Biochar reduced extractable dieldrin concentrations and promoted oligotrophic growth including microbial degraders of chlorinated pollutants.

The role of organic amendments for natural degradation of aged persistent organic pollutants (POPs) in agricultural soils remains controversial. We hypothesised that organic amendments enhance bacterial activity and function at the community level, facilitating the degradation of aged POPs. An incubation study was conducted in a closed chamber over 12 months to assess the effects of selected organic amendments on extractable residues of aged dieldrin. The role of bacterial diversity and changes in community function was explored through sequenced marker genes. Linear mixed effect models indicated that, independent of amendment type, cumulative CO release was negatively associated with decreases in dieldrin concentration, by up to 7% per µmol CO-C respired by microorganisms. The addition of poultry litter led to the highest daily carbon mineralisation, which was associated with low dieldrin dissipation after 9 months. In comparison, biochar resulted in significant decreases in extractable dieldrin residues over time, which coincided with shifts towards aerobic, oligotrophic, gram-negative bacteria, some with dehalogenation metabolism, and with increased potentials for biosynthesis of membrane components such as fatty acids and high redox quinones. The results supported an alternative theory that labile carbon promoted blooms of copiotrophic growth, which suppressed the required community-level traits and oligotrophic diversity to degrade chlorinated pollutants.