Adriana Abril

Nodulation of Lupinus albus by strains of Ochrobactrum lupini sp. nov.

ABSTRACT The nodulation of legumes has for more than a century been considered an exclusive capacity of a group of microorganisms commonly known as rhizobia and belonging to the α-Proteobacteria. However, in the last 3 years four nonrhizobial species, belonging to α and β subclasses of the Proteobacteria, have been described as legume-nodulating bacteria. In the present study, two fast-growing strains, LUP21 and LUP23, were isolated from nodules of Lupinus honoratus. The phylogenetic analysis based on the 16S and 23S rRNA gene sequences showed that the isolates belong to the genus Ochrobactrum. The strains were able to reinfect Lupinus plants. A plasmid profile analysis showed the presence of three plasmids. The nodD and nifH genes were located on these plasmids, and their sequences were obtained. These sequences showed a close resemblance to the nodD and nifH genes of rhizobial species, suggesting that the nodD and nifH genes carried by strain LUP21T were acquired by horizontal gene transfer. A polyphasic study including phenotypic, chemotaxonomic, and molecular features of the strains isolated in this study showed that they belong to a new species of the genus Ochrobactrum for which we propose the name Ochrobactrum lupini sp. nov. Strain LUP21T (LMG 20667T) is the type strain.

Mesorhizobium chacoense sp. nov., a novel species that nodulates Prosopis alba in the Chaco Arido region (Argentina)

Low-molecular-weight RNA analysis was performed for the identification and classification of 20 Argentinian strains isolated from the root nodules of Prosopis alba. SDS-PAGE of total cellular proteins, determination of the DNA base composition, DNA-DNA reassociation experiments and physiological and biochemical tests were also carried out for these strains and the whole 16S rRNA gene was sequenced from one of the strains, strain LMG 19008T. Results of the genotypic and phenotypic characterization showed that the strains isolated in this study belong to a group that clustered in the genus Mesorhizobium. The results of DNA-DNA hybridizations showed that this group is a novel species of this genus. The name Mesorhizobium chacoense sp. nov. is proposed for this species. The type strain is LMG 19008T (= CECT 5336T).

Integral assessment of pollution in the Suquía River (Córdoba, Argentina) as a contribution to lotic ecosystem restoration programs.

The Suquía River lower-middle basin (Córdoba, Argentina) is subject to a strong anthropic impact because it receives pollutants from different sources (industries, wastewaters, heavy traffic, agricultural land use, etc.) We have assessed the degree of watershed degradation of Suquía River lower-middle sections through the analysis of different ecosystem compartments (air, water, riparian soil, sediments and biota), in order to provide useful data to be considered in future river restoration programs. Four study sites were selected along the river (La Calera city, Córdoba city, Corazón de María village and Río Primero city) which were sampled during the low- and high-water flow periods. We analyzed: a) chemical and physical characteristics of water, sediments, and riparian soil; b) heavy metal content of water and sediments, and c) semi-volatile organic compounds in air. Besides, pollutant bioindicators such as fish assemblages, lichens (Usnea amblyoclada), vascular plants (Tradescantia pallida), and microorganisms (fecal coliform and Escherichia coli) were used to further assess the status of the river. All analyzed ecological compartments were affected by water pollution, particularly, fish assemblages, sediments and riparian soils by heavy metal and coliform bacteria. Moreover, we detected a possible contribution of sulfur and a high pollutant content in air that merit further research about other air-water exchanges. Accordingly, we strongly suggest that an action to restore or remediate the anthropic effect on the Suquía River be extended to all possible compartments along the river.

Reliability of the in situ incubation methods used to assess nitrogen mineralization : a microbiological perspective

The reliability of in situ incubation methods to measure soil N mineralization was assessed by comparing the initial and final values for several soil parameters inside and outside the isolated soil containers laid in the ground for 2 weeks. We measured soil water content, ammonifying and nitrifying bacteria, soil respiration, and ammonium and nitrate content. Soil water content did not change inside the container, whereas in the surrounding soil significant differences were found, with varying magnitude and sign according to climatic conditions during the incubation period. Ammonifiers number and ammonium content did not change inside the container, whereas nitrifiers number, nitrate content, and soil respiration decreased significantly (49, 22 and 36%, respectively). When comparing container content and the surrounding soil, a similar pattern was found: ammonifiers number and ammonium content did not change, whereas nitrifiers and soil respiration were lower inside the container than in the outside soil (44 and 33%, respectively). At the end of the incubation period, nitrate contents inside the container correlated significantly with both number of nitrifying organisms and respiration rate. It suggests that aerobic processes inside the cylinder were affected, probably because of oxygen depletion due to microbial activity in a closed environment. We conclude that the in situ incubation methods underestimate the N mineralization rate in soils. Shortening the incubation period may minimize this source of error.

The importance of phyllosphere microbial populations in nitrogen cycling in the Chaco semi-arid woodland

In tropical rain forest, the interface between leaf surfaces and the atmosphere is a fundamental pathway for nutrient cycling (particulary nitrogen), possibly even more important than the soil–plant interface (Parker 1994, Silver et al . 1996). Most important nutrient exchanges in the phyllosphere–atmosphere interface are mediated by microbial populations. For example, some authors have considered that nitrogen fixation in the phyllosphere is the main mechanism for nitrogen gain in humid tropical ecosystems, because of the substantial nutrient demand resulting from a high plant productivity and the constraint imposed by the generally low nitrogen availability in soil (Ruinen 1974, Salati et al . 1982, Silver et al . 1996).

Litter quality and litter removal by the native fauna in the western Chaco woodland of Argentina

Most researchers assume litter decomposition to take place at the site where litterfall occurs, mostly by soil micro-organisms and microfauna of less than 1 mm in size. Accordingly, the litterbag method has become the standard technique for evaluating litter decomposition rates (Schlesinger 1985, Wedderburn & Carter 1999). However, there is evidence indicating that at least in tropical arid ecosystems litter can be removed from the site in substantial amounts by animals, including large vertebrates. For example, litter is removed for forage or as building material for nests by ants (particularly leaf-cutting ants) and termites (Bucher 1982, Scholes & Walker 1993). Moreover, ruminant species (including domestic cattle and goats) supplement their food intake by feeding on falling leaf litter during the dry season in African savannas (Owen-Smith & Cooper 1987) and in the Chaco savannas of Argentina (Morello & Saravia-Toledo 1959). These observations suggest that litter removal may be an important component of the decomposition process, particularly in tropical semi-arid environments where lack of soil moisture decreases microbial activity, increasing litter availability for herbivores.

Sustainability/resilience of soil organic matter components in an Argentinean arid region

Se analizo el grado de resiliencia de los componentes de la materia organica del suelo para establecer la sustentabilidad de las practicas productivas en una region arida de Argentina. Se trabajo en cuatro sitios en el Chaco Arido de la provincia de Cordoba: un sitio testigo, 2 sitios con ganaderia (con desmonte total y selectivo) y un sitio con agricultura. En cada sitio se tomaron tres muestras de suelo y se analizo en contenido de materia organica total, acido fulvicos, acidos humicos y sustancias no humificadas. Se calculo la variacion (%) de cada componente entre los sitios productivos y el sitio testigo, para establecer el grado de resiliencia. En los sitios con ganaderia las sustancias no humificadas fueron moderadamente resilientes, la materia organica total y los acidos humicos escasamente resilientes y los acidos fulvicos no resilientes. En el sitio con agricultura la materia organica total y las sustancias no humificadas fueron escasamente resilientes, mientras que los acidos fulvicos y humicos fueron no resilientes. Se concluye que este metodo de evaluacion de la sustentabilidad es una excelente herramienta para establecer las practicas de manejo de acuerdo a cada situacion en particular, permitiendo mejorar la productividad de las regiones aridas.

Labile and Recalcitrant Carbon in Crop Residue and Soil under No-Till Practices in Central Region of Argentina

We evaluated the effects of the amount and type of C compounds (labile/recalcitrant) of crop residues on the soil organic matter (SOM) components under no-tillage practices in semiarid region of Argentina, with the aim of establishing criteria for sustainable agriculture. Five agricultural situations were evaluated: soybean/wheat rotation (wheat crop and at bare fallow); soybean monoculture; soybean/maize rotation and maize/wheat/potato rotations with biannual ploughing. In each situation crop residues and soil samples were collected. In residue samples, biomass (total and compo- nents), organic carbon, insoluble fibers, and soluble C compounds were evaluated. In soil samples total SOM, non-humic substances (NHS), humic substances (HS), humic (HA), and fulvic acids (FA) were analyzed. The total biomass and its components differed among situations and were lower in the situation with plough, while the fiber contents were higher in situations that includes wheat crop. Total SOM, NHS, HA content were higher in the plowed situation (57.20 g kg -1 , 47.30 g kg -1 , 7.80 g kg -1 respectively). We concluded that the total SOM quantity in continuous no-tillage is not affected by the different crop rotations and sequences, but the HA quantity (stable SOM) seems to be favored by the high recalcitrant compounds amount of the crop residues (particularly wheat). For these reasons, the following criteria for sustainable agri- culture management in semiarid zones can be suggested: a) continuous no-tillage utilization; b) summer crop annual rota- tions; and c) winter wheat crop instead of bare fallow.

Variation in soil biological characteristics on an elevational gradient in the montane forest of north-west Argentina

Montane tropical and subtropical rain forests are complex ecosystems, characterized by marked rainfall and temperature gradients with altitude, which in turn control the vegetation altitudinal zones (Hueck 1978). Montane forests are often referred to as cloud forests in recognition of the important influence of a dense and frequent cloud cover that conditions forest structure and functioning (Bautista-Cruz & del Castillo 2005, Holder 2004).

Is the nitrification a redundant process in arid regions?: activity, abundance and diversity of nitrifier microorganisms

Nosotros testeamos si el concepto de redundancia funcional microbiana (teoria que relaciona los cambios en el funcionamiento del ecosistema y la perdida de especies) se aplica para la comunidad de microorganismos oxidadores de amonio (bacterias nitrificadoras) afectada por el pastoreo, la estacionalidad (estacion seca y humeda), el tipo de habitat (suelo y restos organicos), y las condiciones ecologicas (eco-region del Chaco Arido y el Monte), en la region arida del centro-oeste de Argentina. Se determino: a) la abundancia de bacterias nitrificadoras cultivables, b) la actividad y eficiencia de la nitrificacion, y c) la estructura genetica de la comunidad de nitrificadores mediante PCR-DGGE (riqueza e indice de similitud). El pastoreo no modifico la abundancia de nitrificadores, la actividad y la riqueza, mientras que el efecto de la estacionalidad y la eco-region fue escaso. Contrariamente, el tipo de habitat afecto la actividad nitrificadora, la cual fue mas alta en los restos organicos que en el suelo. Todos los indices de similitud fueron bajos (promedio = 0.5; rango entre 0.76-0.18), lo cual sugiere una alta diversidad de especies en estas regiones aridas. Nuestros resultados indican que el proceso de nitrificacion en la region arida central de Argentina es redundante, pero que se puede aplicar solo para cada tipo de habitat. En nuestro conocimiento, este es el primer estudio de redundancia funcional sobre procesos ecologicos microbianos en zonas aridas disturbadas, en el cual se detecto un importante remplazo de especies, sin modificaciones en la abundancia y actividad de los microorganismos.