Platinum (Pt)-tipped electrodes are frequently employed to measure the soil redox potential (E_H). Thereby, the timely transition from reducing towards oxidising soil conditions is one of the most important biogeochemical changes that can occur in soil. This condition is mainly linked to the air-filled pore volume (ε) and pore geometries. However, even when the Pt electrodes are located in close vicinity to each other, E_H readings behave non-uniformly, presumably due to the millimetre scaled heterogeneity of pore spaces controlling oxygen (O₂) availability and transport. In this study, we examined the ε distribution and pore connectivity in the close vicinity of a Pt electrode during an artificial evaporation experiment using an undisturbed soil sample (Ah-horizon, Calcaric Gleysol). We combined physio-chemical methods with non-destructive X-ray computed microtomography (μCT) and 3D-image analysis. μCT scans were conducted at three-time points, that is, reducing conditions with E_H < −100 mV (CT-1), the transition from reducing towards oxidising conditions with an E_H increase > 5 mV h⁻¹ (CT-2), and oxidising conditions with E_H > 300 mV (CT-3). We observed that the shift from reducing towards oxidising conditions took place at an air-filled porosity (ε_CT) of ~0.03 cm³ cm⁻³, which matches very with gravimetrically calculated data obtained by tensiometry of ε ~0.05 cm³ cm⁻³. Besides the relation of E_H and ε, image analysis revealed that a connected ε_CT (ε_{CT_conn}) of ~0.02 cm³ cm⁻³ is needed to enable enhanced O₂ diffusion from the soil surface towards the Pt surface and facilitate a straightforward E_H response. We conclude that ε_{CT_conn} is a critical parameter to assess aeration processes in temporarily water-saturated soils to characterise a switch in redox conditions.

中文翻译：

---

X 射线显微断层成像揭示了土壤通气和氧化还原电位与孔隙连通性的关系

铂 (Pt) 电极经常用于测量土壤氧化还原电位 ( E _H )。因此，从还原土壤条件向氧化土壤条件的及时过渡是土壤中可能发生的最重要的生物地球化学变化之一。这种情况主要与充气孔隙体积 ( ε ) 和孔隙几何形状有关。然而，即使当 Pt 电极彼此靠近时，E _H读数的表现也不一致，这可能是由于控制氧气 (O ₂ ) 可用性和传输的孔隙空间的毫米级异质性。在这项研究中，我们检查了ε在使用未受干扰的土壤样品（Ah-horizo​​n，Calcaric Gleysol）的人工蒸发实验期间，Pt 电极附近的分布和孔隙连通性。我们将物理化学方法与无损 X 射线计算机显微断层扫描 (μCT) 和 3D 图像分析相结合。μCT 扫描在三个时间点进行，即E _H < -100 mV (CT-1) 的还原条件， E _H增加 > 5 mV h ^-1 (CT-2  ) 从还原到氧化条件的转变)，以及E _H  > 300 mV (CT-3) 的氧化条件。我们观察到从还原条件向氧化条件的转变发生在充满空气的孔隙度_（εCT) 约为 0.03 cm ³  cm ^{-3 ，这与通过}ε ~0.05 cm ³  cm ^-3的张力测量法获得的重量计算数据非常匹配。除了E _H和ε的关系之外，图像分析表明，需要一个约 0.02 cm ³  cm ^-3的连接ε _CT ( ε _{CT_conn} )以增强 O ₂从土壤表面向 Pt 表面的扩散，并促进直接的E _H回应。我们得出结论，ε _{CT_conn}是评估临时水饱和土壤中的通气过程以表征氧化还原条件转换的关键参数。