The Role of Coarse Woody Debris in the Survival of Soil Macrofauna in Metal-Contaminated Areas in the Middle Urals

The soil macrofauna of three microsites in both background and contaminated areas was compared: within decaying trunks of deciduous trees (linden and aspen) in the final stages of decomposition, beneath the trunks, and outside the influence of the trunks (standard soil samples). The macrofauna composition was analyzed at two levels: supraspecific taxa and species, focusing on several taxocenes (earthworms, centipedes, arachnids, ground beetles, click beetles, and mollusks). The study was conducted in the spruce-fir forests of the southern taiga, an area affected by emissions from the Middle Ural Copper Smelter. At the level of supraspecific taxa, the composition of the macrofauna differs little between decaying trunks and standard soil samples. At the species level, the difference between microsites vary depending on taxocene: the species composition within decaying trunks either closely resembles that of standard samples (mollusks), is more distinct (click beetles), is more diverse (centipedes, arachnids, and ground beetles), or is reduced due to the loss of a specific ecological group (earthworms). The ordination of microsites by the generalized list of species for the studied taxocenes aligns with the ordination based on macrofauna composition at the supraspecific taxa level. The total density and abundance of most soil macrofauna groups are higher in the decaying trunks than in the standard samples. This difference in the background area is particularly pronounced (2–6 times) for earthworms, harvestmen, lithobiids, herbivorous Heteroptera, ground beetles, and chironomid larvae. In the contaminated area, the difference is even more striking: 70 times higher for earthworms, 30 times for mollusks, 10 times for Heteroptera, 7 times for lepidopteran larvae, 5 times for spiders, and 4 times for diplopods. The preferential habitation of soil macrofauna in decaying tree trunks within the contaminated area may be attributed to the significantly lower content of potentially toxic metals in decomposing wood compared to forest litter: the difference is 85 times for Pb, 77 times for Fe, 25 times for Cu, 2.6 times for Cd, and 1.7 times for Zn. Thus, the negative impact of pollution on soil macrofauna is less pronounced in decaying trunks compared to standard soil samples.