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

Bromoxynil is a widely used nitrite 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 C-13-labeled bromoxynil for 32 days. A focus was laid on the estimation of biogenic NER based on the turnover of C-13 into amino acids (AA). At the end, 25% of C-13(6)-bromoxynil equivalents were mineralized, 2% assigned to extractable residues and 72.5% to NER. Based on 12% in the C-13-total AA and an assumed share of AA of 50% in microbial biomass we arrived at 24% of total C-13-biogenic NER. About 33% of the total C-13-NER could thus be explained by C-13-biogenic NER; 67% was unknown and by definition xenobiotic NER with potential for toxicity. The C-13 label from C-13(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 C-13-total amino acids hydrolyzed from humic acids, humins and fulvic acids amounted to 5.2%, 6.1% and 1.2% of C-13(6)-bromoxynil equivalents, respectively, corresponding to total C-13-biogenic NER amounts of 10.4%, 12.2% and 2.4%. The humins contained mostly C-13-biogenic NER, whereas the humic and fulvic acids may be dominated by the xenobiotic NER. Due to the high proportion of unknown C-13-NER and particularly in the humic and fulvic acids, future studies should focus on the detailed characterization of these fractions. (C) 2018 Elsevier Ltd. All rights reserved.