Revealing pore connectivity across scales and resolutions with X‐ray CT

Abstract

Connectivity is one of the most important parameters to quantify pore structure and link it to soil functions. One of the great challenges in quantifying connectivity with X‐ray microtomography (X‐ray μCT) is that high resolution, as required for small pores, can only be achieved in small samples in which the connectivity of larger pores can no longer be quantified in a meaningful way. The objective of this study was to investigate the changes in pore connectivity with changing sample size, covering a range of analysed pore diameters of more than three orders of magnitude. With this approach, we wanted to address whether pore types formed by different processes in an agricultural chronosequence leave characteristic traces in certain connectivity metrics. The Euler number, χ, and the connection probability of two random points within the pore system, that is, the Γ‐indicator, were determined as a function of minimum pore diameter. The results show that characteristic signatures of certain pore types overlap with scale artifacts in the connectivity functions. The Γ‐indicator, gives highly biased information in small samples. Therefore, we developed a new method for a joint‐Γ‐curve that merges information from three samples sizes. However, χ does not require such a scale fusion. It can be used to define characteristic size ranges for pore types and is very sensitive to the occurrence of bottle necks. Our findings suggest a joint evaluation of both connectivity metrics to disentangle different pore types with χ and to identify the contribution of different pore types to the overall pore connectivity with Γ. This evaluation on the chronosequence showed that biopores mainly connect pores of diameters between 0.5 and 0.1 mm. This was not coupled with an increase in pore volume. In contrast, tillage led to a shift of pores of diameter >0.05 mm towards pores of diameter >0.20 mm and thus increased connectivity of pores >0.20 mm. This work underlines the importance of accounting for the scale dependence of connectivity measures and provides a methodological approach for doing so. Highlights Scale dependence of connectivity metrics needs to be accounted for. Connectivity metrics can be used to disentangle different pore types across scales. Roots mainly connect the pore system between 0.1 and 0.5 mm. A joint Γ‐connectivity function can be constructed that is free of scale artifacts.

Editorial for the special issue on “Advances in soil scaling: Theories, techniques and applications”

“…Somewhat similar differences in the scale dependence of structural metrics were observed by Lucas, Vetterlein, Vogel, and Schlüter (2020). They used data from X‐ray CT imaging of samples of three distinctly different sizes to observe the scaling behaviour of two pore connectivity metrics: the Euler number and gamma indicator.…”

Root growth and presence of Rhizophagus irregularis distinctly alter substrate hydraulic properties in a model system with Medicago truncatula

“…One important is that AMF cause morphological responses of root systems (Berta et al 1990;Gamalero et al 2004;Wu et al 2010). The growth and activity of root systems will lead to the formation of a highly connected biopore system and will determine the extent of clogging when roots grow into existing pores (Bodner et al 2014;Głąb et al 2013;Lucas et al 2020;Wiersum 1957). When roots grow, they exert mechanical strength and move adjacent soil particles to accommodate their size increment (Bengough and MacKenzie 1994;Vollsnes et al 2010).…”

“…Particle-particle contacts increase in the surrounding soil which are then glued together and stabilized against disintegration by the sticky and repellent properties of the plant-derived organic material (Ahmed et al 2016;Hallett et al 2003;Hallett et al 2009;Materechera et al 1994;Rillig and Mummey 2006;Vollsnes et al 2010). These root-induced processes alter the size, the geometry and the wetting properties of the pore space around roots and, thus, affect soil hydraulic properties (Bodner et al 2014;Kroener et al 2018;Lucas et al 2020;Read et al 2003;Scholl et al 2014). Hence, changes to soil hydraulic properties are expected as soon as AMF alter root system properties.…”

Data Grouping Method for the Purpose of Forecasting the Mechanical Strength of Plastic Soils

“…The soil bulk density determination was performed using the core sampling method (volumetric cylinder method), which is the most common method used to determine bulk density agricultural soils [35]. Researchers use cylinders of different sizes, which may be affected by the result of the soil properties determined [36][37][38]. The Bouyoucos Casagrande method modified by Prószyński was used to measure the particle size distribution of soil, which is one of the methods commonly used, although automated methods are currently gaining increasing popularity [39].…”