The earthworms or Lumbricidae attain the largest biomass in soil among soil invertebrates. They occur in high abundance and diversity in soils worldwide except for deserts and arctic tundra¹. In temperate regions average abundance and biomass is about 100 specimen per squaremeter², and even in intensively managed arable soils still about 50 g fresh earthworm biomass is found per square meter though one order of magnitude less than in pasture and grassland². Earthworms contribute to the functioning of soil ecosystems by consuming decaying plant matter, creating burrows that aerate soil and allow water to infiltrate soil; in its entirety, they contribute considerably to soil formation³. The upper part of soil will have passed through the earthworm gut every tenth year and are therefore entirely coprogenic, i.e. made up of earthworm excrements and casts^4,5. A large body of knowledge and ongoing research is centered on earthworms and this has led to acknowledgement of their crucial role. Thus, most new chemicals with a fate in soil will be tested according to ISO or OECD standards^6,7.

Earthworms respond to land management, agriculture and chemicals in soil, so the species composition and their abundance will reflect any change in soil structure and food availability and quality. Hence, their abundance and diversity is used to calculate soil health indices⁸.

Earthworms can be hand-sorted from soil and their enumeration and identification are highly standardized⁹ and facilitated by concise easily accessible identification keys¹⁰, although some level of expert knowledge is needed for precise identification. Currently, earthworm DNA in soil, i.e. eDNA, has slowly been adopted for assessment of earthworm diversity, thus avoiding the need for taxonomic expertise for earthworm identification¹¹.

Literature:

¹ Petersen, H., Luxton, M., 1982. A comparative analysis of soil fauna populations and their role in decomposition processes. Oikos 39, 287-388.

² Krogh et al. 2021 Krogh, P.H., Lamandé, M., Holmstrup, M., Eriksen, J., 2021. Earthworm burrow number and vertical distribution are affected by the crop sequence of a grass-clover rotation system. Eur. J. Soil Biol. 103.

³ Blouin, M., Hodson, M.E., Delgado, E.A., Baker, G., Brussaard, L., Butt, K.R., Dai, J., Dendooven, L., Pérès, G., Tondoh, J.E., Cluzeau, D., Brun, J.J., 2013. A review of earthworm impact on soil function and ecosystem services. Eur. J. Soil Sci. 64, 161-182.

⁴ Whalen, J.K., Sampedro, L., 2010. Soil ecology and management. CABI.

⁵ Taylor, A.R., Taylor, A.F.S., 2014. Assessing daily egestion rates in earthworms: using fungal spores as a natural soil marker to estimate gut transit time. Biol. Fertility Soils 50, 179-183.

⁶ OECD, 2016. Test No. 222: Earthworm Reproduction Test (Eisenia fetida/Eisenia andrei). DOI: 10.1787/9789264264496-en

⁷ ISO, 2014. International Organization for Standardization. ISO No. 11268-3 Soil quality – Effects of pollutants on earthworms – Part 3: Guidance on the determination of effects in field situations. Geneva (CH).

⁸ Fusaro, S., Gavinelli, F., Lazzarini, F., Paoletti, M.G., 2018. Soil Biological Quality Index based on earthworms (QBS-e). A new way to use earthworms as bioindicators in agroecosystems. Ecol. Indicators 93, 1276-1292.

⁹ ISO, 2018. International Organization for Standardization. ISO 23611-1:2018 ― Soil quality ― Sampling of soil invertebrates. Part 1: Hand-sorting and formalin extraction of earthworms. Geneva (CH).

¹⁰ OPAL (Open Air Laboratories): Citizen science for everyone, 2015. Earthworm Identification Guide: OPAL Soil and Earthworm Survey. URL: imperial.ac.uk/media/imperial-college/research-centres-and-groups/opal/SOIL-4pp-chart.pdf

¹¹ Bienert, F., De Danieli, S., Miquel, C., Coissac, E., Poillot, C., Brun, J.-J., Taberlet, P., 2012. Tracking earthworm communities from soil DNA. Mol. Ecol. 21, 2017–2030.