Background and aims: Acid soils constrain legume growth and biochars have been shown to address these constraints and enhance biological N 2 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 N 2 fixation and crop productivity. Results: Biological N 2 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 N 2 fixation, biochars provided greater benefits to faba bean yield by addressing P nutrition and ameliorating Al toxicity.
