Earthworm bioturbation reshapes salinity gradients and microbial community assembly in saline soils

Soil salinization severely constrains agricultural productivity and biogeochemical functions. However, the regulatory mechanisms of earthworms on water-salt dynamics and microbial community assembly under salt stress remain poorly understood. We simulated salinity gradients driven by different groundwater depths using soil columns of varying heights (30–150 cm). Two earthworm species with distinct ecological strategies, Metaphire guillelmi and Eisenia foetida , were introduced to investigate their regulation of salt migration and microbial communities. Results indicated that salinity strictly limited earthworm survival and activity. We identified a critical tolerance threshold of approximately 1.0% water-soluble salt for Metaphire guillelmi . Below this threshold, earthworm burrowing activity effectively impedes upward salt movement by disrupting capillary pores. Metaphire guillelmi exhibited superior regulatory capacity compared to Eisenia foetida during this process. This bioturbation-mediated buffering alleviated microbial exposure to extreme osmotic stress and consequently maintained higher bacterial and fungal α-diversity. It also stabilized stress-tolerant and copiotrophic communities dominated by Sphingomonadales, while enhancing the complexity and positive connectivity of cross-domain microbial networks. Moreover, the enrichment of beneficial symbiotic fungi, particularly Glomerales, promoted persistent positive legacy effects within the soil microbiome. Salinity also exerted dominant control over carbon mineralization in earthworm casts. Low salinity significantly enhanced mineralization rates and CO 2 emissions. Conversely, high salinity inhibited microbial metabolism, transforming casts into a transient carbon pool. These findings demonstrate that anecic earthworms can shift from “passive responders” to “active regulators” of salt gradients. By mediating microbial resilience and C cycling, these earthworms serve as key biological drivers for the functional restoration of moderately saline soils.