The Drilosphere as a Driver of Microbial Metabolism of Herbicides in Soil: Linking Processes with Populations

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Herbicides are widely used in agriculture. Degradation of phenoxyalkanoic acid (PAA) herbicides and their derivatives is catalyzed in the drilosphere (i.e., earthworm gut content, cast, and burrows). Burrow walls represent interfaces in soil with up to 1.2 m2 m-3 and are ‘hotspots’ for the aerobic degradation of PAA herbicides. Earthworm eco-types (i.e., epigeic, anecic, or endogeic species) might impact on the intensity of burrow formation and thus herbicide degradation. However, the effect of earthworm eco-type on herbicide degradation is unknown. Degradation pathways of many herbicides, associated structural genes, and associated organisms in the drilosphere, are poorly resolved. Based on these deficiencies of information, the following hypotheses will be tested: (i) the drilosphere harbors hitherto unknown, highly active prokaryotes that transform the herbicide bentazon and (ii) the earthworm eco-type impacts on PAA, 2,4-dichlorophenol, and bentazon degradation. Degradation intermediates and products will be determined by high performance liquid chromatography and scintillation counting. Subtractive transcriptome analysis and 454 pyrosequencing will identify structural genes involved in herbicide degradation. 16S rRNA stable isotope probing, quantitative structural gene-targeted PCR, most probable number analyses, and isolation approaches will be utilized to identify and characterize process-associated microbial populations.