Impact of urea and Azospirillum brasilense on soybean nodulation and early growth

The effects of isolated and combined inoculation with Bradyrhizobium japonicum (SEMIA 3079 and 5080) and Azospirillum brasilense (Ab-V5), with and without nitrogen fertilization, on nodulation and early soybean growth were evaluated in a greenhouse under controlled temperature and irrigation conditions, using non-sterilized soil. The experimental design included five treatments: (1) urea (200 kg ha⁻¹); (2) B. japonicum; (3) A. brasilense; (4) co-inoculation (B. japonicum + A. brasilense); and (5) B. japonicum + 100 kg ha⁻¹ of urea. Evaluations were carried out 50 days after sowing and included plant height, shoot and root dry mass, number and fresh mass of nitrogen-fixing nodules, Falker chlorophyll index, and leghemoglobin content. Co-inoculation increased the number of nodules by 26.13% compared to B. japonicum alone (88 → 111 nodules), while A. brasilense alone also promoted nodulation (113.3 nodules), indicating synergy with native soil rhizobia. Higher leghemoglobin levels were observed in nodules under co-inoculation, suggesting enhanced symbiotic functionality, whereas urea application reduced this indicator. Nitrogen fertilization strongly suppressed symbiosis (-75.77% in nodule number and - 83.36% in nodule fresh mass; B. japonicum + N vs. B. japonicum), confirming the inhibitory effect of mineral nitrogen on biological nitrogen fixation (BNF). It is concluded that co-inoculation with B. japonicum (SEMIA 3079 and 5080) and A. brasilense (Ab-V5) enhances nodule functionality and BNF in non-sterile soil conditions, providing a biological basis for reducing reliance on mineral nitrogen in soybean production systems.