Nutritional constraints to degradation of polycyclic aromatic hydrocarbons in a simulated rhizosphere

An industrially polluted soil (3.1 g PAH kg−1) was used to study C, N and P limitations to dissipation of polycyclic aromatic hydrocarbons (PAHs) in a model rhizosphere. Soil columns (50 cm3) were packed with soil and percolated daily with artificial root exudates (ARE) at 100 μg C g−1, 1 mM NH4NO3 or 1 mM NaH2PO4, or any double or triple combination of these for 30 days. At harvest, soil was analyzed for PAHs, and the numbers of total heterotrophic and PAH degrading bacteria estimated by an MPN technique. Dissipation of 3 and 4 ring PAHs was highest in soil receiving ARE+N, followed by the treatments with N+P and ARE+N+P. The complete treatment ARE+N+P had the highest dissipation of 5-ring PAHs. For both 5 and 6 ring PAHs most treatments resulted in increased concentrations at the end of the experiment, which was attributed to desorption of initially unextractable molecules. PAH degrading bacteria reached the highest numbers in the ARE+N+P treatment, followed by the treatment receiving only P, whereas total number of heterotrophs was elevated in the treatments ARE, P, ARE+N and ARE+N+P. Net PAH dissipation in soil was seemingly due to the two counteracting processes biodegradation and desorption. In the presence of ARE, the dissipation of higher molecular weight PAHs apparently involved microbial degradation and/or biotransformation through co-metabolism.