Basil Growth, Soil Chemistry, and Bacterial Community Responses to Compost Tea Alone or Combined with Biochar

The combined compost tea and biochar treatment (BC) showed more distinct responses than compost tea alone, particularly in selected growth-related traits. Plant height, leaf width, and SPAD value increased under the BC treatment at specific growth stages, whereas final fresh weight did not differ significantly among treatments. Antioxidant-related traits also showed no significant differences among treatments. Compared with the control treatment and, where applicable, the compost tea alone treatment, the BC treatment was associated with selective changes in plant ion composition, including increased K+ concentration and decreased Ca2+, Cl−, and NO3− concentrations in basil tissue extracts. In soil, the BC treatment was associated with increased pH, organic matter, NO3−-N, and exchangeable Mg, and decreased NH4+-N and exchangeable Na. Soil bacterial community analysis revealed treatment-related shifts in community structure, and these changes were associated with soil chemical properties, including pH, organic matter, inorganic nitrogen forms, and exchangeable cations. Genus-level analysis within Chloroflexi further indicated that the BC treatment was associated with changes in bacterial taxonomic composition. Overall, the combined compost tea and biochar treatment selectively affected basil growth-related traits, plant ion composition, soil nutrient status, and bacterial community structure. However, because a biochar-only treatment was not included, these responses should be interpreted as effects associated with the combined compost tea and biochar treatment rather than as effects attributable to biochar alone.