Abstract. The modelling of peatland functioning, in particular the impact of anthropogenic warming and direct human disturbance on CO₂, CH₄ and N₂O, requires detailed knowledge of the peat structure and of both water and gas flow with respect to the groundwater table level. To this end, freezing is nowadays increasingly used to obtain small size peat samples for X-ray micro computed tomography (X-ray μ-CT) as required by the need to increase the resolution of the 3D X-ray CT images of the peat structure recorded. The aim of this study was to analyze the structure of a peat material before and after freezing using X-ray μ-CT and to look for possible alterations in the structure by investigating looking at the air-filled porosity. A highly decomposed peat material close to water saturation was selected for study and collected between 25 and 40 cm depth. Two samples 4 × 4 × 7 cm³ in volume were analyzed before and after freezing using an X-ray μ-CT Nanotom 180NF (GE Phoenix X-ray, Wunstorf, Germany) with a 180 kV nanofocus X-ray tube and a digital detector array (2304 × 1152 pixels Hamamatsu detector). 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 after freezing. Many much smaller air-filled pores not detected before freezing were also recorded after freezing with 470 and 474 pores higher than one voxel in volume (60 × 60 × 60 μm³ in volume each) before freezing, and 4792 and 4371 air-filled pores higher than one voxel in volume after freezing for the two samples studied. Detailed analysis showed that this increase resulted from a difference in the whole range of pore size studied and particularly from a dramatic increase in the number of air-filled pores ranging between 1 voxel (216 10³ μm³) and 50 voxels (10.8 10⁶ μm³) in volume. Theoretical calculation of the consequences of the increase in the specific volume of water by 8.7 % when it turns from liquid to solid because of freezing led to the creation of a pore volume in the organic matrix which remains saturated by water when returning to room temperature and consequently to the desaturation of the largest pores of the organic matrix as well as the finest tubular pores which were water-filled before freezing. These new air-filled pores are those measured after freezing using X-ray μ-CT and their volume is consistent with the one calculated theoretically. They correspond to small air-filled ovoid pores several voxels in volume to several dozen voxels in volume and to discontinuous air-filled fine tubular pores which were both detected after freezing. Finally, the increase in the specific volume of water because of freezing appears also be also responsible for the alteration of the already air-filled tubular pores before freezing as shown by the 3D binary images and the pore volume distribution.

中文翻译：

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在 X 射线显微计算机断层扫描之前使用时，冷冻对接近水饱和度的高度分解泥炭材料微观结构的影响

摘要。泥炭地功能建模，特别是人为变暖和人类直接干扰对 CO ₂、CH ₄和 N _{2 的影响}O，需要详细了解泥炭结构以及相对于地下水位水平的水和气流。为此，根据提高泥炭 3D X 射线 CT 图像分辨率的需要，现在越来越多地使用冷冻来获得用于 X 射线显微计算机断层扫描（X 射线 μ-CT）的小尺寸泥炭样品记录的结构。本研究的目的是使用 X 射线 μ-CT 分析冻结前后泥炭材料的结构，并通过观察充气孔隙率来寻找结构中可能的变化。选择接近含水饱和度的高度分解泥炭材料进行研究，并在 25 至 40 厘米深度之间收集。两个样品 4 × 4 × 7 cm ³使用带有 180 kV 纳米聚焦 X 射线管和数字探测器阵列（2304 × 1152 像素滨松探测器）的 X 射线 μ-CT Nanotom 180NF（GE Phoenix X 射线，德国温斯托夫）在冷冻前后对体积进行分析）。结果表明，在冷冻后，直径数百至约一千微米的充气管状孔的连续性和横截面发生了改变。冷冻后还记录了许多在冷冻前未检测到的小得多的充气孔，其中 470 和 474 孔的体积高于一个体素（60 × 60 × 60 μm ³冷冻前的体积分别为 4792 和 4371 个，冷冻后 4792 和 4371 个充气孔的体积高于所研究的两个样品的一个体素。详细分析表明，这种增加是由于所研究的整个孔径范围的差异，特别是由于在 1 体素 (216 10 ³  μm ³ ) 和 50 体素 (10.8 10 ⁶ 微米³) 量。水的比容增加 8.7 % 的后果的理论计算，当它由于冻结而从液体变成固体时，导致在有机基质中产生孔隙体积，当返回室温时，该孔隙仍然被水饱和，并且因此，有机基质的最大孔以及在冷冻前充满水的最细管状孔的饱和度降低。这些新的充气孔是用 X 射线 μ-CT 冷冻后测量的，它们的体积与理论计算的一致。它们对应于体积为几个体素到几十个体素的小充气卵形孔和不连续的充气细管状孔，它们都是在冷冻后检测到的。最后，