Identification of degradation routes of metamitron in soil microcosms using 13C-isotope labeling

Metamitron is one of the most commonly used herbicide in sugar beet and flower bulb cultures. Numerous laboratory and field studies on sorption and degradation of metamitron were performed. Detailed biodegradation studies in soil using C-13-isotope labeling are still missing. Therefore, we aimed at providing a detailed turnover mass balance of C-13(6)-metamitron in soil microcosms over 80 days. In the biotic system, metamitron mineralized rapidly, and (CO2)-C-13 finally constituted 60% of the initial 1(13)C(6)-metamitron equivalents. In abiotic control experiments CO2 rose to only 7.4% of the initial C-13(6)-metamitron equivalents. The C-13 label from C-13(6)-metamitron was incorporated into microbial amino acids that were ultimately stabilized in the soil organic matter forming presumably harmless biogenic residues. Finally, C-13 label from C-13(6)-metamitron was distributed between the (CO2)-C-13 and the C-13-biogenic residues indicating nearly complete biodegradation. The parallel increase of C-13-alanine, C-13-glutamate and (CO2)-C-13 indicates that metamitron was initially biodegraded via the desamino-metamitron route suggesting its relevance in the growth metabolism. In later phases of biodegradation, the Rhodococcus route was indicated by the low (CO2)-C-13 evolution and the high relevance of the pyruvate pathway, which aims at biomolecule synthesis and seems to be related to starvation. This is a first report on the detailed degradation route of metamitron in soil. (C) 2016 Elsevier Ltd. All rights reserved.