Interaction between anecic and polyhumic endogeic earthworms can lead to synergistic effects on soil functioning

Using a mesocosm experiment, we investigated the individual and interaction effects of two earthworm species with contrasting behaviour on soil structure and water transfers. The anecic species Amynthas zenkevichi (Thai, 1982) and the polyhumic endogeic species Pontoscolex corethrurus (Müller, 1857) were incubated in repacked soil columns alone or together for three months under laboratory conditions. The volume of belowground casts, empty burrows and lateral soil compaction were assessed using X-ray computed tomography. The production of surface casts and the amount of food ingested were also recorded. The soil moisture at 7 cm depth and water evaporation of the whole column were monitored regularly. Soil water infiltration was assessed using the Beerkan method at the end of the experiment. A. zenkevichi burrows were less numerous (25 vs. 85), more continuous (41 vs. 0 cm³), more connected from the surface to the bottom of the columns (17 vs. 0 cm³) and more compacted laterally (243 vs. 92 cm³) than those of P. corethrurus. Conversely, P. corethrurus burrows were more abundant in the top 5 cm of the columns and more backfilled by casts than those of A. zenkevichi (36 vs. 5 %). Both species ingested buffalo dung provided at the soil surface and produced surface casts at similar rates. Interactions resulted in an increase in surface activity of more than 40 % and a decrease in the depth and continuity of burrow systems. The water infiltration rate was increased by 3.5 times (compared to the control soil without earthworms) by A. zenkevichi burrows and was not modified by interactions. P. corethrurus increased the cumulative water evaporation by 10 % and decreased soil moisture by 3 % (compared to the control soil without earthworms), whereas A. zenkevichi had marginal effects on these parameters. Globally, interactions led to a slight positive synergistic effect on soil resistance to water loss by evaporation, which was likely related to the increase in surface casting activity. To conclude, this study stresses the importance of considering interactions between earthworms in soil and the need to confirm our findings under natural conditions.