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This paper describes the validation and application of a simple flask-based (14)C-respirometer system designed to assess mineralisation of (14)C-labelled substrates under defined conditions. Validation of this respirometer system indicated stoichiometric CO(2) trapping up to a maximum of 400 micromol of CO(2) (in a single trap). Polycyclic aromatic hydrocarbon (PAH)-degrading bacteria were used to measure growth-linked biodegradation of [(14)C]naphthalene to (14)CO(2). A (14)C activity balance of 101.7+/-8.9% (n=6), after 74 h incubation time and 10 respirometer-opening events, indicated the suitability of the system for monitoring substrate mineralisation. This respirometric apparatus was then successfully applied to assess: (i) the PAH catabolism of microbes in a field contaminated soil, where naphthalene and phenanthrene were rapidly mineralised and (ii) soil-associated organic contaminant bioavailability, where increased soil-phenanthrene contact time resulted in a reduction in phenanthrene mineralisation in the soil. The described respirometer system differs from existing respirometer systems in that the CO(2) trap can be removed and replaced quickly and easily. The system is efficient, reproducible, adaptable to many situations, easy to construct and simple to use, it therefore affords advantages over existing systems.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants frequently associated with light non-aqueousphase liquids (LNAPLs) in soil. Microbial degradation comprises a major loss process for PAHs in the environment. Various laboratory studies, using known degraders, have shown reduced or enhanced mineralisation of PAHs when dissolved in different LNAPLs. Effects due to the presence of LNAPLs on indigenous micro-organisms, however, are not fully understood. A pristine pasture soil was spiked with [14 C]phenanthrene and transformer oil to 0, 0.01 and 0.1%, and incubated for 180 days. The catabolic potential of the soil towards phenanthrene was assessed periodically during ageing. The extent of the lag phase (prior to s 5% mineralisation), maximum rates and overall extents of mineralisation observed during the course of a 14-day bioassay appeared to be dependent upon phenanthrene concentration, the presence of transformer oil, and soil^contaminant contact time. Putatively, transformer oil enhanced acclimation and facilitated the development of measurable catabolic activity towards phenanthrene in a previously uncontaminated pasture soil. Exact mechanisms for the observed enhancement, longer-term fate/degradation of the oil and residual phenanthrene, and effects of the presence of the oil on the indigenous microbes over extended time frames warrant further investigation.

Subsurface high voltage electric cables are commonly insulated using dodecylbenzene in combination with mineral oil. This work assessed the impact of increasing concentrations of cable insulating oil (0-10% dry weight) on soil microbial respiration as determined by mineralisation of [1-(14)C]glucose (11 microg C g(-1) soil). Acute impact was assessed from 0 days to 21 days, and chronic impact was assessed after 300 days. This study found that cable insulating oil increased respiratory activity of soil microflora. The extent of impact was found to depend on both oil concentration and the length of oil-soil contact time. Following acute exposure (21-days oil-soil contact time), it was found that oil concentrations up to 1% promoted a significant (P<0.05) increase in the extent of [1-(14)C]glucose mineralisation to (14)CO(2) relative to the control. In contrast, higher concentrations of cable insulating oil (5% and 10%) promoted no significant (P0.05) increase in the extent of [1-(14)C]glucose mineralisation to (14)CO(2) relative to the control. Following chronic exposure (300-days oil-soil contact time), the extent of mineralisation was greater at all oil concentrations applied relative to the control. For oil concentrations up to and including 1%, there was a decrease in the extent of elevation in mineralisation relative to the values after 21-days exposure. At higher oil concentrations, namely 5% and 10%, the extent of elevation in mineralisation was comparable with that after 21-days oil-soil contact time. We suggest that the increase in mineralisation of glucose indicates that cable insulating oil is a readily available carbon source to the carbon-limited soil microflora.

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