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Degradation of polycyclic aromatic hydrocarbons (PAHs) and survival of bacteria in soil was investigated by applying different inoculation protocols. The soil was inoculated with Sphingomonas paucimobilis BA 2 and strain BP 9, which are able to degrade anthracene and pyrene, respectively. CFU of soil bacteria and of the introduced bacteria were monitored in native and sterilized soil at different pHs. Introduction with mineral medium inhibited PAH degradation by the autochthonous microflora and by the strains tested. After introduction with water (without increase of the pore water salinity), no inhibition of the autochthonous microflora was observed and both strains exhibited PAH degradation.

Experiments were carried out to evaluate the impact of the addition of ripe compost on the degradation of two 14C-labeled hydrocarbon model compounds (anthracene and hexadecane) in soil. The addition of mature compost (20% dry wt./dry wt) stimulated significantly the disappearance of the extractable fraction of both compounds. With compost, 23% of the labeled anthracene was transformed into 14CO2 and 42% was fixed to the soil matrix irreversibly. In the unsupplemented control reactor, more than 88% of the original anthracene could be recovered by either of two organic extraction procedures. THe formation of non-extractable bound residues was significantly less with 14C-hexadecane since only 21% of the labeled carbon had become non-extractable after 103 days. The results presented show that compost could stimulate the depletion of hydrocarbons by either mineralization or the formation of unextractable bound residues (humification). The latter process might be a significant route of depletion in soil especially, for those hydrocarbons that are mineralized only slowly. The meaning of this finding for the assessment of soil bioremediation is discussed.