Functional and structural responses of soil N-cycling microbial communities to the herbicide mesotrione: a dose-effect microcosm approach
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Microbial communities driving the nitrogen cycle contribute to ecosystem services such as crop production and air, soil, and water quality. The responses to herbicide stress of ammonia-oxidizing and ammonia-denitrifying microbial communities were investigated by an analysis of changes in structure-function relationships. Their potential activities, abundances (quantitative PCR), and genetic structure (denaturing gradient gel electrophoresis) were assessed in a microcosm experiment. The application rate (1 × FR, 0.45 μg g−1 soil) of the mesotrione herbicide did not strongly affect soil N-nutrient dynamics or microbial community structure and abundances. Doses of the commercial product Callisto® (10 × FR and 100 × FR) or pure mesotrione (100 × FR) exceeding field rates induced short-term inhibition of nitrification and a lasting stimulation of denitrification. These effects could play a part in the increase in soil ammonium content and decrease in nitrate contents observed in treated soils. These functional impacts were mainly correlated with abundance shifts of ammonia-oxidizing Bacteria (AOB) and Archaea (AOA) or denitrifying bacteria. The sustained restoration of nitrification activity, from day 42 in the 100 × FR-treated soils, was likely promoted by changes in the community size and composition of AOB, which suggests a leading role, rather than AOA, for soil nitrification restoration after herbicide stress. This ecotoxicological community approach provides a nonesuch multiparameter assessment of responses of N-cycling microbial guilds to pesticide stress.
KeywordsHerbicide Soil microcosms Ammonia oxidizers Denitrifiers Microbial ecotoxicology
This work was supported by a Fellowship from the French Ministère de l’Education, de la Recherche et de la Technologie. We thank S. Leininger for providing plasmids with bacterial—amoA and archaeal—amoA DNA inserts used for qPCR standards and to G. Borrel for providing the plasmids containing archaeal—16S rDNA genes used for qPCR standards. We would also like to thank J. Watts, English native translator, for the English review of this paper.
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