Biochar-biostimulant combinations enhance nodulation, yield, and antioxidant defense in mungbean under reduced NPK fertilization

Reducing mineral fertilizer inputs while maintaining crop productivity is a central challenge for sustainable grain legume production. This study evaluated whether biochar-biostimulant combinations could improve mungbean (Vigna radiata L.) performance under reduced NPK fertilization. A polytunnel pot experiment was conducted using a completely randomized 2 × 4 factorial design with two NPK levels, 100% and 50% of the recommended dose, and four treatments: biochar alone, biochar + humic acid, biochar + Bacillus amyloliquefaciens, and biochar + Pseudomonas fluorescens. Biostimulant addition to biochar improved plant growth, pigment status, and antioxidant response, with the strongest responses observed under reduced NPK supply. Under 50% NPK, biochar + P. fluorescens increased grain yield by approximately 90% compared with biochar alone and maintained grain yield (4.00 g plant⁻¹) comparable to the 100% NPK biochar-only control (3.85 g plant⁻¹). It also enhanced nodulation, increased total chlorophyll by approximately 60%, and reduced malondialdehyde by approximately 50%. Grain weight was positively associated with nodule number and ascorbic acid content and negatively associated with H₂O₂ content. These findings support biochar-assisted P. fluorescens supplementation as a promising strategy for sustaining mungbean productivity under reduced NPK input, warranting further field validation for broader applicability.