Assessing the benefit of post-emergence rhizobial re-inoculation for common bean crops across contrasting field conditions

Context Biological N 2 fixation in common bean is often constrained by rhizobial, environmental, and host factors, limiting seed-inoculation effectiveness. Post-emergence re-inoculation can potentially provide an additional opportunity for effective rhizobial establishment during crop development, but its consistency and agronomic effectiveness across field conditions remain uncertain. Objective The objective was twofold: i) to determine whether post-emergence re-inoculation with elite rhizobia strains could improve common bean performance beyond conventional seed inoculation, and ii) to assess whether it could narrow the agronomic response gap between seed inoculation and mineral-N fertilization across four field experiments conducted at two sites with contrasting soil textures and cropping histories. Methods Two experiments were conducted in a coarse-textured Ferralsol (coarse sites) and other two in a fine-textured Nitisol (fine sites) in São Paulo State, southeastern Brazil. Treatments consisted of five N supply strategies: control; seed inoculation; seed inoculation plus post-emergence re-inoculation (1200 mL ha –1 of liquid inoculant, 8.9 × 10 6 CFU plant –1 ); and mineral-N fertilization at 60 or 120 kg N ha –1 . Plant nodulation, N acquisition, seed yield and crude protein in seeds were quantified to evaluate common bean response to post-emergence re-inoculation. Results Fine sites generally outperformed coarse sites in N uptake, biomass accumulation, and seed and crude protein yields, reflecting strong site-season effects on plant N nutrition and productivity. Re-inoculation promoted some improvements in nodulation and plant N acquisition relative to the non-inoculated control, with N uptake 10.9–19.3% numerically greater than with seed inoculation alone, althoug these differences were not significant and did not translate into greater seed yield or consistency in the increase of crude protein concentration in seeds. Both inoculation strategies matched the seed yield achieved with 60 kg N ha –1 mineral-N fertilization, whereas 120 kg N ha –1 resulted in the greatest seed and protrein yields overall. Correlation analyses showed that plant N uptake was more strongly associated with crop productivity than nodulation traits, while canonical variable analysis further demonstrated the strong influence of site-season conditions on plant N uptake and productivity. Significance/implications Inoculation strategies produced seed yields comparable to 60 kg N ha –1 , highlinghting the contribution of biological N fixation to common bean N managemet. Post-emergence re-inoculation enhanced nodulation and N acquisition, but did not consistently improve agronomic performance or seed protein beyond seed inoculation alone. These findings suggests that the agronomic value of re-inoculation is context-dependent, warranting further investigation into the specific soil, seasonal, or conditions under which it is most beneficial.