Avaliação da fixação biológica de nitrogênio em plantios tecnificados de feijão-caupi na região Centro-Oeste do Brasil

Abstract

The planting of cowpea [Vigna unguiculata (L.) Walp.] is growing in the Midwest of Brazil, which is characterized by a technified agriculture, with an average yield of 1000 kg ha-¹. New technologies of inoculation as carriers and the agricultural practice of pre inoculation are presented as alternative to current technologies. Thus the objective of this study was to evaluate technologies for cowpea inoculation in the Midwest region of Brazil. The quality of the polymeric IPC 2.2 inoculant was evaluated for the cells survival at 180 days of storage and the ability to maintain viable cells after five weeks of inoculation. The agronomic performance of the polymeric inoculant and the pre inoculation were compared with technologies already recommended, in three different experiments with the same cultivate (BRS Guariba) in areas of Embrapa Agrobiologia (Seropédica-RJ), Embrapa Agrossilvipastoril (Sinop-MT) and in the New Horizon farm – Seeds Tomazetti (Primavera do Leste, MT). The contribution of biological nitrogen fixation was quantified by 15N natural abundance delta (‰) in the two experiments in Mato Grosso State. In the experiment at Embrapa Agrobiologia the polymeric inoculant IPC 2.2 was compared with peat and liquid carriers, plus nitrogen treatments with 50 and 80 kg N ha-¹ and the absolute treatment. In the experiment at Embrapa Agrossilvopastoril the treatments were pre inoculated with polymeric inoculant IPC 2.2 and peat carrier, with 0, 1, 2 and 5 weeks before planting for both inoculants plus addition of nitrogen (70 kg N ha-¹), and the absolute treatment. The assay at the New Horizon farm was in a central pivot area of 60 ha with the strains: BR3267 in the polymeric and liquid carriers; BR3262 in polymeric; and a consortium of four recommended strains (BR3267, BR3262, and BR3301 BR3302) in the polymeric carrier. The polymer showed inoculant cell concentration higher than 109 cells per gram after 180 days, and in the field experiment the formulation IPC 2.2 provided yield above the reference treatment and equal to the N treatment, and the peat and liquid carriers. The pre-inoculation with the polymeric blend with five weeks had the highest nodule dry mass, however it did not differ from the N control and the absolute, and the treatments did not differ for the productivity. In the essay at the Novo Horizonte farm the inoculation with the consortium of four strains had the highest nodule dry mass, and it differed from the BR3267 in liquid carrier and the BR3262. The contribution of biological nitrogen fixation did not exceed 50%, and the highest averages were obtained with 48% and 39% of N originated from biological fixation, respectively in the pre inoculation essay (14 days in the peat carrier), and with the BR3262 at the New Horizon farm with the polymeric carrier. Thus, it is concluded that the polymer inoculant might be recommended for the traditional form inoculation of the cowpea crop, and inoculation should be a common practice, since it was proven that with the no inoculated seeds there was no biological nitrogen fixation.