A density-based method to objectively quantify earthworm activity

Earthworms are among the most important soil fauna. To exert effects on soils, they need to be active. Earthworm presence is often used as an indicator for activity, but this is not always reliable as earthworms may survive long periods of inactivity or even estivation. Other direct, reliable, and objective measurements for earthworm activity are lacking. Here, we present a novel earthworm activity measurement, based on body-density difference between actively feeding and inactive earthworms. The underlying principle is that active earthworms have a higher density as their gut is filled with soil particles, while inactive earthworms generally have an empty gut. The method therefore separates inactive earthworms from active ones by flotation. To achieve separation, a 1.08 g cm⁻³ sucrose solution is used. Since it is well established that earthworm activity is reduced in dry soil conditions, we set up a soil moisture gradient experiment to gain a range in earthworm activity. We tested our method on four common European earthworms (endogeic species Allolobophora chlorotica [Savigny] and Aporrectodea caliginosa [Savigny], anecic species Aporrectodea longa [Ude] and epigeic species Lumbricus rubellus [Hoffmeister]) in an experiment with three soil moisture levels (84, 126 and 168 mL kg⁻¹) using a sandy topsoil. As additional proxy for earthworm activity, estivation was visually recorded. We found a high inter-method correlation between density-based and visually estimated earthworm activity, regardless of earthworm species and moisture treatment. Furthermore, our novel method detected inactive individuals of L. rubellus, a species without the ability of estivation. We demonstrated that our density-based method allows for easy and quick quantification of active earthworms. This method offers clear advantages over visual assessments of estivation or cast production, in particular objectivity and applicability to a wider range of species, including those that do not enter estivation.