Effects of earthworms on microbial community structure, functionality and soil properties in soil cover treatments for mine tailings rehabilitation

While earthworm inoculation is viewed as a promising strategy to accelerate soil formation and ecosystem development in post-mining substrates, limited studies are field-based and focus on the influence of earthworms on microbial communities. This study investigated the effects of earthworm inoculations on soil microbial catabolic profiles, microbial community structure and physical properties. Large (1 m³) macrocosms were filled with 60–80 cm mine tailings and 40 cm of organic-rich engineered soil (SOM 8.5 g/kg) and inoculated treatments of: (1) without earthworms (Tc), (2) with one endogeic species (Allolobophora chlorotica) (T₁), 5.8 g/m², (3) with a mix of anecic species Lumbricus sp., Lumbricus friendi and Lumbricus terrestris (T₂), 10.5 g/m² (4) with two species of two ecological groups, the endogeic A. chlorotica and the epigeic Lumbricus rubellus (T₃), 4.0 g/m². Earthworm survival was not evaluated due to the large container size and logistics. After six months, soil catabolic profile (MicroResp™), community structure (PLFA and NLFA), and soil physicochemical properties were analysed. MicroResp™ showed that multiple substrate-induced respiration (9.2 μg CO₂–C g⁻¹ soil h⁻¹) and microbial biomass (1.5 mg/kg soil) were higher in the treatment with endogeic and epigeic worms. The decomposition rate (k) of 0.1 was also higher than treatments with no earthworms. Water holding capacity, bulk density, aggregate stability and labile carbon showed no significant difference over time and among treatments. Overall, earthworm inoculation positively influenced microbial respiration. These findings showing the role of earthworms on microbial activity and community structure in soil covers have significant implications for management of ecosystem processes and sustainability in post-mining sites. However, the study also highlights the need for extended monitoring periods under natural field conditions to fully comprehend the complex interactions between earthworm ecological groups and microbial functionality.