Biochar-microbe synergy enhances auxin-mediated soil–plant interactions for canola productivity in alkaline calcareous soil

Abstract

Biochar and microbial bio-fertilizers, such as endophytic fungi and plant growth-promoting endophytes (PGPEs), offer sustainable alternatives to chemical fertilizers by enhancing soil fertility and plant performance. However, their synergistic effects particularly those involving auxin (IAA) biosynthesis and nutrient uptake, remain underexplored in calcareous soils. This study investigates how the integration of biochar with auxin-producing microbial inoculants influences soil–plant interactions and canola productivity. A pot experiment was conducted using two Brassica napus (canola) cultivars (DGL and Faisal canola) grown under alkaline calcareous soil. Treatments included individual and combined applications of A. baumannii MN24 and P. indica, with and without biochar. Physiological traits, auxin concentrations (IAA), root and shoot biomass, nutrient uptake (N, P, K), seed quality, and soil microbial and enzymatic activities were measured. Results revealed that the combined application of biochar, MN24, and P. indica significantly enhanced plant biomass, stem diameter, and grain yield by up to 203%, 127%, and 212%, respectively, in Faisal canola compared to the control. Biochar also amplified the microbial colonization and enzymatic activity in soil, leading to higher microbial biomass, enhanced nutrient uptake, and reduced electrolyte leakage and osmotic potential, indicating improved plant performance. Notably, these improvements translated to higher canola seed quality, with elevated fat, ash, and fiber contents. Enhanced auxin synthesis, particularly in the presence of L-tryptophan, was linked to improved root architecture, microbial colonization, and reduced plant stress indicators. These findings suggest that biochar–microbe–L-tryptophan synergy offers a powerful tool for sustainable crop intensification in challenging soils by optimizing hormone-mediated soil–plant processes.