Objectives This study aimed to screen and construct a compound microbial agent suitable for ameliorating saline-alkali soil in the Songnen Plain, and to investigate the effect of its combination with humic acid on improving soil fertility and promoting rice growth, thereby providing an effective soil conditioner for saline-alkali land improvement.

Methods Functional strains were isolated and screened from saline-alkali paddy soil in Da'an City. Strains with strong acid-producing ability were identified via 16S rRNA or 18S rRNA gene sequencing based on their growth characteristics. Non-antagonistic strains were combined into a compound microbial agent. The effects of the agent on soil fertility and plant growth were evaluated through soil incubation and seedling establishment tests, pot experiments, and field trials. In the soil incubation and seedling establishment tests, four treatments were set: (1) CK, no microbial agent, (2) B, microbial agent B-3 (Staphylococcus xylosus), (3) F, microbial agent F-1 (Aspergillus niger), (4) BF, compound microbial agent B-3 (Staphylococcus xylosus) and F-1 (Aspergillus niger). After 30 days, soil pH and EC values, as well as rice plant height, root length, fresh weight and dry weight were measured. In the pot experiment, the compound microbial agent was applied together with humic acid. Four treatments were designed: no application (CK), humic acid (HA), compound microbial agent (BF), and compound microbial agent+humic acid (HB). Rice plant height and tiller number were measured three times during the tillering stage. Soil physicochemical properties and rice yield were determined after harvest. In the field trial, two treatments were compared: no application (CK) and compound microbial agent+humic acid (HB). Rice yield was measured after harvest.

Results Four salt tolerant bacteria strains (B-1, B-3, B-7, B-8) and four salt tolerant fungi strains (F-1, F-5, F-6, F-12) were isolated and screened from saline-alkali soil. All eight strains could still grow on the medium at pH 12 with 200 g/L NaCl. The growth characteristics, acid production, and antagonistic tests of the strains showed that B-3 and F-1 exhibited the strongest acid-producing capacity among bacteria and fungi, respectively, and no antagonism was observed between them, indicating that they can be formulated into a compound microbial agent. These strains were identified as Staphylococcus xylosus and Aspergillus niger, respectively. In the soil incubation and seeding establishment tests, compared with CK, soil pH values under BF, F, and B treatments decreased by 0.47, 0.28, and 0.16 units, respectively; EC values under BF and F treatments decreased by 17.5% and 9.4%, respectively. Plant height under BF increased by 26.8%; fresh weight of five plants under BF, F, and B treatments increased by 45.1%, 25.1%, and 12.3%; and dry weight of five plants increased by 41.5%, 24.6%, and 13.8%, respectively. In the pot experiment, compared with CK, soil organic matter content under HB and HA treatments increased by 7.5% and 4.3%, respectively, while total nitrogen content increased by 14.9% and 10.6%, respectively. Soil pH under HB, HA and BF treatments decreased by 1.01, 0.66, and 0.43 units, respectively, and water-soluble total salt content decreased by 22.9%, 18.1%, and 8.3%, respectively. Final measurements showed that plant height under HA and HB treatments increased by 10.4% and 5.3%, respectively; tiller number increased by 22.6% and 18.9%, respectively; and rice yield under HB, HA, and BF increased by 29.2%, 14.2%, and 9.1%, respectively. In the field trial, rice yield under HB treatment increased by 18.6% compared with CK.

Conclusions The soil conditioner formed by combining the compound microbial agent consisting of strains B-3 (Staphylococcus xylosus) and F-1 (Aspergillus niger) with humic acid significantly improved soda saline-alkali soil and showed strong application potential.