Emission of nitrous oxide from hydrocarbon contaminated soil amended with waste water sludge and earthworms

Soils in Mexico are often contaminated with hydrocarbons and addition of waste water sludge and earthworms accelerates their removal. However, little is known how contamination and subsequent bioremediation affects emissions of N2O and CO2. A laboratory study was done to investigate the effect of waste water sludge and the earthworm Eisenia fetida on emission of N2O and CO2 in a sandy loam soil contaminated with the polycyclic aromatic hydrocarbons (PAHs): phenanthrene, anthracene and benzo(a)pyrene. Emissions of N2O and CO2, and concentrations of inorganic N (ammonium (NH4+), nitrite (NO2−) nitrate (NO3−)) were monitored after 0, 5, 24, 72 and 168 h. Adding E. fetida to the PAHs contaminated soil increased CO2 production rate significantly 2.0 times independent of the addition of sludge. The N2O emission rate from unamended soil expressed on a daily base was 5 μg N kg−1 d−1 for the first 2 h and increased to a maximum of 325 μg N kg−1 d−1 after 48 h and then decreased to 10 μg N kg−1 d−1 after 168 h. Addition of PAHs, E. fetida or PAHs + E. fetida had no significant effect on the N2O emission rate. Adding sludge to the soil sharply increased the N2O emission rate to >400 μg N kg−1 d−1 for the entire incubation with a maximum of 1134 μg N kg−1 d−1 after 48 h. Addition of E. fetida, PAHs or PAHs + E. fetida to the sludge-amended soil reduced the N2O emission rate significantly compared to soil amended with sludge after 24 h. It was found that contaminating soil with PAHs and adding earthworms had no effect on emissions of N2O. Emission of N2O, however, increased in sludge-amended soil, but addition of earthworms to this soil and contamination reduced it.