Response of earthworm communities to soil disturbance: Fractal dimension of soil and species’ rank-abundance curves

Soil structure degradation and its relationship with soil fauna communities is a crucial issue in soil management. The aim of this work is to analyze soil fractal dimensions and the earthworm community structure along a perturbation gradient in a Typic Argiudoll soil with different combinations of annual crops and pastures. Samples were taken in four sites: a natural grassland (NAT) without cultivation for 30 years, and three pastures of 1–3 years (P1, P2, P3). Five undisturbed soil blocks were taken per site in order to evaluate soil structural complexity. Two fractal dimensions were measured on images of polished sections of the blocks: the mass fractal dimension of pores (Dmp), and the mass fractal dimension of soil solid complement (Dms). Biomass, number and species composition of earthworm communities were assessed by hand-sorting 15 samples of 25 cm × 25 cm × 20 cm on each site. Species rank vs. abundance distribution were fitted to geometric series and broken stick models. Shannon index and expected species number by rarefaction (E(S)) were calculated. Both soil solid and pore space evaluated may be considered as fractals in all sites analyzed. We found that the fractal dimension of pores was significantly (p < 0.03) higher for NAT and P3 (1.63 ± 0.03 and 1.62 ± 0.02, respectively) than for P1 (1.43 ± 0.05). Species richness was P1: 3, P2: 7, P3: 7, NAT: 9. Species abundance vs. rank curves shows better fit to the geometric series model in all sites. We found the highest values of Dmp in sites with the higher diversity and species richness of earthworms, suggesting a trend between the observed biological activity and the fractal dimension of the habitat.