Mr Christian Krohn1, Dr. Jian Jin1, Dr. John Ryan2, Dr. Piotr Fabijański2, Assoc. Prof. Ashley Franks1, Prof. Caixian Tang1
1La Trobe University, Melbourne, Australia, 2Department of Job, Precincts and Regions, Australia
The internationally banned agricultural insecticides dieldrin and dichlorodiphenyltrichloroethane (DDT) continue to exceed government thresholds in some Australian surface soils (0 – 10 cm) – 30 years after their last use. Their extreme persistence in soils is believed to be governed by sorption to soil organic matter (SOM), hence their existence in SOM-rich, high-value pastures limits land use options as grazing animals accumulate residues in their fat. To enhance remediation, more knowledge is needed of soil factors and microbial community dynamics involved in microbial in-situ degradation of these man-made organochlorine chemicals. Furthermore, while DDT-degrading strains have been successfully isolated in the past, there are no known microbial species with substrate specificity to dieldrin. A total of 12 contaminated paddocks with records of dieldrin and DDT concentrations dating from 1980 were sampled again in 2017. We hypothesised that SOM is a key factor affecting microbial biomass and diversity which would in turn affect biodegradation and total loss of the pollutants after 30 years. Correlations between total loss and current concentrations of dieldrin and DDT residues and soil physicochemical measurements, microbial biomass carbon, microbial community diversity indices and microbial community abundance were analysed. Low C:N ratios of SOM, high microbial biomass and high fungal community evenness correlated with an increased loss of dieldrin after 23 – 30 years, but sorption to SOM and clay likely inhibited further degradation. This indicated that co-metabolism of dieldrin and DDT could be enhanced by manipulating the quality of SOM to cater for a broad microbial functional diversity. Future culture studies will help to confirm whether microbial metabolism in these soils has evolved to utilise dieldrin or DDT as a primary carbon source, which taxa are involved and what role SOM composition plays in degradation of these pollutants.