Using Nitrogen and Phosphorus to Stimulate Microbial Degradation of Diesel Oil in Four Ghanaian Soils

Abstract

The amendment of oil-contaminated soils with nutrient elements, especially nitrogen and phosphorus, is a crucial factor for the enhancement of the growth of the indigenous microbes involved in the biodegradation of petroleum hydrocarbons. The present study investigated the(i) effects of nitrogen and phosphorus on diesel oil degradation in four Ghanaian soils, Beach sand (Haplic Arenosol), Toje series (Rhodic Kandiustalfs), Nyankpala series (Plinthic Lixisol), and Oda series (Eutric Gleysol), and (ii) germination and growth of maize and cowpea in diesel oil contaminated soil amended with nitrogen and phosphorus. The soils were contaminated with diesel oil at 10 g oil/kg soil and amended with single super phosphate (P 2 O 5 ) and ammonium nitrate (N) at 0, 30, 60 and 90 kg/ha separately. The treated soils were incubated in the laboratory under room temperature and sampled at 10 days interval for 40 days to monitor the growth of hydrocarbon utilizing bacteria (HUB) and the amount of oil degraded. In the plant culture study, 2 kg of the Toje series was contaminated with diesel oil at 0, 2.5, 5 and 10 g oil/kg soil. The contaminated and uncontaminated soils were amended with N and P, each at a rate of 60kg/ha and maize and cowpea were grown 20 days after contamination. Results showed thatsupplementation of soil with N and P stimulated the growth of HUB and enhanced degradation of diesel oil in the four soils. Increasing the application rate of N and P enhanced both HUB growth and degradation of oil. Although the initial HUB populations of Beach sand and Nyankpala series were lower thanthat of Toje and Oda series, they showed better degradation of the oil. Besides, the HUB populations of the Beach sand and Nyankpala series reached their peaks earlier than Toje and Oda series. The rates of biodegradation of oil in the soils were in the order Beach > Nyankpala > Toje > Oda. These results suggest that (i) the Beach sand and Nyankpala series could contain highly effective HUB useful for the rapid bioremediation of oil-contaminated soils and (ii) HUB populations whose growth peaks early during bioremediation could be more effective degraders than those that peak later. Results also showed that diesel contamination had adverse impact on nodulation of cowpea and growth of both maize and cowpea. However, addition of nitrogen and phosphorus to the oil contaminatedsoils improved germination, shoot dry matter and nutrient uptake. The effect of nitrogen supplementation in the contaminated soils was pronounced in maize whilst that of phosphorus was pronounced in cowpea. This implies that nutrient elements are important for the growth of crops in oil contaminated soils and nutrient elements are specific to crops grown in oil contaminated soils. Further research should be conducted to isolate and identify effective HUB from Ghanaian soils especially from the Beach sand.