Gamma ray computed tomography to evaluate wetting/drying soil structure changes

Abstract

Wetting and drying (W–D) cycles can cause strong modifications of the structure of a soil, especially in pore distribution, which reflects the temporal and spatial distribution of soil water and, consequently, these processes can affect soil water and nutrient retention and movement. These alterations have important practical consequences when calculating soil water storages and matric potentials, widely used in irrigation management. The present paper has as objective to use gamma ray computed tomography (GCT) as a tool to investigate possible modifications in soil structure induced by W–D cycles and to analyze how these alterations can affect soil water retention. The GCT scanner used was a first generation system with a fixed source–detector arrangement, with a radioactive gamma ray source of 241Am. Soil samples were taken from profiles of three different soils characterized as Xanthic Ferralsol (Fx), Eutric Nitosol (Ne) and Rhodic Ferralsol (Fr). Eighteen samples (50cm3), six from each soil, were submitted to none (T0), three (T1) and nine (T2) wetting/drying cycles. Based on image analysis it was possible to detect modifications in soil structure in all samples after wetting/drying cycles for all treatments. Tomographic unit profiles of the samples permitted to identify an increase on soil porosity with the increase in the number of wetting/drying cycles and it was also possible to quantify the average porosity values. The statistical test (Duncan test) indicates that there are significant differences between treatments for all samples at the 5% significance level.