Earthworm burrowing activity of two non-Lumbricidae earthworm species incubated in soils with contrasting organic carbon content (Vertisol vs. Ultisol)

The aim of this study was to investigate the burrowing activity of two earthworm species: the endogeic Drawida sinica and one undescribed Amynthas species incubated in Vertisol and Ultisol presenting different soil organic C content. Because of their contrasting feeding behaviours, we hypothesised that soil type would have a bigger influence on the burrowing activity of the endogeic than the anecic species. Repacked soil columns inoculated with earthworms for 30 days were scanned using X-ray tomography and the compiled images used to characterise the burrow systems. After scanning, the saturated hydraulic conductivity (K _sat) was also measured. The Amynthas species burrows were less numerous (30 vs. 180), more vertically oriented (57 vs. 37°), more connected from the surface to the bottom of the columns (73 vs. 5 cm³) and had a higher global connectivity index (83 vs. 28%) than those of D. sinica. The K _sat was threefold faster in columns incubated with Amynthas and was linked to the volume of percolating burrows (R ² = 0.81). The soil type did not influence Amynthas burrow characteristics. In contrast, there were 30% more D. sinica burrows in the Vertisol than in the Ultisol while other burrow characteristics were not affected. This result suggests that these burrows were more refilled with casts leading to shorter and discontinuous burrows. The K _sat was negatively related to the number of burrows (R ² = 0.44) but was not statistically different between the Vertisol and the Ultisol, suggesting a constant impact of this species on the K _sat. We found that a decrease in the amount of soil organic C by 50% had only a small influence on earthworm burrowing activity and no effect on the K _sat.