Contrasting acidic and alkaline biochar reprogram alfalfa metabolism and rhizosphere microbiomes in saline-alkali soils

Abstract Soil salinization severely limits crop productivity and ecosystem sustainability, particularly in arid and semi-arid regions. Here, we investigated the potential of targeted biochar amendments to rehabilitate saline-alkali soils and promote alfalfa ( Medicago sativa ) performance through integrative analyses of soil, plant, and rhizosphere microbiome. Two contrasting biochars—acid-modified biochar (AC-biochar, pH 2.3) and alkaline biochar (AL-biochar, pH 8.8)—were applied at 1%, 2%, and 5% (w/w) in controlled pot experiments. Our findings identified low-dose AC-biochar (1%) and high-dose AL-biochar (5%) as the most effective treatments for ameliorating soil geochemical constraints, including enhancing nutrient levels, reducing electrical conductivity, and modulating pH. These amendments significantly promoted alfalfa biomass and forage quality. A closer examination showed that AL-biochar primarily stimulated plant growth by enhancing amino acid metabolism, reactive oxygen species detoxification, and nitrogen assimilation, coupled with increased bacterial diversity and enrichment of beneficial taxa linked to nutrient cycling (e.g., Rhizobium and Firmicutes). In contrast, AC-biochar enhanced root development via activation of secondary metabolite biosynthesis (e.g., flavonoids and alkaloids) and recruitment of Actinobacteria known for pathogen suppression and organic matter decomposition. Integrative metabolic and microbiome analyses reveal that biochar-induced plant benefits transcend soil amelioration, exerting targeted regulation on plant physiology and rhizosphere ecology. These findings uncovered a previously underappreciated functional dichotomy in biochar effects and demonstrated the potential of precision biochar application as a scalable, sustainable approach for rehabilitating saline-alkali soils and improving crop resilience under environmental stress. Graphical Abstract