Spatial pattern of soil properties and earthworm communities in oak- based traditional wood pastures of Romania

Solitary trees such as oak ( Quercus spp.) play a central ecological role in wood pastures. They alter the surrounding soil and abiotic conditions, with largely unclear consequences for the soil fauna such as earthworms. We investigated how solitary oak trees alter soil properties and whether those alterations impact earthworm communities. Soils and earthworms were sampled across 30 oak-based traditional wood pastures in Transylvania, Romania. At each of the 30 wood pastures, a southern transect was established from the trunk of the focal tree. Four soil profiles were opened along each transect at distances of 1, 3, 7, and 24 m from the tree trunk, resulting in a total of 120 soil profiles, which were described and sampled at three depths: 0–5 cm, 5–25 cm, and 25–45 cm. At each profile location, earthworms from the top 25 cm were hand-sorted from a 25 × 25 cm soil block, providing paired soil and earthworm data. We analysed the contents of soil organic carbon (SOC) and total nitrogen (TN), soil pH, soil moisture, and bulk density for all three depths. The cover of plant functional groups and litter was visually recorded at all sites. We found the highest SOC content (23.4 ± 7.2 g kg −1 ) and the widest C:N ratio (10.2) at 1 m from the tree trunk compared with those further away. Soil pH, soil moisture content, and bulk density were similar at all distances from the tree trunk. Earthworm abundance was lowest at 1 m (173.3 ± 31.8 ind m −2 ) and highest at 24 m (225.1 ± 30.8 ind m −2 ) distance from the tree trunk, but did not differ significantly with distance from the tree trunk or with any of the analysed soil properties. Earthworm composition, however, varied along the distance gradient, with anecic species occurring closer to the tree trunk and endogeic species dominating the open grassland, though these patterns were only marginal after correction for multiple testing. Our results show that solitary oak trees create distinct chemical fertility islands in wood-pasture topsoils, but this enrichment does not ensure greater earthworm abundance. The findings suggest that organic matter quality and accessibility, rather than quantity alone, may shape the observed functional-group patterns.