Local variations in soil compaction in the drilosphere of a wide range of earthworms and links with ecological categories and functional traits

Abstract

Earthworms can compact the soil through the production of casts and by pushing the soil when burrowing. However, how different ecological categories differently affect these processes is poorly known. This study aimed to expand our knowledge on the compaction within the drilosphere and to examine similarities and differences between ecological categories. We sampled 21 earthworm species in Vietnam whose bioturbating behaviours and morpho-anatomical traits had been previously studied. One individual was incubated in soil core for four weeks under laboratory conditions. After incubation, the following drilosphere properties were assessed using X-ray computed tomography: the volume and compaction of casts, the volume of empty burrows, the compaction of burrow walls and the radius of lateral compaction around burrows. The drilosphere volume decreased in the following order: anecic (8.5%), endogeic (2.5%), intermediate (1.5%) and epigeic species (0.5%). Together, casts and lateral compaction occupied more than 70% of the whole drilosphere regardless of the ecological category. Both anecic and endogeic species had relatively similar effects on soil compaction. They compacted surface casts (up to 1.45-fold), belowground casts (up to 1.20-fold), and burrow walls (up to 1.25-fold) and induced the widest lateral compaction around burrows (up to 11 mm). Conversely, epigeic species had markedly fewer effects, increasing compaction from 1.10- to 1.20-fold and producing the narrowest lateral compaction (1 mm). Intermediate species showed large variability, which included species with little and large effects on compaction. Finally, we showed that food ingestion, muscle thickness and body size were the most related to the compaction of the drilosphere. It is concluded that the soil volume compacted by the earthworms is larger than the volume of empty burrows. Additionally, our study encourages further investigations using functional traits to overcome some limitations associated with the traditional ecological category approach.