X-ray microtomography analysis of representative elementary volume (REV) of soil morphological and geometrical properties

Abstract

The representative elementary volume (REV) is related to the minimum size of a sample that is necessary for representative measurements of a property. The REV is frequently investigated in samples with characteristic sizes extending from millimeters to few centimeters. Most of the time, the obtained information in this kind of study does not reveal any important detail of the inner structure of a porous medium. Something that can only be attained using some high spatial resolution image analysis technique, as, for instance, the X-ray computed microtomography (μCT). In this research, 3D μCT images (60 μm of resolution) were used for investigating the REV for: soil macroporosity (MAP), number of macropores (NMAP), pore tortuosity (τ) and connectivity (Euler-Poincare characteristic, EPC) of a Rhodic Ferralsol under no-tillage (NT) and conventional (CT) tillage systems. Consecutive increasing volumes, centrally and randomly, with volumes ranging from ≈0.40 to ≈36,591 mm³, were selected inside the 3D images. Results showed that MAP and NMAP presented high variability for small investigated subvolumes and for the random selection scheme. The REV for MAP (under NT) was reached at volumes of ≈10,841 mm3 (centered scheme) and ≈17,216 mm3 (random scheme), respectively. For the CT, the REV for MAP occurred at volumes of ≈17,216 mm3 (random and centered schemes). The REV was not reached for NMAP, while the ratio between NMAP and sample subvolumes (here called specific NMAP, NMAPe) presented a REV of ≈17,216 mm3. The tortuosity and EPC were characterized by high variability for smaller subvolumes and random selection scheme. The REV for EPC was not found and for τ it was reached at volumes of ≈17,216 mm3 (both subvolume selection schemes and tillage systems). The results of our work present for the first time the REV definition for soil properties such as NMAPe, τ and EPC at the micrometer scale as well as the effect of tillage systems in the REV. The obtained results surprisingly demonstrated that the REV was not influenced by the tillage systems as well as it was the same for NMAPe, τ and EPC. In addition, the complexity of the soil porous system did not allow the REV determination for EPC, at least for the sample volume here analyzed.