Genotype-dependent phosphorus acquisition responses of two soybean genotypes under combined low phosphorus and salt stress: evidence from root traits, exudates and rhizosphere microbiome.

Soil salinization and phosphorus (P) deficiency often co-occur in coastal saline-alkali soils, severely limiting crop productivity. Although root exudates are known to mediate rhizosphere microbiome assembly, the coordinated associations among root exudates, rhizosphere microbes, and P acquisition under combined salinity stress and phosphate deficiency remain unclear. In this study, we used two salt-tolerant soybean genotypes differing in P efficiency (A74, P-efficient; A6, P-inefficient) as materials. In a controlled pot experiment, four treatments were established: control (CK), low P (LP), salt (S), and combined stress (LPS). By integrating root phenotypes, P acquisition traits, root exudate metabolomics, and rhizosphere metagenomics, the genotype-dependent rhizosphere responses were investigated. Results showed that, compared to A6, A74 maintained a more extensive root-soil interface and superior P nutritional status under LP and LPS, exhibiting more robust P acquisition responses. Metabolomics and metagenomics identified 16 candidate metabolites and 12 candidate microbial taxa, respectively. Effect-size and correlation analyses prioritized naringenin and Sinorhizobium as candidate components associated with genotype-dependent P acquisition responses. KO annotation identified Sinorhizobium-associated genes with potential roles in phosphonate and phosphinate metabolism and benzoate degradation (e.g., phnJ, pcaC, and mhpD). In summary, these results support a genotype-dependent rhizosphere response pattern involving naringenin and Sinorhizobium, in which coordinated changes in root traits, exudate profiles, and microbial functional potential were associated with the stronger P acquisition response of A74 under combined stress. These findings identify candidate relationships for future functional validation under saline conditions.