Fixação biológica de nitrogênio em cultivares de feijão-caupi recomendadas para o Estado de Roraima Biological nitrogen fixation in cowpea cultivars recommended for Roraima state, Brazil

O objetivo deste trabalho foi avaliar a fixação biológica de nitrogênio (FBN) em cinco cultivares de feijão-caupi: BR 17 Gurguéia, BRS Guariba, BRS Mazagão, UFRR Grão Verde e Pretinho Precoce 1. Em 2007, foram conduzidos um experimento em casa de vegetação e outro em campo, em esquema fatorial com cinco cultivares de feijão-caupi e quatro fontes de nitrogênio: adubação com ureia (50 kg ha-1 de N), inoculação com a estirpe de Bradyrhizobium BR 3262 ou BR 3267 e um controle absoluto. Aos 35 dias após a emergência das plantas, foram avaliados número e massa de nódulos secos, massa de matéria seca e N total da parte aérea, eficiência nodular em casa de vegetação e rendimento de grãos na colheita em campo. Em casa de vegetação, foi observada alta nodulação e eficiência nodular para ambas as estirpes em todas as cultivares. Em campo, a nodulação e o N total foram menores para todas as cultivares, comparativamente à casa de vegetação, o que indica interferência de fatores edafoclimáticos na FBN. Ocorreu aumento no rendimento de grãos em todas as cultivares em decorrência da inoculação, especialmente com a estirpe BR 3262.<br>The objective of this work was to evaluate the biological nitrogen fixation (BNF) in five cowpea cultivars: BR 17 Gurguéia, BRS Guariba, BRS Mazagão, UFRR Grão Verde and Pretinho Precoce 1. In 2007, one greenhouse and one field experiment were performed using a factorial design with five cowpea cultivars and four nitrogen sources: urea fertilization (50 kg ha-1 of N), inoculation with BR 3262 or BR 3267 Bradyrhizobium strains, and absolute control. Thirty-five days after plant emergency, the number and dry weight of nodules, the dry matter weight and the total N of the shoots, the nodule efficiency in greenhouse plants and the grain yield for the field experiment plants were evaluated. In the greenhouse, high nodulation and nodule efficiency were observed for both strains in all cultivars. In the field, the nodulation and total N were lower for all cultivars in comparison with the greenhouse experiment, which indicates the interference of edaphoclimatic factors in the BNF. An increase in grain yield was observed for all cultivars as a consequence of the inoculation, especially for the BR 3262 strain

Bacterial exo-polysaccharides: a biological tool for the reclamation of salt-affected soils

International audienceAccumulation of salts on soil surface and in the root zone damages physico-chemical and biological properties of salt-affected soils. Exo-polysaccharides (EPS), the polymers of monosaccharides, are synthesised and released in soil by microorganisms inhabiting rhizosphere, roots of the plants and the decomposing organic residues. The bacterial EPS are involved in formation and stability of soil micro-aggregates, a factor that ensures fertility of the cultivated soils. The rhizosheaths formed around roots by bacterial EPS contribute to build up soil physical structures, regulate nutrients and water flow from rhizosphere soil to the plants, promote growth and protect the roots against pathogens. Thus, the bacterial EPS are directly and indirectly involved in and impacted both physico-chemical soil characteristics and growth of the plants. However, the role of the bacterial EPS in improving soil fertility and interaction with constituents of the salt-affected soils has rarely been explored. Research studies, therefore, were conducted to observe the effect of the bacterial EPS extracted from Microbacterium sp. MAS133 isolated from a salt-affected soil on soil moisture release and aggregate stability at three (native, acidic and alkaline) pH values of clay fraction of a saline-sodic soil compared to a normal soil. Results showed that the processes and phenomenon of soil aggregate formation and stability in the colloidal form with interactive effect of biopolymers and SMA (suspended micro-aggregates) as well as water retention and release of a saline-sodic soil were all influenced by the bacterial EPS. Although extent and nature of the bacterial EPS-micro-aggregates interactions varied with pH, soil type and EPS concentration, the effect was consistent and persistent for extended time periods. Moreover, 16S rRNA gene sequence analysis showed that MAS133 belonged to Microbacterium hominis of the lineage of the Firmicutes and the EPS produced on sucrose medium were fructose biopolymers. Interaction of the bacterial biopolymer with soil constituents and a positive impact of the bacterial inoculation on soil aggregation around roots and mitigation of negative effects of salinity on plant growth observed in earlier studies suggest the EPS-producing bacteria a useful biological tool for reclamation of the salt-affected soils. Additionally, a strategy of provision of carbohydrate substrates to foster growth and production of the EPS by the bacterial populations living in the salt-affected soils through field water distributaries and the ‘bioretention’ or ‘biomonitoring’ cells could help overcome the economy and the environmental concerns associated with application and use of the bacterial inoculum in the field