Abstract
The effects of crude oil on soil nitrogen dynamics and cycling in plant-soil ecosystems and its effect on the growth of legumes (Calopogonium mucunoides, Centrosema pubescens and Pueraria phaseolodes) grown in wetland ultisols were investigated. The test plants species were grown on wetland soil simulated with 0.35, 10.8, 20.5, and 50 g.kg-1levels of crude oil contamination. The results showed time and species dependent variation in mineral N content of the treated soils. The variation is indicative of significant interaction between the hydrocarbon content and plant species. Variations in microbial N and microbial C were similar and correlation between the microbial N and the total C (Organic matter (C) + hydrocarbon content (C)) in soil was highly significant (r = 0.96, n = 12, P ≤ 0.01). The presence of hydrocarbon contaminant widens the C:N ratio in soil and leads to more available N being immobilized by soil microorganisms, which reduces available N for plant uptake. This result implies that crude oil contamination significantly reduces N uptake by plants but increases N accumulation in soil microbial biomass. The findings show that N dynamics, transformation and cycling in soil are influenced by hydrocarbons and that the interactions between hydrocarbon content and plant species in contaminated soil are remarkable. The use of plant Centrosema pubescens with poultry manure or NPK fertilizer for bioremediation is more effective than that of Calopogonium mucunoides and Pueraria phaseoloides. However, the selective attributes of the various treatment approaches adopted here may be exploited for enhanced remediation of contaminated wetlands in the Niger Delta region of Nigeria.
Highlights
The Niger Delta terrestrial and aquatic systems are generally the main recipients of crude oil spillages, sometimes resulting in large scale contamination of the environment
In spite of all the harmful effects widely reported, [6] [7] have revealed improved growth of tropical legumes grown on soil contaminated with crude oil in wetland soil. [8] noted that organic matter of soils polluted with crude oil increased through the activities of nitrogen-fixing microorganisms
The prevalent species of legumes Calopogonium mucunoides, Centrosema pubescens and Pueraria phaseoloides found in the wetland sites were selected for the study
Summary
The Niger Delta terrestrial and aquatic systems are generally the main recipients of crude oil spillages, sometimes resulting in large scale contamination of the environment. The approach that has been exploited most consists of stimulation of the soil indigenous microflora by adding an electron acceptor and/or nutriments, especially nitrogen in the form of ammonium salts [2]-[4]. This nitrogen source is exploited mainly by microbial biomass for growth and production of pollutants-degrading enzymes. Nitrogen dynamics and cycling play important roles in bioremediation and reclamation of hydrocarbon-contaminated soils, because nitrogen is an essential element for microbial activity and plant growth. [8] noted that organic matter of soils polluted with crude oil increased through the activities of nitrogen-fixing microorganisms. The nitrogen-fixing capacity of legumes stabilizes the soil nitrogen and organic carbon content in the rhizosphere than non-rhizosphere soils [9]
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