Formation of humus-bound residues in the course of BTX biodegradation in soil

To examine whether the xylene component of BTX (benzene, toluene, xylene) mixture is cometabolized and residues are produced in soil, C-14-labeled-o-xylene was added to sandy loam in combination with unlabeled benzene and toluene. After 4 weeks of incubation in a sealed system connected to an oxygen reservoir, 55.1% of the radiocarbon was converted to (CO2)-C-14, 3.0% was solvent-extracted, and 33-35% was recovered by wet combustion of the extracted soil, adding up to 95.8% radiocarbon recovery. Biomass incorporation of o-xylene radiocarbon which was detected by fumigation/extraction was usually low (5.6%), but 32.1% radiocarbon became associated with soil humus. Most of the humus-bound radiocarbon was found in humin fraction. In addition to o-xylene, p-xylene and toluene also showed similar results. The evidence shows that some of their reactive methylcatechol biodegradation intermediates