The area under investigation is the European part of the Northern Palaearctic, specifically European Russia. It encompasses a diverse range of biome types, including tundra, boreal forests, temperate forests, grasslands, wetlands and coastal areas. Our research primarily focuses on the East European Plain, although some sampling sites extend beyond this region to include the Northern Caucasus Mountains, Novaya Zemlya Archipelago and Franz Josef Land. Therefore, we limit our research area by Franz Josef Land to the north, the Caucasus Mountains to the south and the Ural Mountains to the east. In total, we examined 204 sites located within various biomes as classified by WWF (Olson et al. 2001), including:

1. tundra;
2. boreal forest;
3. temperate broadleaf and mixed forest;
4. temperate grassland, savannah and shrubland;
5. desert and xeric shrubland.

In each of the biomes, we sampled a different number of sites due to logistical constraints and various extraction capacities. Comprehensive information about each site is given in Table 1.

The material for the dataset was collected between 2019 and 2023. We selected sampling sites in areas that were not heavily disturbed by human activity. In arid regions, we chose the most humid (yet not flooded) spots. The sampling protocol was developed, based on widely recognised methods (Ghilarov 1975, Coleman et al. 2004) and in accordance with international standards (ISO 23611-3 2019). At each site, we collected a random selection of 1-8 soil cores. These soil cores were taken using a 5-cm-diameter steel corer down to a depth of 10 cm. After collection, the soil was carefully placed into plastic bags and transported to the laboratory at the A.N. Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences, Moscow. Subsequently, the soil samples were stored in a refrigerator at +4°C until extraction. Enchytraeids were extracted from the soil without heating, using the Graefe's wet funnel technique (Didden et al. 1995). We placed a sieve in each funnel and a soil sample in each sieve. Then tap water was poured into the funnel so that the soil was completely covered. A test tube was attached to each funnel and placed in a container with water at room temperature. This precaution aimed to prevent potential overheating of the extracted enchytraeids, considering the possibility of random and sudden temperature fluctuations in the extraction room. Extraction was carried out for 16 to 24 hours, after which the tubes were detached from the funnels and the contents of the tubes were poured into Petri dishes.

The samples were collected by a number of soil zoologists and ecologists from the A.N. Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences, Moscow, colleagues from other scientific institutes and trained volunteers. In total, 73 different enchytraeid species were collected. Enchytraeid species were identified in vivo immediately after the extraction procedure, according to Schmelz and Collado (2010). For species not included in this guide or described later, comparisons with original descriptions were used.

A total of 747 soil samples were collected from 204 sites, resulting in 61,254 records, of which only 1,419 indicated a "present" status, while the remaining 59,836 were classified as "absent". These "absences" refer specifically to the soil samples collected within our project and should be interpreted with caution when extrapolating the distributions of certain species at larger spatial scales. Nevertheless, we believe that including data records with an "absent" status may be beneficial for future biogeographic and soil ecological research.

It is important to note that the dataset presented does not claim to provide a comprehensive and objective account of enchytraeid diversity in the studied area due to several limitations related to sampling and extraction constraints. Our focus was primarily on the most typical habitats, which may have resulted in the omission of certain species, particularly those associated with aquatic environments. Additionally, the number of soil samples and the depth at which they were collected in each habitat may not have been sufficient to capture the full range of species diversity.

Despite these limitations, the presence data for specific species remain ecologically and biogeographically significant. All samples were collected during the vegetation period (spring to autumn), deliberately avoiding particularly dry summer periods. The population dynamics of many enchytraeid species are still largely unknown; therefore, species presence data are considered more reliable and informative than abundance data, as the latter are specific to a particular timeframe. Finally, while the extraction method employed does not guarantee 100% efficiency, it is comparable in effectiveness to other commonly-used enchytraeid extraction techniques (Kobetičová and Schlaghamerský 2003).

The taxonomy follows the WoRMS database (Martin et al. 2024). Scientific names were checked using the GBIF species matching tool. Since most individuals were used for in vivo identification without permanent preparation, for subsequent isotopic analysis, all instances of enchytraeid occurrences within the studied sites were documented as dwc:basisOfRecord=HumanObservation. Juvenile specimens were identified at the genus level. The identification of all enchytraeids was conducted by Maxim Degtyarev.

1. Selection of study sites, choosing undisturbed areas displaying minimal or no signs of human activity.
2. Site sampling was carried out at a distance of no less than 100 m from the borders of selected natural sites within one of the seven biome types according to the WWF map of biomes (Olson et al. 2001).
3. At each site, soil cores were collected using a steel corer with a diameter of 5 cm, reaching a depth of 10 cm.
4. The transportation of soil samples was conducted in isothermic containers to prevent soil overheating, which could lead to the mortality of organisms present.
5. Enchytraeids were extracted from the soil using the wet funnel method as described by Didden et al. (1995).
6. Following the extraction procedure, enchytraeids were identified in vivo to the species level using an Olympus BX-43 microscope. Subsequently, they were preserved in 96% alcohol for further isotopic analyses.
7. Identified individuals were counted to determine abundance within the 10 cm soil core and then extrapolated from a 5 cm diameter to 1 m². Extrapolation was performed by multiplying the number of individuals by a coefficient of 512 to account for sampling bias induced by the slight ellipsoidality of the corers, which arises from their production technology peculiarities. Given the variance in the number of soil samples across sites, the dataset includes abundance expressed as individuals per square metre.

The research area is located in the East European part of the Northern Palaearctic, with most sampling sites situated in the East European Plain (Fig. 1). Additionally, the research area includes the northern macroslope of the Caucasus Mountains to the south, as well as the Novaya Zemlya Archipelago and Franz Josef Land to the north. This extensive geographic area encompasses diverse habitat types, including tundra, boreal forest, steppe, deciduous forest and alpine habitats. Spanning a large latitudinal gradient, the region transitions from a tundra climate (ET) in the north to a subarctic climate (Dfc), warm-summer humid continental climate (Dfb), hot-summer humid continental climate (Dfa), humid subtropical climate (Cfa) and cold semi-arid climate (Bsk) in the south, according to the Köppen-Geiger climate classification (Rubel et al. 2017). Furthermore, in the mountainous regions of the Caucasus, the climate gradually changes from a hot-summer humid continental climate (Dfa) at lower elevations to a tundra climate (ET) at the peaks.

Figure 1.  

Enchytraeidae sampling locations in the Eastern European part of the Northern Palaearctic. Polygons in different colours illustrate biomes according to Olson et al. (2001). The map was created using QGIS 3.40.1 - Bratislava software (QGIS.org 2024). The map layer was created using Yandex Maps 4x with the WGS 84/Pseudo-Mercator projection. The shapefile with biomes was downloaded from www.ecoregions.appspot.com (Dinerstein et al. 2017).

The geographical references were obtained by recording the coordinates of the sampling sites using a mobile phone and the Organic Maps app (Organic Maps 2024) or using Garmin GPSMAP 66s. The measurement error of the coordinates was approximately 25 m. The WGS84 coordinate system was used for all records.

41.7411 and 81.129 Latitude; 30.676 and 68.8152 Longitude.

Across the 204 sampling sites studied within five biomes in the European Russian part of the Northern Palaearctic, we identified a total of 73 species belonging to 17 genera (Table 2). The most frequently observed species were Enchytraeus buchholzi s.l. (present in 79 out of 204 sites), Cognettia sphagnetorum s.l. (62 sites), Buchholzia appendiculata (48 sites) and Fridericia bulboides (46 sites).

Several species were identified with the "cf." designation, indicating that they were similar to, but not definitively identified as, those species (Table 2). Specimens of Fridericia cf. ulrikae were much smaller compared to the original description (5 mm vs. 13–18 mm in total length; 36–37 vs. 50–55 segments) and had five pairs of preclitellar nephridia instead of four. The shape of the Achaeta cf. danica spermatheca more closely resembles that of the newly-described A. florens, but careful re-investigation is impossible due to the loss of specimens. Fridericia cf. ilvana had colourless blood instead of pink-yellow. In the case of Achaeta cf. diddeni, specimens from Ingushetia were not fully mature, while specimens from Yaroslavl Oblast were significantly larger compared to the original description (6–7 mm vs. 4 mm in total length). Specimens of Marionina cf. magnaglandulosa were subadult (the spermatheca was not yet fully formed) and very small (1.5–2 mm long), possibly representing exceptionally small M. filiformis. For Henlea cf. nasuta, yellowish epidermal gland cells were present, which is unusual for the genus. All specimens of Cernosvitoviella cf. atrata were subadult. In the case of Henlea cf. montana, the quality of the obtained material does not allowed us to make a definitive conclusion.

The highest species richness was recorded in temperate broadleaf and mixed forests (73 species in total, see Table 3). Boreal forests hosted significantly fewer species; we found only 27. The number of species in the tundra and temperate grasslands, savannahs and shrublands varied between 17 and 21, respectively. The lowest species richness was observed in xeric shrublands and deserts (Table 3). The average species richness of enchytraeids varied between 5.4 species per site in temperate broadleaf and mixed forests and 0.3 species per site in deserts and xeric shrublands.

The highest average abundance of enchytraeids, reaching up to 13,000 individuals per square metre, was observed in temperate broadleaf and mixed forests (Table 3). A relatively lower density was found in boreal forests, with approximately 10,000 individuals per square metre, as well as in temperate grasslands, savannahs and shrublands, which had about 6,000 individuals. The lowest abundance was recorded in deserts and xeric shrublands, with only 1,000 individuals per square metre or fewer.

This work is licensed under a Creative Commons Attribution 4.0 International Licence (CC BY 4.0).