Multifractal description of nitrogen adsorption isotherms

The specific surface area (SSA) of a soil is commonly estimated by the Brunauer-Emmett -Teller (BET) equation, which implies linearity between applied pressure and volume of adsorbate for a restricted scale range. Dinitrogen adsorption isotherms provide useful data for BET analysis, but they also contain additional information that may render useful a multifractal analysis. The objectives of this work were: (i) to find out if differences in soil use intensity cause changes in SSA assessed by the BET method, and (ii) to evaluate the usefulness of multifractal analysis for characterizing N2 adsorption isotherms also used for SSA determination. The study soils were a Vertisol and a Mollisol from Entre Ríos, Argentina. Treatments included: (i) native land never previously cultivated; (ii) permanent pasture; (iii) crop-pasture rotation; and (iv) continuous cropping. Vertisols had significantly greater SSA than Mollisols. Continuous cropping resulted in significant losses of organic matter (OM) content and aggregate stability decay in both soil types. Losses of OM by land use intensification significantly increased SSA in the Mollisol, but this trend was reversed in the Vertisol. All N2 adsorption isotherms exhibited multifractal behavior. Singularity spectra showed strongly asymmetric concave parabolic shapes with a left-hand side much wider than the right-hand one. Entropy dimension, D1, values were in the range 0.357 to 0.558 for the Vertisol and 0.401 to 0.658 for the Mollisol, which indicates that most of the measure concentrates in a small size domain. Several multifractal parameters were significantly different between soil types and soil use intensities.