Evaluation of the impact of freezing technique on pore-structure characteristics of highly decomposed peat using X-ray micro-computed tomography

The modelling of peatland functioning requires detailed knowledge of the peat structure. To this end, freezing is nowadays increasingly used to obtain X-ray micro computed tomography (X-ray-CT) images. The aim of this study was to analyze the structure of a peat material before freezing and post-defreezing using X-ray-CT and to look for possible alterations in the structure by analyzing the air-filled porosity. A highly decomposed peat material close to water saturation was select-ed for study. Three samples were analyzed before freezing and post-defreezing using an X-ray-CT Nanotom 180NF. Results showed that the continuity and cross section of the air-filled tubular pores several hundreds to about one thousand micrometers in diameter were altered post-defreezing. Many much smaller air-filled pores not detected before freezing were also recorded post-defreezing. Detailed analysis showed a dramatic increase in the number of air-filled pores ranging between 1 voxel (216 10(3) mu m(3)) and 50 voxels (10.8 106 mu m(3)) in volume. The volume of these pores newly occupied by air using X-ray-CT and their total volume was found to be consistent with the one calculated as resulting from the increase in the specific volume of water when it turns into ice.