Quantifying soil macropore morphology and biopore distribution at different subsoil horizons in European agricultural soils using X-ray CT

The volume and connectivity of the soil macropore network play a key role in soil functioning. Cylindrical pores formed by biotic processes, known as biopores, are essential components of the network because they are typically continuous over longer distances and connect different soil regions. They are therefore fundamental to near-saturated hydraulic and gas-exchange properties, to delineating habitats for soil fauna, and to low-resistance pathways for root growth to deeper soil layers. So far, detailed studies quantifying soil macropore network morphologies have predominantly focused on topsoils. Little is yet known about macropore networks in the subsoil with respect to soil depth, land use, and soil management, especially at depths greater than 0.5 m. This study addresses this knowledge gap by examining the average and variability of macropore network morphologies and the distribution of biopores across different soil horizons down to 1.5-2 m at three agricultural sites located in Belgium, Germany, and Switzerland. Each site included three sampling pits, two in croplands under two contrasting management systems (e.g., conventional tillage vs. reduced tillage) and one in adjacent grassland. Eight undisturbed 250 cm³ aluminium soil cores were sampled from every soil horizon identified in the respective sampling pits, as well as from the transition area between the A and B horizons, resulting in a total of 434 samples [3 sites × 3 pits × (5 – 7) horizons × 8 replicates]. X-ray computed tomography was performed at a voxel resolution of 90 µm. All imaged air-filled macropores were segmented, and cylindrical pores were extracted as biopores. The effects of soil depth, land use, and cropland management on the imaged pore and biopore network morphologies and on the variability of pore structure across soil horizons will be investigated using linear mixed-effect models. We hypothesize that i) the variability of the soil macropore network morphology will decrease with depth; ii) the diameter and volume of biopores will decrease with depth; and iii) the effects of land use and management will be limited to the uppermost B subhorizon. The results from this study provide insight into how land use, agricultural management, and soil depth influence soil macropore structures, which is crucial for understanding and predicting subsoil health, specifically soil functioning related to air, water, and solute transport properties, and soil habitat quality for roots and fauna.