Preparation of Acid-Modified Biochar and Remediation Mechanisms on Soda–Saline–Alkali Soil

Soil salinization poses significant threats to food security and ecosystem functions, while acid-modified biochar functions as an effective carbon-based material for the reclamation of saline–alkali soils. In this study, acid-modified biochars prepared using boric (BC), sulfuric (SC), hydrochloric (HC), acetic (AC), phosphoric (PC), and oxalic (OC) acids were analyzed. The ameliorative effects of acid-modified biochar on soda–saline–alkaline soils were evaluated through adsorption and pot experiments, with preliminary insights into its mechanism of action. The results indicated that the specific surface area and maximum adsorption capacities (Qm) of conventional biochar and OC were 5.91, 35.39 m2 g−1 and 21.62, 41.00 mg g−1, respectively. After the addition of conventional biochar, OC and SC in pot experiments, soil pH, (CO32− + HCO3−) content, and exchangeable sodium percentage (ESP) were significantly reduced. Compared to conventional biochar, SC increased the relative abundance of Bacillus, Adhaeribacter, and Preussia, while OC increased the relative abundance of Antarcticbacterium, Diezia, and Peziza. OC and SC maximally increased both the aboveground and belowground biomass of Medicago falcata L., while simultaneously reducing sodium content. This study demonstrated that biochar modified with SC and OC significantly reduced soda–saline–alkali stress. SC and OC exhibited greater potential in remediating soda–saline–alkali soils.