Fixação biológica de nitrogênio em cultivares de feijoeiro estimada pela abundância natural de 15N

Abstract

The 15N natural abundance technique to measure the biological nitrogen fixation (BNF) in legumes is based on the slight enrichment of 15N observed in soils to the atmosphere and does not require the application of labelled fertilizers. The objectives of this work were: to quantify the BNF contribution in common bean (Phaseolus vulgaris) cultivars under field conditions through the 15N natural abundance technique; to evaluate the agronomic performance of these cultivars under inoculation; to estimate the isotopic fractionation value during BNF (B value) of common bean cultivars grown in nutrient solution. Two experiments were conducted in greenhouse in nutrient solution without N, in the years 2012 and 2013 in Seropédica, RJ. Three common bean cultivars were evaluated. Plant biomass was sampled at different growth stages, where the N natural abundance was analyzed. In 2012 experiment, the nodules showed positive δ15N, with deltas of 6.63‰ and 8.18‰ in Grafite and Radiante cultivars, respectively. The B value in shoot was –0.919‰, –0.734‰ and –0.724‰ for cultivars Grafite, Ouro Negro and Radiante, respectively. In 2013 experiment, the nodules showed great enrichment in 15N, with 9.11‰ and 8.36‰ deltas in Radiante and Grafite cultivars. The B value in shoot was -1.06‰ and -1.40‰ for cultivars Radiante and Grafite, respectively. The average B value in shoots of common bean cultivars was –1.23‰, and the B value of –1.2 was used in the calculation of the BNF contribution in the field experiments of this study. Eight common bean cultivars were grown in field conditions in 2011 and 2012, with two N sources (inoculation with commercial inoculant with rizhobia or fertilization with 90 kg ha-1 of mineral N), with four replications, at Embrapa Rice and Bean in Santo Antonio de Goias, GO. Biomass of the shoot, root, nodules and grains were sampled, and total N and 15N natural abundance were analyzed in shoots and grains. In the experiment in 2011, the Ouro Negro cultivar produced higher nodule mass under inoculation, also at reproductive stages, and the Jalo Precoce and Radiante cultivars showed the lowest nodule mass. The grain yield of the eight cultivars was 1614 and 2942 kg ha-1 under inoculation or mineral N, respectively. Under inoculation, the Ouro Negro cultivar showed the highest N accumulation in shoots and grains and the highest grain yield, reaching 2200 kg ha-1, equivalent to 73% of yield under mineral N. In 2011, the N accumulation derived from BNF in grains of the eight cultivars ranged from 5.8 to 16.4 kg ha-1, which corresponded to a BNF contribution of 17% and 33% in Marfim and Ouro Negro cultivars, respectively. In 2012, the higher nodule mass occurred in Vereda and Estilo cultivars, in the first and second samples, respectively, whereas Radiante cultivar had the lowest nodule mass. The average grain yield of the eight cultivars was 3284 and 3919 kg ha-1 under inoculation and mineral N, respectively. The N accumulation derived from BNF in grains was 68.4, 38.7, and 21.6 kg ha-1 in Ouro Negro, Grafite and Radiante cultivars, respectively, corresponding to a BNF contribution of 49, 29 and 32%. The method of 15N natural abundance provided the identification of BNF potential for the supply of N in different common bean genotypes, arising as a tool for providing subsidies to guide breeding programs in order to increase BNF efficiency.