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 soybean, maize and canola, has further increased herbicide consumption worldwide. The environmental fate of herbicides is a matter of recent concern given that only a small fraction of the chemicals reaches the target organisms. While most herbicides are not intentionally applied onto soil, they can enter the soil environment from direct interception of spray by the soil surface during early season or post-harvest applications, from runoff or leaching of the herbicide from vegetation and/or from dead plant material. 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, on their processes and interactions, will be also discussed. The enormous variety of herbicides commercially available today makes it impossible to review all of them. Thus, this work will focus on some of the herbicides most used in the (semiarid) Pampa region of Argentina and worldwide (i.e., glyphosate, 2,4-dichlorophenoxyacetic acid, atrazine, metsulfuron-methyl), based on our own research data. 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 herbicide present in the soil solution. These processes are dependent on several factors related to soil characteristics such as mineral composition, organic matter content, soil solution chemistry and to chemical characteristics of the herbicide. Soil-bound herbicide or residues are temporarily inactivated, which prevents harmful effects on soil biota but also makes them less bio-available for microbial degradation because most microbial species are not able to utilize herbicides in the sorbed state. Herbicide degradation will be discussed both in terms of their use as carbon and nutrient sources. Most isolated herbicide-degrading microorganisms belong to bacterial species, but fungi are also well-known for their capacity to degrade complex substrates, and may be more important than present isolation approaches have suggested. Differential toxicity of herbicides to soil microorganisms may alter community structure, including potential increases in plant or animal pathogens. Herbicides may also cause changes in microbial community function and concomitant impacts on soil health and ecosystem processes.Fil: Zabaloy, Maria Celina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentina. Universidad Nacional del Sur. Departamento de Agronomía; ArgentinaFil: Zanini, Graciela Pilar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentina. Universidad Nacional del Sur. Departamento de Agronomía; ArgentinaFil: Bianchinotti, Maria Virginia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentina. Universidad Nacional del Sur. Departamento de Biología, Bioquímica y Farmacia; ArgentinaFil: Gomez, Marisa Anahi. Universidad Nacional del Sur. Departamento de Agronomía; ArgentinaFil: Garland, Jay L.. Dynamac Corporation; Estados Unido

El método de supresión de un cultivo de cobertura influye sobre el microbioma rizosférico

Los cultivos de cobertura (CC) son cultivos invernales que se establecen entre dos cultivos de cosecha, usualmente entre el inicio del otoño y el inicio de la primavera, en reemplazo del barbecho, mejorando la eficiencia del uso del agua y del nitrógeno (N). Su uso se ha extendido como una alternativa para proveer de residuos ricos en carbono (C), evitar pérdidas de nitratos por lixiviación y disminuir riesgos de erosión al mantener la cobertura de los suelos. En general, se emplean gramíneas como centeno, avena, cebada o raigrás. La supresión del CC previo a la siembra del cultivo estival se realiza mediante desecación con un herbicida sistémico de amplio espectro, fundamentalmente glifosato, o en mucho menor medida, mediante implementos mecánicos.Son escasos los trabajos que hayan indagado en los efectos de la desecación química sobre la microbiota de la rizosfera de plantas sensibles al glifosato, y mucho menos aún, que analizaran los potenciales cambios en la rizosfera tras la aplicación de los dos sistemas empleados para suprimir los CC. La aplicación foliar de glifosato en los CC puede tener impactos significativos sobre las comunidades microbianas de la rizosfera y las funciones ecológicas asociadas a ellas, por la alteración en la composición de exudados radicales y el herbicida mismo exudado y/o acumulado en las raíces senescentes del CC desecado, en comparación con el CC suprimido en forma mecánica. En esta presentación se discutirán resultados recientes obtenidos por nuestro grupo de trabajo en la rizosfera de avena (Avena sativa L.) como modelo de CC, en ensayos de invernadero bajo condiciones controladas. Específicamente, nos enfocamos en el análisis de las comunidades de bacterias y arqueas mediante cuantificación y secuenciación de amplicones del gen de ARNr 16S y amoA.Fil: Zabaloy, Maria Celina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentina. Universidad Nacional del Sur. Departamento de Agronomía; ArgentinaFil: Allegrini, Marco. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; ArgentinaFil: Morales, Marianela Estefania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; ArgentinaFil: Villamil, M. B.. Comisión Nacional de Investigación Científica y Tecnológica; ChileXII Reunión Nacional Científico-Técnica de Biología de SuelosBuenos AiresArgentinaUniversidad de Buenos Aires. Facultad de Agronomí

Primer design to assess bacterial degradation of glyphosate and other phosphonates

Phosphonates are organic phosphorous (P) compounds frequently detected in the environment due to a very stable C[sbnd]P bond that render them relatively recalcitrant. Glyphosate [N-phosphonomethyl glycine] is the most widely used and best-known synthetic phosphonate, and one of the most concerning herbicides in the world today. Microbial degradation of glyphosate and organophosphonates in general, is the main dissipation mechanism operating in most environments. One microbial metabolic pathway in this process is the C[sbnd]P lyase pathway, entailing an enzymatic complex encoded by about 14 genes (the Phn operon). Our goal was to develop a quantitative polymerase chain reaction (qPCR) assay for a key enzyme, the C[sbnd]P lyase that breaks down the C[sbnd]P bond, via quantification of the codifying phnJ gene. The primers designed in this study fulfill the requirements for a successful qPCR assay, with high efficiency and sensitivity, as well as specific detection of the target sequence in a wide range of taxonomic groups. This is, to our knowledge, the first report of primers designed to target phnJ in both pure cultures and metagenomic DNA from different environmental sources. Direct quantification of phnJ may be a cost-effective proxy to determine glyphosate degradation potential in different matrixes.Fil: Morales, Marianela Estefania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; ArgentinaFil: Allegrini, Marco. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; ArgentinaFil: Basualdo, J.. Universidad Nacional del Sur; ArgentinaFil: Villamil, M. B.. University of Illinois; Estados UnidosFil: Zabaloy, Maria Celina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentin

Examining N-limited soil microbial activity using community-level physiological profiling based on O2 consumption

Nitrogen-limited soil microbial activity has important implications for soil carbon storage and nutrient availability, but previous methods for assessing resource limitation have been restricted, due to enrichment criteria (i.e., long incubation periods, high substrate amendments) and/or logistical constraints (e.g. use of radioisotopes). A microtiter-based assay of basal and substrate induced soil respiration based on O2 consumption may be a rapid, ecologically relevant means of assessing N limitation. The present study evaluated this approach by examining 1) the extent and duration of N limitation on soil respiratory activity following different levels of N fertilization in the field, and 2) the relationship between N-limited activities and growth under the assay conditions. Fertilization rate and the time since fertilization had significant impacts on the degree of N limitation of soil microbial activity. The highest fertilization rate showed the earliest and most persistent reduction in N limitation, as would be predicted from the higher concentration of extractable inorganic soil N observed with this treatment. Bacterial growth under the assay conditions, as estimated by quantitative-PCR of 16S rRNA genes, was less than twofold in soils demonstrating a rapid respiratory response (i.e. peak within 6-8 h of initiating incubation) to up to fourfold in soils demonstrating a slower respiratory response (i.e., peak response after 14 h of incubation). Increased respiratory response with N amendment was usually associated with increased cell growth, although for rapidly responding soils some C sources showed N-limited use without growth. This was likely due to exhaustion of the relatively low levels of available C amendment before growth was detected. The method appears useful for assessing N-limited microbial growth, and it may be effective as a rapid indicator of bioavailable soil N. It may also be a tool to evaluate the complexity of N limitation among various metabolic pathways found in soil microbial communities, particularly if linked to dynamics in community structure and gene activation.Fil: Garland, J. L.. Dynamac Corporation; Estados UnidosFil: Zabaloy, Maria Celina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiarida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiarida; ArgentinaFil: Birmele, M.. Dynamac Corporation; Estados UnidosFil: Mackowiak, C. L.. University of Florida; Estados UnidosFil: Lehman, R. M.. North Central Agricultural Research Lab; Estados UnidosFil: Frey, S. D.. University Of New Hampshire; Estados Unido