Propriedades físicas de um cambissolo submetido à calagem usando espectroscopia de fluorescência e microtomografia de raios X

Abstract

In the present study, effects of surface liming on: chemical attributes, soil-radiation interaction properties and the structure (porous system) of a Dystrudept soil were investigated. The soil structure was analyzed in micrometric scale, considering monoliths (8×8×8 cm) and tiny aggregates (2-4 and 1-2 mm in diameter). The trial consisted of five stripes, one of them under pasture, considered here as reference (REF), representing the soil conditions before the no-till system (NTS) implementation, and the remaining, under NTS, received the following lime rates: 0 (C0), 10 (C10), 15 (C15) and 20 (C20) t ha-1 on the soil surface. Samples were collected thirty months after the liming procedure, at the 0-10 cm (A) and 10-20 cm (B) soil layers. The soil chemical attributes were characterized according to standard experimental procedures and the soil oxide composition, obtained by X-ray fluorescence spectroscopy (XRF), was used for the calculation of the soil mass attenuation coefficient (μm), by means of the computer code XCOM, for the energies of ≈ 60 keV (241Am) amd ≈ 662 keV (137Cs). The μm values were used to calculate the remaining soil-radiation interaction properties and to simulate variations caused on soil density (D) and total porosity (Pt). For the soil structure analysis, 3D images of X-ray computed microtomography (μCT), with spatial resolution of 60 μm (monoliths), and 3D images of synchrotron radiation based X-ray computed microtomography (SR-μCT), with spatial resolution of 1,64 μm (aggregates) were used. The image visualization, processing and analysis were performed in the Avizo Fire software. The following micromorphological and geometrical properties were determined: image-based porosity (P’), number of pores (NP), distributions of P’ and NP as function of volume intervals, length, elongation, shape, connectivity, and tortuosity of pores (monoliths); and: P’, distributions of P’ and NP as function of volume intervals, connectivity, tortuosity, and fractal dimension (aggregates). Additionally, the elemental composition of the soil aggregates was evaluated by XRF. In the majority of cases, liming effects were concentrated at layer A, where there was an improvement of soil chemical attributes and, therefore, of soil acidity level, as well as there was an increase in the soil-radiation interaction properties as function of lime rates. The increase in soil-radiation interaction properties was more accentuated for the energy of ≈ 60 keV in relation to ≈ 662 keV and, in the former case, the μm variation promoted considerable variation in D and Pt, demonstrating the relevance of the study. Highlighted instances of liming effects on the porous system of the soil monoliths, at layer A, were an increase in P’ and in the number of pores into which the main soil pore was separated, which were identified as being longer and more connected due to liming. Moreover, liming promoted changes in the arrangement of the separated pores, at both soil layers, with the formation of cylindrical pores in the horizontal orientation, which can be attributed to stimulation of the soil fauna activity. Regarding the soil aggregates, at layer A, calcium was raised to a higher percentage in aggregates of 1-2 mm when compared to those of 2-4 mm, as function of the lime rates. Liming affected negatively the structure of the soil tiny aggregates, decreasing P’ and increasing the tortuosity of pores for 1-2 mm aggregates. Besides, liming decreased the fractal dimension in aggregates from both size classes, in accord to the fact that, with liming, larger pores were replaced by smaller ones in 1-2 mm aggregates, as both quantitatively and qualitatively analyzed.