Cloransulam-methyl (CM) is an herbicide which undergoes decreased extraction recovery during soil incubation, with up to 76% of applied material remaining as non-extractable residues (NER) after one year.  Herein, experiments were performed to examine the nature and plant toxicity of CM NER, to determine whether CM carbon can be assimilated by soil microorganisms, and whether the compound may be predisposed toward oxidative couple reactions, such as those mediated by peroxidases.  Reactions of <sup>14</sup>C-cloransulam-methyl with dissolved soil organic carbon in the presence of peroxidase produced radiolabeled high molecular weight residues (&gt; 5 kDa), suggesting oxidative coupling may be possible in soil.  Exhaustive extraction of aged residues recovered 7-21% of the applied radiocarbon as metabolites, with recovery being inversely correlated with soil organic carbon content.  Accumulation of NER, as well as formation of metabolites diminished in sterile controls.  A portion (up to 19%) of the non-extractable soil residue was associated with high molecular weight (&gt;5 kDa) material, and up to 6.5% of the non-extractable residue was in the form of amino acids, with nearly six times as much amino acid carbon recovered from the phenyl versus the pyrimidine ring, the latter exhibiting significantly greater mineralization.  Movement of metabolites into biologically inaccessible soil pools (via biological and non-biological mechanisms), may explain limited mineralization of the herbicide, despite a short half-life and susceptibility to microbial transformation.  The presence of NER was not associated with plant injury in a corn plant bioassay after extractable CM had been removed.  Results suggest that the phenyl ring system may be more prone toward coupling with soil organic matter or incorporation into microbial biomass, whereas the pyrimidine ring may be converted to energy or used as a nitrogen source.  Non-extractable residues of CM appeared not to cause plant injury within the context of the experiment.

Nature and bioavailability of non-extractable soil residues of the herbicide cloransulam-methyl

256th National Meeting (2018)

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