Soil texture effects on multifractal behaviour of nitrogen adsorption and desorption isotherms

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Nitrogen adsorption (NAI) and desorption (NDI) isotherms have been reasonably well described by multifractal analysis. This study aimed to assess effects of soil texture on the scaling heterogeneity of NAIs and NDIs. Contrasting medium textured and clayey soils, developed over parent materials with felsic or mafic compositions respectively, were sampled. These two soil groups also showed significant differences in specific surface area (SSA) and cation exchange capacity (CEC), but not in organic matter content (OMC). The scaling properties of all NAIs and NDIs studied exhibited a well-defined multifractal structure, which was assessed by generalised dimension, D-q, and singularity spectra, f (alpha), versus alpha functions. Width of Dq given by (D-5-D-5) and therefore scaling heterogeneity was significantly higher for NAIs than for NDIs. Also, the former was less evenly distributed than the latter. There was significant interaction between isotherm branch (NAI versus NDI) and texture (medium to heavy) for several indices obtained from D-q, namely D-_5, D-1 D-2, D-5 and (D-5-D-5), so that the values of these indices were greater for clayey soils during adsorption and for medium textured soils during desorption. Therefore, NAIs of clayey soils exhibited higher scaling heterogeneity and were more evenly distributed than those of medium textured soils and the reverse was true for their respective NDIs counterparts. Differences in multifractality of NAIs and NDIs were consistent with a wider hysteresis loop of the medium textured soils compared to that of the clayey soils; this is because features of this loop influenced the proportion of high and low values in the probability distribution of the isotherms. Multifractal analysis of N-2 isotherms was useful to characterise soil types with contrasting textures, accounting for peculiar agronomical and environmental characteristics; therefore, it may help in assessing links between soil quality and inherent soil properties.