Distribution and location of polycyclic aromatic hydrocarbons (PAHs) and PAH-degrading bacteria within polluted soil aggregates

A study was conducted to determine the location and distribution of PAH and PAH-degrading bacteria in different aggregate size fractions of an industr ially polluted soil. The estimation of PAH-degrading bacteria using an MPN microplate technique indicated that these bacteria are most numerous in the aggregate size fractions corresponding to fine silt (2-20 mum) and clay (< 2 mum) compared to larger fractions or unfractionated soil. PAH concentrati ons were also highest in the aggregate size fraction corresponding to fine silt. Similar results were found in a spiked soil (incubated for 6 months) with similar carbonated minerals. Transmission electron microscopy observat ions showed that the autochtonous PAH-degrading bacteria were embedded in t he aggregates where PAHs were abundant. In spite of this extensive co-local isation PAH degradation was limited during 6 months incubation. This indica tes that factors other than spatial distribution and PAH degrading ability control degradation rates. The fine silt fraction of the industrial soil ha d an elevated C/N ratio (35) compared to the clay fraction (C/N: 16). Thus the fraction which assumably had the highest specific surface area containe d less PAH but similar numbers of PAH-degraders. N thus seem to play an imp ortant role in the long term, but as PAH degradation was low in fine size f ractions, other sources/factors were probably limiting (easily degradable C , P org, O-2 etc.). Based on these findings, soil particle organization and structure of soil aggregates appear to be important for the characterizati on of a polluted soil (localization and sequestration). Manipulations that modify aggregation in polluted soils could thus potentially influence the a ccessibility and biodegradability of PAHs.