Relationships between earthworm community, bioturbation and soil detachment: A one-year outdoor experiment

Abstract

We conducted a one-year outdoor experiment to evaluate how the functional diversity of earthworms influences soil erosion. Three species with contrasting bioturbating behaviors were selected: the polyhumic endogeic Pontoscolex corethrurus (feeding and casting on the soil surface and constructing shallow burrows), the endogeic Amynthas alluxus (geophagous and permanently living in the subsoil), and the anecic Amynthas zenkevichi (feeding and casting on the soil surface and constructing deep burrows). A total of 27 mesocosms (1 m³ each) planted with grass were inoculated with none to all three species at two biomass levels (30 and 60 g m⁻²). Soil detachment, water runoff, drainage, soil water potential, and grass biomass were monitored throughout the whole experiment, while burrow volume, water infiltration rate, surface casts, and earthworm communities were measured at the end. A. zenkevichi was the only surviving species and colonized all the mesocosms. Consequently, the impact of each species and their interactions on soil detachment could not be assessed. Nevertheless, mesocosms initially inoculated with earthworms showed, on average, a 7.7-fold decrease in soil detachment compared to the control mesocosm, where no earthworms were introduced at the beginning but was later colonized by dispersed earthworms. Structural equation modeling explained 77 % of the variance in soil detachment. It revealed that both surface casts and burrows (measured by X-ray tomography) indirectly reduced soil detachment by increasing water infiltration and reducing water runoff. However, surface casts also promoted water runoff, partially counteracting these benefits. This study highlights the challenges of managing earthworm communities in long-term mesocosm experiments under natural conditions. Despite these limitations, our findings emphasize the crucial role of anecic earthworms in reducing soil detachment.