A potential link between the turnover of soil organic matter and the release of aged organic contaminants.

Soil column experiments were conducted to investigate the release of aged organic contaminants (adsorbable organohalogens, [AOX]) and the potential effects of microbial turnover of soil organic matter onto the contaminant release from two different soils of former wastewater infiltration sites. Under reference conditions, the total carbon flux ranged from 6 to 9 mg/kg/d from the two soils. More than 92% of it was due to mineralization (CO2) and below 8% to the dissolved organic carbon (DOC) in the column effluent. Despite very different organohalogen levels in the two soils, the fluxes of AOX were similar (1 microg/kg/d). Microbial activity was altered by changing temperature and the solution pH and by drying. In all experiments where increased microbial respiration (CO2 production) was observed, the release of organic contaminants in the column effluent also increased. Fluxes of nickel, zinc, and phosphate showed a completely different pattern, suggesting that their retention in soil is determined by chemical parameters. A linear correlation analysis provided clear evidence that AOX and DOC fluxes are closely linked to each other. However, the superimposition of abiotic and biotic processes in the soils did not allow a causal relation to be established between the mineralization of soil organic matter and the release of organic contaminants sequestered in the soil. Our findings suggest that microbial activity may have a significant impact on the extent to which contaminated soils may act as a source of persistent organic pollutants.