Unraveling microbial turnover and non-extractable residues of bromoxynil in soil microcosms with 13C-isotope probing

Abstract Bromoxynil is a widely used nitrile herbicide applied to maize and other cereals in many countries. To date, still little is known about bromoxynil turnover and the structural identity of bromoxynil non-extractable residues (NER) which are reported to occur in high amounts. Therefore, we investigated the microbial turnover of 13 C-labeled bromoxynil for 32 days. A focus was laid on the estimation of biogenic NER based on the turnover of 13 C into amino acids (AA). At the end, 25% of 13 C 6 -bromoxynil equivalents were mineralized, 2% assigned to extractable residues and 72.5% to NER. Based on 12% in the 13 C-total AA and an assumed share of AA of 50% in microbial biomass we arrived at 24% of total 13 C-biogenic NER. About 33% of the total 13 C-NER could thus be explained by 13 C-biogenic NER; 67% was unknown and by definition xenobiotic NER with potential for toxicity. The 13 C label from 13 C 6 -bromoxynil was mainly detected in the humic acids (28.5%), but significant amounts were also found in non-humics (17.6%), fulvic acids (13.2%) and humins (12.7%). The 13 C-total amino acids hydrolyzed from humic acids, humins and fulvic acids amounted to 5.2%, 6.1% and 1.2% of 13 C 6 -bromoxynil equivalents, respectively, corresponding to total 13 C-biogenic NER amounts of 10.4%, 12.2% and 2.4%. The humins contained mostly 13 C-biogenic NER, whereas the humic and fulvic acids may be dominated by the xenobiotic NER. Due to the high proportion of unknown 13 C-NER and particularly in the humic and fulvic acids, future studies should focus on the detailed characterization of these fractions.