Soil Calcium Gradients Drive Divergent Responses in Bacterial and Fungal Communities in Brassica Rhizosphere

Calcium (Ca) is one of the most important elements determining vegetable yield, but the driving factors that regulate microbial community structure, microbial network system stability, and metabolic pathways along the soil Ca gradient remain unclear. In this work, the relationship between soil physicochemical properties and bacterial and fungal communities was investigated under distinct Ca gradients in well-established Chinese cabbage fields located in Shijiazhuang, Hebei Province, China, with sites named Group 1 (G1), Group 2 (G2), and Group 3 (G3) from lowest to highest along the soil Ca gradient. This study demonstrated that Ca exerts dual effects by modulating pH, electrical conductivity (EC), and soil organic carbon (SOC) dynamics, enhancing bacterial diversity while reinforcing fungal network stability through distinct metabolic adaptations. Bacterial networks showed reduced stability despite increased diversity, perhaps linked to the downregulation of ATP-binding cassette (ABC) transporters. Notably, Fe-Mn oxides counteracted Ca influences through selective nutrient adsorption, creating antagonistic selection pressures. Under calcium stress, both Ca and total P (TP) emerge as key drivers of microbial community restructuring, with fungal networks exhibiting significantly greater stability compared to their bacterial counterparts. This study bridges the knowledge gap in the driving mechanisms of microbial communities under soil Ca stress and provides a theoretical basis for improving vegetable yields, with implications for soil management in Ca-rich ecosystems.