Impact of Chronic Crude Oil Pollution on Nitrification in Edaphic Systems

Abstract

Nitrification plays a key role in regulating soil fertility, concentration of soil inorganic nitrogen and production of the highly potent greenhouse gas, nitrous oxide, from soil. This study investigated the effect of long-term crude oil pollution on the activity, abundance and diversity of autotrophic nitrifying bacteria in impacted soils from Bomu and Abonema in the Nigerian Niger Delta compared to unpolluted systems. Nitrifier activity was investigated by determining the soil oxidation potentials while the most probable number microtechnique using Winogradsky media was used to ascertain nitrifying bacteria abundance and soil DNA analysis alongside plate cultures defined nitrifying bacterial diversity. Nitrifier activity was found to be higher in the polluted soils than in the unpolluted soils while quantification of the nitrifying bacteria revealed that polluted samples had higher nitrifier counts. Statistically significant differences existed amongst nitrifier counts and activity (p≤0.05) between polluted and unpolluted systems with positive correlation between nitrifying bacteria counts and oxidation potential. The results highlighted the dominance of cultivable Nitrosomonas and Nitrobacter in both polluted and unpolluted soils while DNA analysis revealed that Nitrospira was the dominant nitrifier genus in unpolluted soils and Nitrobacter in polluted soils. Nitrospira was not detected the polluted ecosystems. No ammonia oxidisers were detected via metagenomics analysis. The ammonia oxidisers proved to be more sensitive to chronic crude oil pollution than the nitrite-oxidising bacteria. Long term crude oil exposure produced a 3 – 5 times increase in the activity of nitrifying bacteria in polluted soils with approximately twice the abundance when compared to the unpolluted systems.