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En el presente trabajo se estudiaron los cambios que provocó el lavado de cinco suelos afectados por sales sobre la actividad biológica (número de bacterias g-1y producción de CO2) y enzimática (catalasa, deshidrogenasa, ureasa y fosfotriesterasa) de los mismos. El lavado disminuyó la conductividad eléctrica (CE) y modificó el tipo de sales dominantes en los suelos. La producción de CO2 y la actividad de la fosfotriesterasa fue significativamente mayor (pChanges in the biological activity (number of bacteria g-1and CO2 production) and in the enzymatic activity (catalase, deshidrogenase, urease and phosphotriesterase) caused by the leaching of five soils affected by salts have been studied. The leaching decreased the electric conductivity (CE) and modified the type of dominant salts in the soils. Production of CO2and the activity of the phosphotriesterase was significantly higher (p

Changes in the biological activity (number of bacteria g(-1) and CO2 production) and in the enzymatic activity (catalase, deshydrogenase, urease and phosphotriesterase) caused by the leaching of five soils affected by salts have been studied. The leaching decreased the electric conductivity (CE) and modified the type of dominant salts in the soils. Production of CO2 and the activity of the phosphotriesterase was significantly higher (p<0.05) in a leached soil (Torrifluventes Typical Centennial); the increment were 88% and 71%, respectively. The results showed that the decrease of the salinity by leaching did not produce significantly different results in most of the biotic parameters analysed.

A survey of the changes in populations of heterotrophic bacteria, coliform microorganisms and S. meliloti was conducted in samples taken from the water irrigation channels of the Neuquén River (Argentina). Fifty-six water samples were collected during the spring-summer seasons of 1997-1999 years. Both the heterotrophic plate count bacterial and the number of coliforms oscillated between 110-5050 CFU/ml and 8-1400 CFU/100 ml, respectively, during the period this study was carried out. Fecal coliforms were detected in 91.1% of the water samples investigated. Moreover, the results showed that S. meliloti capable of nodulating alfalfa (Medicago sativa L.) Cuf 101 were present in 68% of the water samples and in effectiveness studies, no isolate out of 25 evaluated could be classified as superior N fixers. That is, they did not produce plants equal in weight to nitrate-grown plants (KNO3 0.05%). All the S. meliloti strains were resistant to novobiocin and bacitracin, while 72% of the microsymbionts demonstrated resistance to between seven and ten antibiotics. Results presented in this study showed that irrigation waters of the Neuquén river could act as dispersal agents of both ineffective S. meliloti strains and thermotolerant coliform bacteria.

Little is known about native populations of Bacillus thuringiensis (Bt) isolated from soils of Argentina. We undertook this study to determine the resistance to different pesticides of two commercial and fourteen native strains of Bt under in vitro conditions. An agar plate bioassay test conducted with ten pesticides and sixteen strains of Bt showed that Basagran, Scepter, Fungoxan and Decis were not toxic for the bioinsecticide bacteria at recommended application rates (RAR). In contrast, low concentrations (3.2% RAR) of Agil, Select and Isomero showed a deleterious effect on the bacteria investigated. Simultaneously, four of the pesticides were able to produce phenotypical changes on the Bt colonies grown on nutrient agar. Moreover, in a greenhouse experiment, seven pesticides applied at 1.6%, 12.5% and 100% RAR on soybean leaves were not as inhibitory as under in vitro conditions for two Bt strains (HD-1 and A61). However, survival of these strains in the phyllosphere of soybean differed significantly between untreated leaves and leaves treated with pesticides after 20 days of study (P < 0.05). Finally, and from an ecological point of view, these findings suggest that the addition of some pesticides to soybean leaves in lower concentrations than those recommended could be favourable for the persistence of Bt in this environment.