Activating microbial traits: Vitamin C byproduct alleviates carbon limitation and enhances plant-soil-microbe...

Carbon limitation and microbial functional degradation constrain nutrient cycling and vegetation establishment in mine dump soils. Residue after evaporation (RAE) from vitamin C production is a potential labile carbon source, yet its role in restoring microbial functionality in nutrient-poor substrates remains unclear. Here, we conducted a pot experiment using sterilized, low-organic-matter mine soil and perennial ryegrass (Lolium perenne L.) to evaluate the individual and combined effects of RAE and two Bacillus species (Bacillus subtilis and Bacillus megaterium). RAE alone increased soil dissolved organic carbon by 78.02% but had limited effects on microbial activity and plant growth, indicating that carbon input alone is insufficient under depleted microbial conditions. Bacillus inoculation increased microbial biomass carbon (MBC) and nitrogen (MBN) by 40.58-98.83%, enhanced soil available nitrogen and phosphorus by 22.76-32.86%, and increased plant biomass by 24.37-24.85%, although these effects were constrained by carbon availability. In contrast, combined RAE and Bacillus application produced strong synergistic effects, markedly enhancing rhizosphere carbon-acquiring enzyme activity and extracellular polymeric substance production. MBC and MBN increased by up to 166.52% and 139.72%, soil available nitrogen, phosphorus, and potassium increased by 25.75-68.79%, and plant biomass increased by 45.00-50.73%. Path analysis identified rhizosphere carbon acquisition as the central hub linking labile carbon inputs, microbial processes, nutrient supply, and plant growth. Overall, RAE acts as a microbial function-activating carbon source, enhancing Bacillus-driven processes, alleviating carbon limitation, and restoring plant-soil-microbe coupling.