Field and laboratory responses of earthworms to use of wood ash as a forest soil amendment

Abstract

Wood ash is produced by burning wood residues for energy and is often landfilled but can be more sustainably used as a forest soil amendment. We investigated how different fly and bottom wood ashes applied at a moderate 10 Mg∙ha−1 (dry) affected earthworms, a key group of forest soil biota. We conducted a field experiment with different wood ashes and application timings to determine the effects on population densities of different earthworm functional groups three weeks and one year post-amendment. We also used three laboratory microcosm experiments focused on the geographically widespread and ecologically influential anecic earthworm Lumbricus terrestris to test how wood ashes affect acute survivorship as well as habitat avoidance and surface feeding behaviors. In the field, wood ash amendment generally decreased or had no impact on earthworm densities in the first three weeks following amendment (with density decreases up to − 52 m−2), but had neutral or minor positive impacts one year later (up to + 21 m−2). In the laboratory, wood ashes generally had no effects on short-term earthworm survivorship or growth, but earthworms did respond behaviorally to certain wood ashes by habitat avoidance and reduced surface activity, with decreases in the overall frequency of aboveground events (−63%), the individual duration of each event (−63%), and the percentage of events that involved contact with litter (−53%). Across all experiments, fly ash tended to have a stronger effect than bottom ash. Overall, our study suggests that while some wood ashes may have limited short-term impacts on earthworm abundances, they may cause more subtle but ecologically important behavioral changes. Additionally, these earthworm-wood ash interactions are likely influenced by highly scenario-specific circumstances, including wood ash type, application site, and timing of application.