Organic–mineral fertilization modulates microbial communities and nutrient-cycling genes in saline–alkali soil

Introduction Soil salinization constrains crop production in arid regions, yet the microbial and functional mechanisms underlying organic–mineral co-application in saline–alkali soils remain unclear. Methods A pot experiment with sorghum–sudangrass was conducted in a saline–alkali soil under five fertilization regimes with equal total N but different proportions of organic N. Soil physicochemical properties were measured at the seedling and maturity stages, and rhizosphere bacterial communities and C, N and P cycling genes at maturity were characterized by 16S rRNA gene sequencing and SmartChip high-throughput qPCR. Results Organic–mineral fertilization decreased soil pH and total salt content and increased soil organic matter, total N and available P relative to mineral fertilizer alone, with the strongest improvements under the 50% organic–50% mineral N regime. Organic inputs increased bacterial Shannon diversity and evenness and shifted community composition, enriching Actinobacteriota , Firmicutes , Bacillus and Pseudarthrobacter . The balanced regime increased genes involved in C degradation/fixation, N fixation and P mineralization/polyphosphate metabolism (e.g., xylA, acsA, mct, nifH, phoD, ppx ), whereas mineral-only fertilization favored nitrification/denitrification and methane oxidation genes (e.g., amoA2, nirK, nirS, pmoA ), indicating a higher potential for N losses. Discussion Multivariate analyses identified soil pH, total salt, organic matter and total N as primary regulators of bacterial communities and functional gene profiles. Moderate organic–mineral co-application, particularly the 50%–50% regime, improves soil conditions and strengthens nutrient-cycling potential in saline–alkali sorghum–sudangrass systems.