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

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 (mu CT). In this research, 3D mu CT images (60 mu m of resolution) were used for investigating the REV for: soil macroporosity (MAP), number of macropores (NMAP), pore tortuosity (tau) and connectivity (EulerPoincare 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 approximate to 0.40 to approximate to 36,591 mm(3), 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 approximate to 10,841 mm(3) (centered scheme) and approximate to 17,216 mm(3) (random scheme), respectively. For the CT, the REV for MAP occurred at volumes of approximate to 17,216 mm(3) (random and centered schemes). The REV was not reached for NMAP, while the ratio between NMAP and sample subvolumes (here called specific NMAP, NMAP(e)) presented a REV of approximate to 17,216 mm(3). 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 tau it was reached at volumes of approximate to 17,216 mm(3) (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 NMAP(e), tau 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 NMAP(e), tau 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.