Objectives Saline-alkali soil has unfavorable physical and chemical properties that restrict the nutrient absorption and growth of maize. Some beneficial salt-alkali-tolerant microorganisms exhibit a significant promoting effect on crop growth and development. We compounded some strains and studied their ability and promotion mechanism under moderate saline-alkali soil conditions.

Methods Three multiple species inoculants: GF-S1, GF-S2, and GF-S3, were prepared using salt-tolerant bacterial strains in our lab, and then used as test materials. Laboratory petri dish experiments were conducted to determine the secretion ability of anti-stress components of the three inoculants. A maize field experiment was conducted by applying the three inoculants, taking no agent as control. The seed germination characteristics, plant and root morphology, and biomass were investigated at the main growing stages of maize. The basic properties of soil before the experiment and the rhizosphere soils at the main growing stages were tested.

Results 1) Petri dish experiment　　Under moderate saline-alkaline conditions, all the three inoculants grew well and possessed the ability of secreting indole-3-acetic acid (IAA), exopolysaccharides (EPS), iron carriers, and 1-aminocyclopropane-1-carboxylate deaminase (ACC), and the phosphorus soluble capacity. Among these inoculants, GF-S3 demonstrated a stronger metabolic capacity. 2) Field experiment　　When compared with the control, the effect of enhancing rhizosphere soil urease, alkaline phosphatase, and sucrase activity in GF-S1, GF-S2, and GF-S3 treatment maintained to V6 and R1 stage, respectively; all the three inoculants reduced rhizosphere soil pH, with the effect persisting until the R1 stage; GF-S1 treatment decreased total salt contents up to V12 stage, whereas GF-S2 and GF-S3 treatment maintained this reduction until the R1 stage. Application of the inoculants boosted the germination potential and rate, increased plant height, stem diameter, leaf area index, and dry matter accumulation per plant, ultimately leading to higher maize yields. Remarkably, GF-S3 showed the highest values for these growth indicators, resulting in an 8.59% yield increase at R6 stage. Further analysis revealed that the inoculants’ secreted EPS, phosphorus solubilization ability (PSA) and ACC enhanced maize germination potential, seedling height, stem thickness, and yield; the secreted EPS, IAA, and ACC increased germination rate, root length, and fresh root weight; the EPS, IAA, and spermidine (SPD) increased plant height, while the EPS, ACC, and alkaline phosphatase (AKP) increased dry weight per plant (DWP). EPS, IAA, and ACC were identified as key factors mediating corn stress resistance and promoting growth.

Conclusions Under moderate saline-alkaline conditions, the salt-tolerant multiple species inoculants thrive and secrete substances that are resistant to stress and promote growth (such as EPS, IAA, phosphorus solubilization substances, iron carriers, and ACC). When applied into moderate saline-alkaline soil, they effectively reduce rhizosphere soil pH and total salt content, while increasing soil enzyme activity. This, in turn, benefits maize seed germination, seedling and plant growth, and ultimately enhances yield. Among the inoculants tested, GF-S3 demonstrated the most robust stress resistance and growth promotion effects, owing to its superior secretion capacity of stress-resistance substances and its prolonged ability to reduce soil pH and salt content, as well as increase soil enzyme activities