Effects of Soil Substrates and Microbial Inoculants on Earthworm-Mediated Modification of Soil Structure and Physicochemical Properties

Earthworms play a crucial role in enhancing soil structure and nutrient cycling, especially in degraded lands. However, the interactive effects of soil amendments and microbial inoculants on their ecological functions remain underexplored. This study employed a full-factorial design to evaluate how different soil substrates (control soil, coal gangue-incorporated soil, organic fertilizer-incorporated soil, organic fertilizer surface-applied soil) and microbial inoculants (Bacillus megaterium and compound Bacillus) influence earthworm biomass and their engineering functions in soil. Our results demonstrated that the mean change in earthworm biomass in the control soil was −1.92 g, while the combined application of surface-applied organic fertilizer and compound Bacillus promoted earthworm growth, increasing the biomass change to 1.50 g, which was significantly higher than that in the control soil. Coal gangue addition did not adversely affect earthworms compared with the control soil. Furthermore, earthworm burrowing activity effectively improved soil structure and facilitated nutrient mixing. Notably, microbial inoculants affected soil electrical conductivity and large-aggregate content but had no significant effect on soil nutrients, while their interaction with soil amendments significantly affected only soil electrical conductivity. This study reveals that the co-application of organic fertilizer and earthworms can effectively improve soil nutrients and structure and that the regulatory effect of microbial inoculants on soil structure is substrate-dependent. These findings provide valuable insights for sustainable land management.