Introduction of the twin ring method for collection of earthworms – Method study on seven agricultural sites across Germany

Earthworm populations are investigated as bioindicators due to their crucial role in supporting sustainable land management. Effective research methods should (a) accurately assess abundance, (b) yield reproducible results, (c) minimize environmental harm, and (d) require minimal time and resources. This study compared the standard combo method (manual sorting and chemical extraction) with two alternative chemical-only methods. The new twin ring method was introduced, using two concentric rings (39.9 cm and 47.9 cm diameters) instead of a single square frame (square method). The design aims to prevent horizontal migration and improve sampling efficiency in the inner ring. Results showed that the square method significantly underestimated earthworm abundance compared to the combo method (mean reduction: −31.98%, p < 0.001), particularly for endogeic earthworms (−38.53%, p < 0.001). In contrast, the twin ring method produced results similar to the combo method, with a non-significant mean increase of 5.27% ( p = 0.53). Anecic and epigeic groups were unaffected in both cases. The twin ring method meets key criteria for effective sampling: it is accurate, reproducible, and less resource intensive. These findings advance applied soil ecology by enabling low-cost agroecological monitoring and supporting policy frameworks that rely on robust indicators of soil ecosystem integrity. Further research is necessary to gain a deeper understanding of horizontal migration behavior and to enhance comparability between the three methods.