Earthworm and Plant Root Bioturbation Succession in Compacted Soil Revealed by 2D Rhizobox and X-ray CT Imaging

Soil structure plays a vital role in ecosystem functioning. Earthworms and plant roots are key bioturbation agents crucial to building and maintaining soil structure suitable for agriculture. However, following soil compaction, the succession of biophysical activity between these agents remains unclear and understanding this dynamic is critical for sustainable soil management.  This study utilised imaging techniques to assess how compaction affects bioturbation by endogeic earthworms and barley roots and their impact on soil functionality (e.g. hydraulic conductivity, water retention, etc.). To this end, two experimental systems were established: (i) rhizoboxes for 2D imaging, photographed regularly over a six-weeks, and (ii) PVC cylinders for X-ray computed tomography (XCT), scanned at trial end. Each system included compacted and uncompacted treatments, with earthworms and barley co-incubated. Compacted systems were surface loaded at 150kPa. Rhizobox imaging tracked biopore formation and interactions between bioturbation agents, while XCT provided high resolution 3D structural data subsequent to bioturbation. Image analysis involved segmenting biopores using thresholding and filtering techniques, such as median and Gaussian for the 2D images and non-local means for 3D XCT images. These methods enabled us to compare the structural characteristics of the biopore systems (i.e. number of biopores, branches, thickness, branch length, etc.). Both image types were skeletonised and combined with local thickness maps to extract the structural metrics assessed.  Results showed compaction reduced mean trends in earthworm bioturbation activity, while root activity largely stayed the same. The results from the XCT data showed that hydraulic conductivity increased markedly after bioturbation, increasing two orders of magnitude in uncompacted and three orders of magnitude in compacted soil. We concluded that for soil restoration, this suggests a sequential approach, with initial cover crop planting to alleviate compaction stress, enabling earthworms to proliferate and create the structure needed to maintain healthy soil functioning and productivity.