Effect of N and P fertilisation and aeration on biodegradation of crude oil in aged hydrocarbon contaminated soils

We conducted two laboratory experiments to examine the effects of fertilisation and agitation (aeration) on crude oil degradation in two soils with differential nutrient (nitrogen, phosphorus) availability. Two soils that had been spiked with crude oil two years before were mixed with nitrogen and/or phosphorus at three different levels and subsequently incubated 28 days (Exp. 1). In experiment 2 we investigated the effect of repeated agitation (manual mixing) on hydrocarbon degradation with and without fertilisation. One of the soils was also freshly spiked to assess the impact of ageing. Heptane-extractable hydrocarbon concentrations were determined in both experiments and substrate-induced respiration in Exp. 2. The generally small changes of hydrocarbon concentrations during 28 days of incubation in Exp. 1 are likely attributed to low bio-accessibility of hydrocarbons as a consequence of long-term ageing. Fertilization of nitrogen, phosphorus or combinations thereof was ineffective in most treatments of Exp. 1, which may be explained by limited oxygen supply due to the high proportions of clay and silt. However, agitation enhanced HC biodegradation in the sandy-loamy soil by about 15% (Exp. 2) compared to non-agitated treatments. In contrast, we observed no effect of agitation in the sandy soil.