Synergistic promoting mechanism of modified biochar combined with functional microbial agent on saline-alkali soil improvement

While both modified biochar and microbial agents individually show promise in ameliorating saline-alkali stress, their synergistic effects remain underexplored. This study investigates the efficacy of a novel composite, oxalic acid-modified biochar loaded with halotolerant plant growth-promoting rhizobacteria (PGPR), in enhancing wheat growth and soil quality in coastal saline-alkali soil. A pot experiment with six treatments revealed that the combined application (T5) yielded superior results, significantly increasing wheat plant height, chlorophyll content, above-ground fresh weight, and dry weight by 25.58 %, 25.51 %, 57.89 %, and 42.55 %, respectively, compared to the control. This synergy was attributed to the modified biochar, which served as an effective "micro-shelter," enhancing bacterial survival and activity. The T5 treatment also markedly improved soil quality by reducing pH and sodium adsorption ratio (SAR), increasing soil organic matter (SOM) by 152.31 %, and enriching beneficial genera like Bacillus. Furthermore, it enhanced plant antioxidant capacity (e.g., increased SOD and POD activities by 58.06 % and 57.15 %) and reduced oxidative damage (87.10 % reduction in MDA). Structural Equation Modeling indicated that the composite primarily enhanced plant biomass by directly improving soil quality and indirectly by reshaping the soil microbiome. This study demonstrates that the modified biochar-based microbial agent operates through multi-site synergism ('carrier-functional bacteria-soil-plant'), providing an efficient strategy for saline-alkali soil remediation. • Oxalic acid modification enhanced the sodium ion adsorption capacity of biochar. • The modified biochar significantly improved bacterial colonization and survival rate. • Modified biochar-based bacterial agent strengthened wheat's resistance in saline-alkali soil.