. Assessment of microbial community function and structure in soil microcosms exposed to glyphosate.

ZABALOY MC; GARLAND J; GOMEZ M

Villa Carlos Paz, Córdoba

Congreso; VI CONGRESO ARGENTINO DE MICROBIOLOGÍA GENERAL; 2009

SAMIGE

Glyphosate is widely used in Argentina to control weeds in glyphosate-resistant soybean and in no-till management systems. Glyphosate- resistant crops can be treated post-emergence with glyphosate without killing the crop. Thus, these systems are often subjected to repeated use of glyphosate. This herbicide inhibits the shikimic acid pathway which leads to synthesis of aromatic amino acids in plants and microbes. The herbicide may change the soil environment due to toxicity to soil microorganisms or through the influx of carbon (C), phosphorus (P) and nitrogen (N) in the form of glyphosate or plant residues, which would support rapid growth of opportunistic microbes. The aim of this work was to study changes in soil functional and structural diversity due to two different glyphosate doses in laboratory incubations. We also assessed if glyphosate mineralization occurs within the incubation period, and whether this releases N and/or P to the soil nutrient pools. The experiment consisted of soil microcosm exposed to low and high rates of herbicide (LG= 15 mg kg-1, HG= 150 mg kg-1) or un-treated (Control= 0 mg kg-1), and incubated at 28oC for 7 d. The BD Oxygen Biosensor System (BDOBS) is a microplate platform with an oxygen sensitive fluorophore that allows monitoring O2 consumption in soil slurries amended with the researcher‟s choice of substrates, nutrients, etc. We used BDOBS as a community-level physiological profiling (CLPP) tool to examine herbicide-derived changes in functional diversity. Microbial community structure was analyzed using quantitative PCR (qPCR) and terminal restriction fragment length polymorphism (T-RFLP), in order to quantify bacterial and fungal numbers as well as to detect shifts in community composition in the assay conditions. CLPP was slightly modified by HG treatment, resulting in decreased use of coumaric acid, while glyphosate itself was not metabolized as a C source in soil slurries. Moreover, there is no indirect evidence that glyphosate mineralization occurs in soil in less than 7 d, as we did not detect significant increases in microbe-available N or P in soil slurries. Cluster analysis of T-RFLP data showed that Control microcosms grouped together but there was no pattern for glyphosate treatment, which appeared to decrease the predictability of community structure. Q-PCR analysis showed that bacterial counts almost doubled in glyphosate-treated microcosms with no change in fungal abundance. Summarizing, the function of the community was slightly impacted, but not due to mineralization of the glyphosate itself. Bacteria biomass responded to glyphosate treatment, but consistent selective enrichment for specific bacteria species did not appear to occur.