Enhanced biological N2 fixation and yield of faba bean ('Vicia faba' L.) in an acid soil following biochar addition: dissection of causal mechanisms

Background and aims Acid soils constrain legume growth and biochars have been shown to address these constraints and enhance biological N₂ fixation in glasshouse studies. A dissection of causal mechanisms from multiple crop field studies is lacking. Methods In a sub-tropical field study, faba bean ('Vicia faba' L.) was cultivated in rotation with corn ('Zea mays') following amendment of two contrasting biochars, compost and lime in a rhodic ferralsol. Key soil parameters and plant nutrient uptake were investigated alongside stable ¹⁵N isotope methodologies to elucidate the causal mechanisms for enhanced biological N₂ fixation and crop productivity. Results Biological N₂ fixation was associated with plant Mo uptake, which was driven by reductions in soil acidity following lime and papermill (PM) biochar amendment. In contrast, crop yield was associated with plant P and B uptake, and amelioration of soil pH constraints. These were most effectively ameliorated by PM biochar as it addressed both pH constraints and low soil nutrient status. Conclusions While liming resulted in the highest biological N₂ fixation, biochars provided greater benefits to faba bean yield by addressing P nutrition and ameliorating Al toxicity.