Marisa Anahí Gómez

Diversity of rhizobia isolated from an agricultural soil in Argentina based on carbon utilization and effects of herbicides on growth

Seventy-six rhizobial isolates belonging to four different genera were obtained from the root nodules of several legumes (Vicia sativa, Vicia faba, Medicago sativa, Melilotus sp., Glycine max and Lotus corniculatus). The action of five commonly used herbicides [2,4-dichlorophenoxyacetic acid (2,4-D), glyphosate (GF), dicamba, atrazine and metsulfuron-methyl] on the growth of rhizobial strains was assessed. Subsequently, GF and 2,4-D were tested in a minimum broth as C and energy sources for 20 tolerant strains. The ability of these strains to metabolize different carbon sources was studied in order to detect further differences among them. Tolerance of the bacteria to agrochemicals varied; 2,4-D and GF in solid medium inhibited and diminished growth, respectively, in slow-growing rhizobial strains. Among slow-growing strains we detected Bradyrhizobium sp. SJ140 that grew well in broth + GF as the sole C and energy source. No strain was found which could use 2,4-D as sole C source. The 20 strains studied exhibited different patterns of C sources utilization. Cluster analysis revealed three groups, corresponding to four genera of rhizobia: Rhizobium (group I), Sinorhizobium (group II) and Mesorhizobium–Bradyrhizobium (group III). On the basis of the results obtained on responses to herbicides and C sources utilization by the isolates investigated, it was possible to differentiate them at the level of strains. These results evidenced a considerable diversity in rhizobial populations that had not been previously described for Argentinean soils, and suggested a physiological potential to use natural and xenobiotic C sources.

Microbial Respiration in Soils of the Argentine Pampas after Metsulfuron Methyl, 2,4‐D, and Glyphosate Treatments

Abstract Short‐term response of microbial respiration after treatment with different doses of the herbicides metsulfuron methyl (MET), 2,4‐D, and glyphosate (GLY) was studied in microcosms of soils collected in three agricultural sites of the Southern Pampas region, Buenos Aires, Argentina. The influence of diammonium phosphate [(NH4)2PO4] on carbon dioxide (CO2) evolution, when applied with the highest doses of the herbicides, was also investigated. MET had no effect on microbial respiration of an acidic soil of San Román (pH 6.06), even at the highest rate. However, MET inhibited microbial respiration in soils of Bordenave (pH 7.44), at a rate of 0.1 mg kg−1 soil. Low application rates of GLY and 2,4‐D produced only transitory effects on CO2 evolution, whereas the addition of high doses of these herbicides stimulated microbial activity. On the other hand, the addition of fertilizer to soil treated with a high dose of GLY temporarily inhibited CO2 release.

Soil ecotoxicity assessment of glyphosate use under field conditions: microbial activity and community structure of Eubacteria and ammonia-oxidising bacteria

BACKGROUND A plot-scale experiment was conducted to assess the impact of field application rates of glyphosate on soil microbial communities by taking measurements of microbial activity (in terms of substrate-induced respiration and enzyme activity) in parallel with culture-independent approaches to assessing both bacterial abundance and diversity. Two rates of glyphosate, alone or in a mixture with 2,4-dichlorophenoxyacetic acid, were applied directly onto the soil surface, simulating normal use in chemical fallow in no-till systems. RESULTS No consistent rate-dependent responses were observed in the microbial activity parameters investigated in the field plots that were exposed to glyphosate. Denaturant gradient gel electrophoresis (DGGE) of the overall bacterial community (Eubacteria) and ammonia-oxidising bacteria (AOB) revealed no effects of the high rate of glyphosate on the structure of the communities in comparison with the control. No treatment effects were observed on the abundance of Eubacteria shortly after treatment in 2010, while a small but significant difference between the high rate and the control was detected in the first sampling in 2011. The abundance of AOB was relatively low during the study, and treatment effects were undetectable. CONCLUSIONS The absence of negative effects on soil microbial communities in this study suggests that glyphosate use at recommended rates poses low risk to the microbiota.

Modeling and Specifying Requirements for Cyber-Physical Systems

Cyber physical systems have to do with a strong interaction between the physical world and a computing system, which should be transparent to an external observer. In this paper, an approach for modeling and specifying requirements for these systems is presented. It is called 3D Approach and it is based on the well known 4 Variable Model. The proposed extension intends to tackle the particularities involved in the development of cyber-physical systems by a multidisciplinary team and consequently it emphasizes a practical and applied point of view. The 3D Approach came out as the result of systematizing lessons learned in the design of a biodigester, which allows the anaerobic digestion of biomass for the production of biogas.

Herbicides in the Soil Environment: Linkage between Bioavailability and Microbial Ecology

Modern agriculture relies heavily on herbicides for the control of weeds in crops and pastures to maximize yields and economical benefits to sustain an increasing world population. The introduction of herbicide-resistant traits in several crops, such as glyphosate-resistant (GR) soybean, maize and canola, has further increased herbicide consumption worldwide (Cerdeira & Duke, 2006). United States consumed roughly 200 million kg in 2001, with glyphosate representing 20 % of the total. Glyphosate is, undoubtedly, the most used herbicide worldwide (Woodburn, 2000). In Argentina, where GR soybean accounts for almost 90 % of planted soybean, it was estimated that 160 million l of glyphosate were used with this crop in 2004, representing 37 % of the total herbicide consumed in agriculture (Altieri & Pengue, 2006; Pengue, 2004). The environmental fate of herbicides is a matter of recent concern given that only a small fraction of the chemicals reach the target organisms (Pimentel, 1995), leading to potential impacts of residual herbicides in soil and water have on human, animal and crop health. Bunce (1993) wrote in 1993 ”It is useful to keep in mind the concept that a pollutant is a substance in the wrong place, at the wrong time, or in the wrong amount”. While herbicides are very important to agriculture, under certain circumstances they may act as pollutants that can deteriorate soils, ground waters and surface waters. While most herbicides are not intentionally applied onto soil, they can enter the soil environment from 1) direct interception of spray by the soil surface during early season or post-harvest applications, 2) runoff of the herbicide from vegetation and 3) leaching from dead plant material. The herbicide concentration may vary from a few μg to mg per kg soil, as most of the applied chemical is retained within the top 5 cm of soil. This chapter will present aspects of the behavior of herbicides in soils, focusing on soil retention and microbial degradation as main factors controlling persistence. The potential impact of herbicides on non-target soil microbes, including their processes and interactions, will be also discussed. Adsorption to soil is of critical importance for the regulation of herbicide persistence and mobility throughout the environment because sorption processes control the amount of

Denitrifying capacity of rhizobial strains of Argentine soils and herbicide sensitivity

Agrochemical application in soils is a matter of environmental concern, and among soil microorganisms, rhizobia and their action before different pesticides are interesting to study, due to their taxonomic and functional diversity. The objectives of the present work were to assess the capacity of rhizobial populations to use herbicides as source of nutrients, as well as their ability to reduce nitrates and / or denitrify. Eighty-one strains belonging to four populations of different genera of rhizobia (Rhizobium, Mesorhizobium, Ensifer and Bradyrhizobium) were assessed. The effect of glyphosate, 2,4-dichlorophenoxyacetic acid, and atrazine on growth of the strains, as well as the ability of the strains to act on herbicide transformation to reduce nitrate and denitrify, were evaluated. The genera studied showed different responses to pesticides. Bradyrhizobium had the greater capacity to utilize the herbicides and among the compounds evaluated, atrazine was the most used as a source of energy. To conclude, some Bradyrhizobium strains were able both to denitrify and to use the atrazine herbicide. The results obtained in this study increase expectations of the use of rhizobia as inoculants, causing changes at the agricultural and environmental level and allowing an appropriate management of agricultural soil fertilization, efficiency in nitrogen fixation and a faster biodegradation of pesticides in soil.

Microbial enumeration of different functional groups and bacterial behavior in acid basic conditions of a biotoxic landfill leachate of Bahía Blanca, Argentina.

Leachate is liquid waste from refuse biological decomposition or rainwater percolation in a landfill. This research focused on leachate produced by a landfill in Bahia Blanca, Buenos Aires, Argentina. The research studied the main microbial populations involved in wastewater treatment, analyzed the behavior of bacteria isolated from leachate at different pH values, and appraised leachate biotoxicity. The number of bacteria varied by type, ranging from 1 x 10(4) to 1 x 10(5) CUF/mL aerobic heterotrophic bacteria (AHB); 1 x 10(3) to 1 x 10(5) CUF/mL anaerobic heterotrophic bacteria (ANHB); 1 x 10(5) to 1 x 10(6) CUF/mL sulfite-reducing bacteria (SRB); 1 x 10(3) to 1 x 10(6) NMP/mL nitrate-reducing bacteria (NRB); and 1 x 10(2) to 1 x 10(4) NMP/mL ammonium-oxidizing bacteria (AOB). Several microbial strains developed at pH 5, 7, and 10. These pH values changed to 9 in the culture media after a 48-hour incubation. Leachate was used to water lettuce seeds (Lactuca sativa capitata). Its toxicity was proved by full inhibition of plant development.

Denitrification Activity in Soils for Sustainable Agriculture

Denitrification is a microbial process in the nitrogen cycle in which oxidized nitrogen compounds are used as alternative electron acceptors for energy production. Denitrifying bacteria occur in practically every sort of environmental niche: soil, water, and sediment. Denitrification decreases the leaching of nitrate to ground and surface waters and is the main biological process responsible for the return of fixed nitrogen to the atmosphere. On the other hand, it contributes to the greenhouse effects, to the destruction of the ozone layer, and is a loss of nitrogen otherwise available for the growth of plants. A short overview of denitrification activity in soils leading to a sustainable agriculture is given. The genus Bradyrhizobium and its relevance in the process of denitrification are described.

Rapid characterisation of agro-industrial effluents for environmental fate by UV–visible and infrared spectroscopy from fractions obtained by centrifugation

ABSTRACT Agro-industrial systems (e.g. dairy farms, feed lot, pig breeding and food processing plants) provide large quantity of organic wastes that could be recycled within the productive systems. However, the basic chemical characterisation is not enough to predict the effect that they may generate on the environment. In this study, a centrifugation process was applied at various speeds between 3000 and 15,000 rpm and carried out separately on two different livestock effluents (dairy farm and pig anaerobic digestate), in order to obtain supernatants and precipitates, which were studied separately. The more water soluble fractions, with lighter components and/or simpler structures, remained as liquid supernatants, while the more complex fractions, with higher molecular weight and/or water insoluble fractions, constituted the solid precipitates. An increase in the centrifugation rate did not produce the differential precipitation of dissimilar functional groups. Hence, 5000 rpm was the most adequate velocity since it generated clear supernatants without denaturation of the organic matter. A basic cost-effective chemical analysis, complemented with ultraviolet–visible and Fourier transform infrared spectroscopy, enables a set of properties to be established qualitatively and quickly for the multiple components of the organic matter for its later use as fertilisers or amendments. This rapid and economical technique allows for a characterisation prior to the reuse of the effluents, which is necessary to optimise their application and avoid environmental problems.

Isolation and characterization of onion degrading bacteria from onion waste produced in South Buenos Aires province, Argentina.

Onion production in Argentina generates a significant amount of waste. Finding an effective method to recycle it is a matter of environmental concern. Among organic waste reuse techniques, anaerobic digestion could be a valuable alternative to current practices. Substrate inoculation with appropriate bacterial strains enhances the rate-limiting step (hydrolysis) of anaerobic digestion of biomass wastes. Selection of indigenous bacteria with the ability to degrade onion waste could be a good approach to find a suitable bioaugmentation or pretreatment agent. We isolated bacterial strains from onion waste in different degradation stages and from different localities. In order to characterize and select the best candidates, we analyzed the growth patterns of the isolates in a medium prepared with onion juice as the main source of nutrients and we evaluated carbon source utilization. Nine strains were selected to test their ability to grow using onion tissue and the five most remarkable ones were identified by 16S rRNA gene sequencing. Strains belonged to the genera Pseudoxanthomonas,Bacillus, Micrococcus and Pseudomonas. Two strains, Bacillus subtilis subsp. subtillis MB2-62 and Pseudomonas poae VE-74 have characteristics that make them promising candidates for bioaugmentation or pretreatment purposes.