Long-term dynamics of the abundance of earthworms and enchytraeids (Annelida, Clitellata: Lumbricidae, Enchytraeidae) in forests of the Central Urals, Russia

Study Extent

Study sites were located on gentle slopes of ridges in forests with a different stand composition (spruce-fir, pine, birch, and mixed forests) and arable lands. Loam and heavy loam soddy-podzolic soils (Albic Retisols) and podzolized burozems (Leptic Retisols) prevail. A total of seven study sites (=dwc:locationID) were established corresponding to local aggregations of different biotopes. The number of sampling plots within each study site were unequal: R-E30-Sol (spruce-fir forest) included seven sampling plots, R-E20-Pmay (spruce-fir forest) included six plots, R-B20-Pmay (birch forest) and R-S20-Pmay (pine forest) included one sampling plot each, R-E17-Kryl (spruce and birch forest) included four plots, R-Fp17-Kryl (floodplain forest) and R-A16-Kryl (arable land) included three plots each. Study sites R-E30-Sol and R-E20-Pmay were permanent throughout all years of the study. However, the exact position of the sampling plots within these study sites differed between 1990–1991 and 2004–2020 (exact position varied within a range of 300–500 m due to refinement of methodical procedures and positioning inaccuracy). The current position of the sampling plots has been established since 2004. Study sites R-B20-Pmay and R-S20-Pmay are additional and were included in the study only in 1991. Study sites near Krylosovo (R-E17-Kryl, R-Fp17-Kryl, and R-A16-Kryl) have been explored since 2019.

Sampling

Earthworms were collected in summer months of 1990–2020 at sampling plots (10 × 10 m in size) established in seven study sites.The sampling process included hand-sorting of annelids (earthworms and enchytraeids) out of soil monoliths 20 × 20 cm in area and 25–30 cm in depth. During sampling, monoliths were divided into two layers, corresponding to the samples: the "O" horizon (forest litter) and the "A" horizon (organic-mineral). Monoliths were not subdivided into layers and were analyzed as a whole sample (the A horizon) in R-A16-Kryl (arable land). The sampling and hand sorting procedures were the same in all years (with a few exceptions); a total of 284 soil monoliths and 553 samples (upper and lower layers of the monoliths) were collected for over 30 years of study.

Quality Control

A total of more than 3300 individuals of earthworms, 7200 egg cocoons and cocoon exuvia of earthworms, and 6900 individuals of enchytraeids were collected. All specimens were wet-preserved in 70% alcohol and stored (with the partial exception of materials from 1990–1991) in the depository of the Laboratory of Population and Community Ecotoxicology of the Institute of Plant and Animal Ecology, Ural Branch, Russian Academy of Sciences (IPAE UB RAS). Earthworms were identified by Evgenii Vorobeichik and Dina Nesterkova from IPAE UB RAS and Elena Golovanova from the Laboratory of Invertebrate Systematics and Ecology of Omsk State Pedagogical University.

Method steps

1. Fieldwork and processing of soil monoliths. Earthworms were collected in June, July, and August of 1990–2020. Sampling plots 10 × 10 m in size were established in seven study sites. Annelids (earthworms and enchytraeids) were hand-sorted out of soil monoliths 20 × 20 cm in area and 25–30 cm in depth, depending on the occurrence of macroinvertebrates. The time interval for extracting one soil monolith from the sampling plot was approximately 5 minutes. Ten monoliths were collected from each plot randomly, excluding nearby trunk areas with a radius of 0.5–1 m around large trees (more than 30 cm in diameter) and any visible pedoturbations. During sampling, each monolith was divided into two layers, corresponding to the samples: the O horizons (forest litter) and A horizon (organic-mineral). Monoliths were not subdivided into layers and were analyzed as a whole sample (the A horizon) in R-A16-Kryl (arable land). Monoliths were placed in plastic bags (separately for the layers), delivered to the laboratory, and stored before processing at 12°C for no more than five days (as a rule, 1–2 days). The collected earthworms were carefully washed with water, fixed with 10% formalin, and then wet-preserved in 70% ethanol. Enchytraeids and earthworm cocoons were fixed with 70% ethanol. The sampling and hand sorting procedures were the same in all years, except that the number of samples taken in 1990 reached 80 per plot. Thus, a total of 284 soil monoliths and 553 samples (upper and lower layers of the monoliths) were collected over all these years. Unfortunately, the materials collected in 1990 and 1991 were not preserved in full until now. Therefore, in the dataset, unlike all others, these years marked with dwc:basisOfRecord = "HumanObservation."
2. Identifying earthworm species. Adult earthworms were identified to species level using the taxonomic key of Vsevolodova-Perel (1997). Juvenile specimens were identified to species level using external characteristics such as the coloration, the prostomium shape, the pattern of setae, and examination of the internal structure during autopsy (a shape of nephridial bladders, a presence and location of diverticula). Almost all earthworms (3236 of 3305, 98%) were identified to species level. Earthworm cocoons and enchytraeids were identified only to the family level.

General taxonomic coverage is 1 phylum, 1 class, 2 orders, 2 families, 8 genera, 10 species of annelids.

1. Clitellata
   rank: class
2. Crassiclitellata
   rank: order
3. Lumbricidae
   common name: earthworms rank: family
4. Enchytraeida
   rank: order
5. Enchytraeidae
   common name: pot worms rank: family

The Ural Mountains are a north-south-oriented mountain system located between the East European plain and West Siberian plain. The study area is situated in the lowest uplands of the Urals (altitudes are 150–400 m above sea level) and belongs to the southern taiga subzone. Primary coniferous forests (Picea abies, Abies sibirica, and Pinus sylvestris) and secondary deciduous forests (Betula pendula, Betula pubescens, and Populus tremula) prevail. Spruce and fir forests with nemoral flora on loam or heavy loam soils dominate on the western slope of the Urals, and pine forests on sandy loam or light loam soils prevail on the eastern side. The ground vegetation layer is dominated by Oxalis acetosella, Aegopodium podagraria, Gymnocarpium dryopteris, Dryopteris carthusiana, Asarum europaeum, Maianthemum bifolium, Cerastium pauciflorum, and Stellaria holostea. Soil formation occurs on eluvium and eluvium-diluvium of bedrock metamorphic rocks (shales, sandstones, quartzites, and silicified limestones). Soil cover is formed mainly by soddy-podzolic soils (Albic Retisols, Stagnic Retisols, and Leptic Retisols), burozems (Haplic Cambisols), and grey forest soils (Retic Phaeozems). Zoogenically active humus form (Dysmull) prevail (Korkina and Vorobeichik, 2021). The climate is warm-summer humid continental, Dfb according to Köppen-Geiger classification (Peel et al., 2007). The average annual air temperature is +2.0 °С; the average annual precipitation is 550 mm; the warmest month is July (+17.7 °С) and the coldest month is January (–14.2 °С) (mean values for the last 40 years, 1975–2015, according to the data of the nearest meteorological station in Revda). The snowless period is about 215 days (from April to October), the maximum height of the snow cover is about 40–60 cm.