ABSTRACT The initial development of the common bean plants (Phaseolus vulgaris L.) that rely on symbiotic nitrogen (N) can be limited when active N2 fixation is delayed. Thus, adequate plant growth with rhizobium inoculation could require supplemental mineral N, which in turn may inhibit the symbiosis. Five experiments were performed using hydroponics to identify the initiation of nodulation and nitrogenase activity in common bean cultivars, and effects of mineral N addition on plant nodulation and growth. Three experiments evaluated the initial growth of five inoculated bean cultivars in the absence or presence of mineral N, while two experiments evaluated the effect of mineral N addition until the beginning of the reproductive stage. The first root nodules appeared 10 days after transplanting (DAT), while nitrogenase activity was initiated at 11 DAT. Large seed cultivars had lower levels of initial nodulation and nitrogenase activity than those of small seeds. Inoculated plants showed limited shoot growth that lasted until 21-25 DAT relative to the inoculated plants that received mineral N. Moreover, adding mineral N greatly reduced the nodule mass more than the nodule number, and caused an even greater reduction in nitrogenase activity. Mineral N that was applied until 15 DAT enhanced nodulation and nitrogenase activity without limiting shoot growth relative to the plants that received N throughout their growth. In contrast, plants that received N after 15 DAT had lower levels of nodule mass and nitrogenase activity than those of the plants that were only inoculated. Hence, these results indicate that symbiotic N was not sufficient for an adequate initial growth of the common beans. Therefore, some supplemental N is necessary, which should be added at sowing to stimulate plant growth without inhibiting further nodulation and N fixation.

Flemingia macrophylla é uma leguminosa arbustiva asiática, pouco conhecida no Brasil. Este trabalho objetivou avaliar a produção de massa seca, o acúmulo de nutrientes e a fixação biológica de nitrogênio (N2) de flemingia. Conduziram-se dois experimentos de campo, entre dezembro de 2006 e dezembro de 2007, em Seropédica, RJ, e em Avelar, RJ. O delineamento experimental utilizado em ambos os experimentos foi de blocos casualizados, com quatro repetições, e os tratamentos foram as diferentes épocas de amostragens. A área foliar, a produção de matéria seca e o acúmulo de nutrientes da parte aérea foram determinados em 12 coletas mensais. A contribuição da fixação biológica de N2 foi estimada pelo método da abundância natural de 15N. A massa seca da parte aérea atingiu 4,0 Mg ha-1 em Seropédica e 2,3 Mg ha-1 em Avelar, aos 360 dias após transplante (DAT). O menor crescimento em Avelar foi associado ao inverno mais frio e seco, em relação a Seropédica. A taxa de crescimento da cultura atingiu valor máximo aos 165 DAT, na primeira floração. O acúmulo de N, P, K, Ca e Mg na parte aérea foi de, respectivamente, 74, 5, 33, 25 e 8 kg ha-1 aos 360 DAT. A percentagem de N, proveniente da fixação biológica de N2 em flemingia, foi de aproximadamente 76%, o que propiciou 57 kg ha-1 de N oriundo da fixação biológica, na parte aérea, aos 360 DAT. Os resultados indicam que flemingia é uma espécie que pode contribuir com quantidades significativas de biomassa e nutrientes, especialmente N, sendo promissora para uso como adubo verde e em cultivos em aleias, em sistemas de produção.

Flemingia macrophylla is an Asian shrub legume scarcely known in Brazil. The objective of this work was to evaluate the growth, nutrient accumulation and biological N2 fixation of flemingia. Two field experiments were conducted from December 2006 to December 2007, in Seropédica and in Avelar, Rio de Janeiro State. Both experiments were arranged in a completely randomized block design with four repetitions, and the treatments consisted of the different sampling times. Leaf area, biomass and nutrient accumulation in leaves and branches were measured in twelve monthly samplings. Biological N2 fixation was estimated by the natural abundance of 15N. Shoot biomass yield reached 4.0 Mg ha-1 in Seropédica and 2.3 Mg ha ¹ in Avelar at 360 days after transplanting (DAT). The lower growth in Avelar was associated with the colder and drier winter than in Seropédica. The highest crop growth rate was observed at 165 DAT, at the first flowering. Accumulation of N, P, K, Ca and Mg in shoots was, respectively, 74, 5, 33, 25 and 8 kg ha-1 at 360 DAT. The percentage of N derived from biological N2 fixation was approximately 76% which provided 57 kg ha-1 of N originated from symbiosis in shoots at 360 DAT. The results indicate that flemingia may provide significant amounts of biomass and nutrients, especially N, arising as a promising species to be introduced as green manure and alley cropping in agricultural systems.