Subsoil compaction is a serious threat to the fertility of our urban and agricultural lands. In severe cases it impedes aeration of and water infiltration into the subsoil, increasing risks of water-logging and overland run-off with erosion. It also may prevent roots from growing to larger depths, being unable to exploit subsoil water resources in the case of droughts. In this study we investigated soil structural properties in the aftermath of a heavy compaction event caused by storage of excavated soil in an approximately 10 m tall heap over several years. In addition, we evaluated mechanical loosening with a digger to ameliorate the subsoil physical properties. We contrasted soil physical properties of undisturbed soil samples (100 cm3 volume, sampled approximately 9 months after the subsoil loosening) as well as X-ray image-data of compacted, mechanically loosened and pristine subsoil down to approximately 80 cm below the soil surface. We found that the soil loosening improved penetration resistances, porosities and soil aeration properties, especially in the deepest investigated soil layer at 60 cm depth. At this depth, the loosened soil had similar or even better properties than the pristine soil. The compacted soil was almost completely devoid of X-ray imaged macropores. In contrast, the loosened soil featured similar imaged porosities as the pristine soil, but was lacking biopore networks, which resulted in a less well-connected imaged pore system and decreased soil aeration under wet conditions. Note that classical pore-network connectivity measures like the Gamma connectivity or the Euler number turned out to be unsuited as indicators of severely degraded pore networks like in our study. Instead, we encourage the use of approaches from percolation theory to quantify loss of macropore connectivity in compacted soil, for example the critical pore diameter or the number of samples with an imaged connection from top to bottom surface per treatment. Our results demonstrate the beneficial effects of mechanical soil loosening of severely compacted subsoil to soil macropore-networks and associated soil functions. However, our results also confirm that natural and mechanically-loosened soil structures are clearly dissimilar. Time is required until the loosened subsoil has consolidated and re-developed a biopore network, albeit the loosening is expected to shorten the time for subsoil recovery considerably. Future studies are necessary to evaluate the persistence of effects of comprehensive soil loosening of heavily compacted subsoil.
