiles sampled at three depths. Soil organic carbon (SOC) increases linearly with clay or [clay+silt] contents, and predictably decreases with depth. While cation exchange capacity (CEC) at pH 7.0 is mostly associated with SOC, effective CEC measured by summation of exchangeable cations, largely accounted for by exchangeable aluminum, increases with SOC and with the proportion of kaolinite in the clay fraction. The opposite occurs with the pH measured in water and salt solutions, increasing with the proportion of oxide mineraIs, and with [clay/SOC] ratio associated with depth. CEC and AEC (anion exchange capacity) measured at native soil pH as adsorbed K(+) and CL(-) at 0.01 moI L(-1) KCL revealed significant positive surface charge development in the subsoil, reasonably explained by amounts and proportions of variable-charge constituents. Nevertheless, these subsoils do not develop any significant net positive surface charge (AEC&gtCEC), which makes measured positive (^)pH (pH in l moI L(-I) KCI &gt pH in water) a misleading index of the net surface charge. The salt adsorption phenomenon, in which indifferent anion and cation electrolytes are equivalently adsorbed, is determined by AEC, as assessed by the drop in electrical conductivity of 0.01 moI L(-1) CaCl(2) in 1 :2.5 (w/v) suspensions, effectively integrating the soil factors accounting for the ionic strength-induced surface charging. Nitrate adsorption is closely associated with AEC, and could be predicted by isotherm models with parameters estimated from AEC, [clay+silt], and SOC, or exclusively from AEC and CEC, with [clay+silt]/SOC or [CEC/AEC] ratios explaining the isotherm convexity. Simulations with mechanistic models showed that nitrate transport is significantly retarded in these subsoils as a consequence of adsorption, and so is the root nitrate uptake at low root densities, but not at higher densities. The decrease of nitrate adsorption in limed or gypsum-amended soils can be predicted from different variables. Surface charge development and associated properties of these soils could be qualitatively explained from the expected interactions of mineraIs and humics, and in light of the electrostatic theory of surface complexation models.  aCerrado  aNitrogênio  aPropriedade Físico-Química  aQuímica do Solo  aSolo