Nitrogen fixing capacity of legumes and their Rhizospheral microflora in diesel oil polluted soil in the tropics

摘要

The nitrogen fixing capacity of legumes cowpea(Vigna unguiculata)and groundnut(Arachis hypogea)andtheir microflora grown in diesel oil simulated utisol was investigated. Result revealed that concentration as low as 1% v/w of diesel oil significantly affected the densities of nitrogen fixing bacteria, bacteriods, actinomycetes and fungi associated with the legumes. The heterotrophic bacteria count in the rhizosphere of cowpea reduced from 2.46 ± 0.72 × 107to 1.5 ± 0.37 × 107cfu/g after a growth duration of 12 weeks while it reduced from 3.4 ± 1.25 × 107to 1.52 ± 0.36 × 107cfu/g for groundnut in the same growth period. Nitrifying bacteria count reduced from 3.25 ± 1.19 × 104to 4.5 ± 0.18 × 103cfu/g for cowpea and 3.43 ± 1.23 × 104to 2.7 ± 0.21 × 103cfu/g for groundnut. Bacteriods count also significantly (P 0.05) reduced from 3.85 ± 2.30 × 105cfu/g for the control treatment to 1.25 ± 2.23 × 105cfu/g in 1% level of pollution with no bacteriod formed in both 4 and 8% pollution due to inhibition of nodule formation by the diesel oil. Significant reduction (P 0.05) was also observed in fungal and actinomycetes counts. Generally, organisms in the rhizosphere of groundnut exhibited more tolerance to diesel oil pollution than those found in the rhizosphere of cowpea. This study revealed that diesel oil adversely affected nitrogen fixing bacteria and bacteriods and consequently the nitrogen fixation in the soil.