Regulation pathways of biochar and endophytic bacteria in Na+ translocation and salt stress alleviation in the soil-plant system

Biochar and endophytic bacteria have synergistic effects in improving saline soil properties and plant growth. However, the regulatory pathways underlying this effect, particularly concerning Na+ translocation and salt stress responses, remain unclear and unquantified. This study investigated the influence of biochar prepared from Enteromorpha prolifera and the endophytic bacteria (Sphingomonas prati) on Na+ transfer process, growth and physiological responses of Suaeda salsa through pot experiments and multivariate analysis. Results showed that the combination of biochar and Sphingomonas prati resulted in a significant reduction of exchangeable Na+ content in both soil (by 24.9%) and plants (by 49.8%). Moreover, this combined application led to superior plant growth and biomass accumulation, with enhancements in height, root length, and fresh weight of Suaeda salsa by 57.3%, 65.9%, and 149.4%, respectively. Sphingomonas prati exhibited notable efficacy in enhancing osmoregulatory substance content and antioxidant enzyme activities within the plant. Specifically, soluble sugar content, peroxidase, and catalase activities were observed to be 51.8%, 46.6%, and 34.4% higher, respectively, with Sphingomonas prati inoculation compared to biochar treatment alone. Furthermore, the regulatory pathways of biochar and Sphingomonas prati were quantified through partial least squares path modeling. The detoxification of Na+ in soil was primarily influenced by biochar through mechanisms such as electrostatic attraction and cation retention, which inhibited the transportation of Na+ from saline soil to Suaeda salsa. Conversely, Sphingomonas prati triggered protective functions related to plant osmoregulation and antioxidant mechanisms, thereby detoxifying reactive oxygen species generated by Na+. These findings underscore the importance of distinguishing between the roles of biochar and endophytic bacteria in regulating Na+ translocation, guiding their appropriate application in remediating saline soil.