Cultivar-specific responses to post-emergence rhizobial reinoculation in common bean

Abstract

Common bean (Phaseolus vulgaris L.) relies heavily on nitrogen (N) and partially meets its needs through rhizobial N2 fixation. Post-emergence rhizobial reinoculation may enhance this process. Under controlled conditions, we assessed the efficiency of liquid Rhizobium inoculant rates applied post-emergence on nodulation, dry matter, N nutritional status, and agronomic traits. Treatments consisted of three common bean cultivars (IPR Andorinha, IPR Campos Gerais, and TAA Dama) cultivated under six N input methods: control; seed inoculation; seed inoculation + post-emergence reinoculation with spraying of 400, 800, and 1200 mL ha−1 of liquid inoculant (equivalent to 2.9, 5.9, and 8.9 × 106 colony-forming units plant−1); and mineral-N fertilization with 120 kg N ha−1. At the R8 stage, post-emergence reinoculation, especially at 800 and 1200 mL ha−1, increased nodule number and dry matter relative to seed inoculation and control treatments. Mineral-N fertilization, however, resulted in higher chlorophyll indices, shoot dry matter, and seed weight per plant compared with inoculation treatments, particularly for IPR Andorinha and TAA Dama. Cultivars and N input methods interacted significantly for shoot N concentration, N accumulation, and seed weight, evidencing genotypic variability in the response to biological N fixation and mineral N. TAA Dama stood out for nodulation and biomass accumulation. For IPR Campos Gerais, the highest shoot N accumulation at R8 (0.43 mg plant−1) occurred with 1200 mL ha−1 of post-emergence inoculant. These findings demonstrate that post-emergence reinoculation can enhance biological N fixation in common bean, but its effectiveness depends on cultivar selection and alignment with mineral-N management.