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ABSTRACT: To quantify the BNF contribution to legumes using the 15N natural abundance technique, it is important to know the abundance of 15N of the plants grown entirely dependent on BNF (value ‘B’). The aim of the study was to determine the 15N natural abundance of N2 fixed by different Bradyrhizobium strains in symbiosis with one soybean cultivar. Treatments consisted of soybean plants cultivated with and without inoculation with ten Bradyrhizobium strains, in five replicates planted in Leonard jars in a sand/vermiculite mixture. Plants were harvested after 46 days. The ‘B’ values of the aerial tissue (‘Bs’) ranged from -2.6 to -3.9 ‰. There was a tendency for the ‘Bs’ values of plants inoculated with strains of B. elkanii to be more negative than plants inoculated with other strains. All ‘B’ values of the whole plant were less than 1 unit of δ15N (‰) different from zero, suggesting that the symbioses have little tendency to show significant isotopic fractionation during N2 fixation, but there is considerable depletion in 15N of the N translocated to the shoot tissue.

To quantify the BNF contribution to legumes using the 15N natural abundance technique, it is important to know the abundance of 15N of the plants grown entirely dependent on BNF (value 'B'). The aim of the study was to determine the 15N natural abundance of N2 fixed by different Bradyrhizobium strains in symbiosis with one soybean cultivar. Treatments consisted of soybean plants cultivated with and without inoculation with ten Bradyrhizobium strains, in five replicates planted in Leonard jars in a sand/vermiculite mixture. Plants were harvested after 46 days. The 'B' values of the aerial tissue ('Bs') ranged from -2.6 to -3.9‰. There was a tendency for the 'Bs' values of plants inoculated with strains of B. elkanii to be more negative than plants inoculated with other strains. All 'B' values of the whole plant were less than 1 unit of d15N (‰) different from zero, suggesting that the symbioses have little tendency to show significant isotopic fractionation during N2 fixation, but there is considerable depletion in 15N of the N translocated to the shoot tissue. [ABSTRACT FROM AUTHOR]

Background and aims: The 'B' value, the N abundance of plants depending completely on biological nitrogen fixation (BNF) for growth, is required for quantifying BNF using the N natural abundance technique. This study aimed to investigate how common bean ( Phaseolus vulgaris L.) cultivars with different growth patterns could affect the 'B' value calculated for the plant shoot. Methods: Two experiments were conducted in N-free solution. Experiment I had a factorial design with three cultivars and two rhizobium inoculants. Plants were harvested at pre-flowering stage. Experiment II was also factorial with two cultivars and two times of sampling: full flowering and mid-pod filling. Total N and δN analyses of different plant parts were carried out to estimate the 'B' value. Results: There were differences between cultivars for N accumulation and N abundance, but no difference between rhizobium inoculants. Results revealed differences in 'B' value between cultivars of small and large seeds, the growth stage being also relevant. The ' B' values of −2.08 and −1.34‰ could be used for BNF estimates in small-seeded cultivars at full flowering and mid-pod filling stages, respectively, while such values should change, respectively, to −1.73 and −1.03‰ for large-seed cultivars. The mean 'B' value in shoots was −1.90 and −1.20‰ at full flowering and mid-pod filling stages, respectively. Conclusions: There are variations in 'B' value between cultivars of different cycle length and seed sizes, which are associated with plant architecture, thus a 'B' for each growth stage of common bean plants is required. [ABSTRACT FROM AUTHOR]

In commercial plantations of soybean in both the Southern and the Cerrado regions, contributions from biological nitrogen fixation (BNF) are generally proportionately high. When using the 15N natural abundance technique to quantify BNF inputs, it is essential to determine, with accuracy, the 15N abundance of the N derived from BNF (the 'B' value). This study aimed to determine the effect of four recommended strains of Bradyrhizobium spp. (two B. japonicum and two B. elkanii) on the 'B' value of soybean grown in pots in an open field using an equation based on the determination of δ15N natural abundance in a non-labelled soil, and estimate of the contribution of BNF derived from the use of 15N-isotope dilution in soils enriched with 15N. To evaluate N2 fixation by soybean, three non-N2-fixing reference crops were grown under the same conditions. Regardless of Bradyrhizobium strain, no differences were observed in dry matter, nodule weight and total N between labelled and non-labelled soil. The N2 fixation of the soybeans grown in the two soil conditions were similar. The mean 'B' values of the soybeans inoculated with the B. japonicum strains were -1.84 ‰ and -0.50 ‰, while those inoculated with B. elkanii were -3.67 ‰ and -1.0 ‰, for the shoot tissue and the whole plant, respectively. Finally, the 'B' value for the soybean crop varied considerably in function of the inoculated Bradyrhizobium strain, being most important when only the shoot tissue was utilised to estimate the proportion of N in the plant derived from N2 fixation.Em plantações comerciais de soja na região Sul e do Cerrado, as contribuições da fixação biológica de Nitrogênio (FBN) são geralmente elevadas. Quando usamos a técnica da abundância natural de 15N para quantificar a FBN, é essencial determinar com exatidão a abundância de 15N do N derivado da FBN (valor 'B'). Este trabalho buscou determinar o efeito das quatro estirpes de Bradyrhizobium spp. (duas B. japonicum, duas B. elkanii) sobre o valor 'B' de soja crescida em vasos em ambiente aberto usando uma equação na determinação da abundância natural de 15N em um solo não enriquecido com 15N, e estimativas da contribuição da FBN derivado do uso da técnica de diluição isotópica de 15N em solo enriquecido com 15N. Para avaliar a fixação de N2 pela soja três plantas referenciam foram crescidas nas mesmas condições. Independente da estirpe de Bradyrhizobium, não foi observada diferença para matéria seca, massa de nódulos e N total entre solo marcado e não marcado. A fixação de N2 em soja crescida nas duas condições de marcação do solo foi semelhante. Os valores médios de 'B' para plantas de soja inoculadas com estirpes de B. japonicum foram, em média, de -1,84 ‰ e -0,50 ‰ enquanto as inoculadas com B. elkanii apresentaram médias de -3,67 ‰ e -1,00 ‰, para parte aérea e planta inteira, respectivamente. Finalmente, o valor 'B' para a cultura da soja variou consideravelmente em função da estirpe de Bradyrhizobium inoculada, sendo mais importante quando se utiliza somente a parte aérea da planta para estimar a proporção do N da planta derivado da fixação de N2.

Abstract: The 15N natural abundance technique is one of those most easily applied ‘on farm’ to evaluate the contribution of biological N2 fixation (BNF) to legume crops. When proportional BNF inputs are high, the accuracy of this technique is highly dependent on an accurate estimate of the 15N abundance of the N derived from N2 fixation (the ‘B’ value). The objective of this study was to determine the influence of soybean variety on ‘B’ value. Plants of five soybean varieties were inoculated separately with two Bradyrhizobium strains (one Bradyrhizobium japonicum and one Bradyrhizobium elkanii) grown in pots of soil virtually free of bradyrhizobia capable of nodulating soybean. The proportion of N derived from BNF (%Ndfa) was estimated in separate pots where a small quantity of enriched 15N ammonium sulphate was added. The %Ndfa was then used with the 15N natural abundance data of the nodulated soybean and non-N2-fixing reference plants, to determine the ‘B’ value for each soybean variety/Bradyrhizobium association. The varieties nodulated by the B. japonicum strain showed significantly greater N content and %Ndfa than those nodulated by the B. elkanii strain, and in all cases the ‘B’ value of the shoot tissue (‘B s’) was higher. The differences in ‘B s’ values between varieties nodulated by the same Bradyrhizobium strain were insignificant, indicating that this parameter is influenced much more by the Bradyrhizobium strain than by the variety of the host plant. [ABSTRACT FROM AUTHOR]