Individual and combined effects of earthworms and Sphingobacterium sp. on soil organic C, N forms and enzyme activities in non-contaminated and Cd-contaminated soil

Earthworms and Sphingobacterium sp. are known for their strong organic compound decomposition ability and wide distribution in soil. However, interactions of soil organic matter decomposition with soil properties and whether microbial species such as Sphingobacterium sp. could assist earthworms in carbon and nitrogen transformation in soil remain poorly understood. Earthworms (Eisenia fetida, Amynthas gracilis) and Sphingobacterium sp. were introduced in non-contaminated and cadmium-contaminated soils under controlled laboratory conditions for 20 days. We examined their individual or combined effects on carbon and nitrogen forms and related enzyme activities to assess their influence on soil C and N cycling. Individual Sphingobacterium sp. inoculation led to significantly decreased organic carbon (SOC) contents, reducing it by 16.5% in non-contaminated soil and by 3.77%, in Cd-contaminated soil. It resulted in an increased microbial biomass carbon (MBC) contents, reaching 1685 ± 292 mg·kg⁻¹ in non-contaminated soil. Individual introductions of E. fetida and A. gracilis caused a decline in SOC content in non-contaminated soil, but increased significantly dissolved organic carbon (DOC) and alkali-hydrolysable nitrogen (AN) contents by 75.8%, 53.6% and 32.9%, 20.9%, respectively. In contrast, in Cd-contaminated soil, only the significant combined effects of earthworms and Sphingobacterium sp. were linked to significant increase in SOC contents, raising by 7.22% and 9.64% in E. fetida + Sphingobacterium sp. and A. gracilis + Sphingobacterium sp. treatments, respectively. In non-contaminate soil, the combined effects of earthworm and Sphingobacterium sp. further increased DOC and AN content by 212%, 134% and 31.3%, 25.4% in the treatments of E. fetida + Sphingobacterium sp. and A. gracilis + Sphingobacterium sp., respectively; the highest ratios of DOC to SOC and AN to total Nitrogen (TN) were found in the earthworm + Sphingobacterium sp. treatments as well. In non-contaminated soil, Sphingobacterium sp. and earthworms mainly influenced β-glucosidase (BG), urease (URE), N-acetyl-β-d-glucosaminidase (NAG) activities and fluorescein diacetate hydrolysis (FDA) hydrolysis, while in Cd-contaminated soil, they mainly influenced invertase (INV), NAG, URE, and protease (PRO) activities. Principal component analysis indicated that in non-contaminated soil, the earthworm activities dominated the mineralization processes of soil carbon and nitrogen, and Sphingobacterium sp. can intensify this process when it was inoculated in soil along with earthworms. Furthermore, both earthworm species increased C and N levels by elevated INV and PRO activities in combined inoculation. However, in contaminated soil, the impact of earthworm inoculation on soil C stabilization showed a species dependent pattern. E. fetida reduced C mineralization by decreasing URE activities, while A. gracilis enhanced C stabilization by increasing INV activities and decreasing PRO activities. In conclusion, earthworms played a key role in enhancing C and N mineralization in non-contaminated soil and promoting C stabilization in contaminated soil. Both earthworm species followed similar strategies in the former process but adopted different strategies in the latter. When introduced individually, Sphingobacterium sp. was able to promote mineralization in both soils, primarily assisting earthworms in improving carbon and nitrogen mineralization in non-contaminated soil but hindering these processes in Cd-contaminated soil. These findings provide insights into the combined effects of earthworms and microorganisms on carbon and nitrogen cycling.