[en] Metamitron and its main metabolite desamino-metamitron are frequently detected in surface waters. To date, there are no studies targeting metamitron degradation in water-sediment systems. Therefore, the aim of this study was to trace the fate of metamitron in a water-sediment system using ¹³C-isotope labeling. Mineralization of metamitron was high and accounted for 49% of ¹³C₆-metamitron equivalents at the end. In contrast, only 8.7% of ¹³C₆-metamitron equivalents were mineralized in the water only system demonstrating the key role of sediment for biodegradation. Metamitron disappeared from the water on day 40 and was completely removed from the sediment on day 80. This agrochemical was utilized as carbon source by microorganisms as shown by the incorporation of the ¹³C label into microbial amino acids and finally into biogenic residues. The latter amounted to 24% of ¹³C₆-metamitron equivalents at the end. However, 17% of ¹³C₆-metamitron equivalents were detected in xenobiotic non-extractable residues (NER) with a release potential and delayed risk for the environment. Metamitron was degraded via two pathways, initially via 4-(dimethylimino)-3-methyl-6-phenyl-1,2,4-triazin-5(4H)-one, which might be related to growth, and later via desamino-metamitron, which can be attributed to starvation. - Highlights: • First study on ¹³C₆-metamitron turnover mass balance in water-sediment • Biogenic residues make a great contribution to non-extractable residues formation. • Small portion of ¹³C₆-metamitron is assigned to xenobiotic non-extractable residues. • Metamitron is biodegraded via two pathways.