Bradyrhizobium spp. Strains in Symbiosis with Pigeon Pea cv. Fava-Larga under Greenhouse and Field Conditions

Optimization of symbiosis between nitrogen-fixing bacteria and legumes has been extensively studied, seeking agricultural sustainability. To evaluate the symbiotic efficiency of nitrogen-fixing bacterial strains belonging to the Bradyrhizobium genus with pigeon pea (Cajanus cajan (L.) Millsp.) cv. Fava-Larga, experiments were conducted in Leonard jars (axenic conditions), pots with soil, and in the field. Ten strains were tested in Leonard jars, and three strains, in addition to BR 29, were selected according to their ability to promote the growth of pigeon pea, for further tests in pots with different soil types (Inceptsol and Oxisol) and in the field (Oxisol). Treatments were compared with strains BR 2003 and BR 2801 (approved as inoculants for pigeon pea), with a non-inoculated control with mineral N fertilization, and with another non-inoculated control (absolute control) with low mineral N concentration (Leonard jars) or without mineral N fertilization (soil). The efficiency of Bradyrhizobium strains in axenic conditions varies among strains, being higher when pigeon pea cv. Fava-Larga establishes symbiosis with the strains UFLA 03-320, UFLA 03-321, UFLA 04-212, BR 2801, and BR 2003. The soil type influences the symbiotic efficiency of Bradyrhizobium-pigeon pea in soil in the greenhouse, mainly in Inceptsol, in which strains UFLA 04-212, BR 2801, and BR 2003 increased N accumulation in the plant. The strain UFLA 03-320 increased shoot dry matter and N accumulation in the shoot equivalent to the mineral N treatment under field conditions. UFLA 03-320, BR 29, UFLA 03-321, and UFLA 04-212 promoted yields similar to those of the reference strain (BR 2801), and of the mineral N treatment with 70 kg ha-1 urea-N. These results confirm that pigeon pea establishes efficient symbiosis, which provides the N required for its growth. All strains, except for BR 2003, show potential for recommendation as inoculants for grain production. The strain UFLA 03-320 also shows potential for use in green manure crops.