Cowpea Induced Physicochemical and Biological Rhizosphere Changes in Hydrocarbon Contaminated Soil

Abstract Purpose To understand the influence of cowpea on its rhizosphere physicochemical and biological conditions. Methods Pristine soil samples were contaminated with Bonny-Light crude oil and viable seeds of cowpea were planted to establish rhizosphere soil. Cowpea root exudates were collected and characterized while soil metabolic activities, physicochemical properties and rhizosphere effect were monitored following plant emergence. Results Cowpea root exudates were composed of organic acids, phenolics, carbohydrates and hydrocarbons. High rate of soil respiration and microbial biomass carbon were observed in the contaminated rhizosphere reaching its peak on 12th week (70.56 mgCO2g−1day−1) and 10th week (23.18mg/Kg) respectively. Lower rates of soil respirations and microbial biomass carbon were observed in contaminated (10.28 mgCO2g−1day−1; 1.24 mg/Kg) and uncontaminated (0.23 mgCO2g−1day−1; 0.37 mg/Kg) non-rhizosphere control soils respectively. The metabolic properties were positively correlated with soil organic matter contents and microbial size (r = 0.98; p < 0.05). There was considerable improvement in soil physicochemical properties in the cowpea rhizosphere as compared to non-rhizosphere soil Microbial populations were generally improved with positive rhizosphere effect values (>1) presumably due to the presence of compounds in exudates that promote microbial growth. Conclusion The results highlighted the influence of cowpea on its rhizosphere conditions which is a good indication for its ability to promote plant growth and environmental cleanup. Therefore, there is the need to further understand the microbial community dynamics in cowpea rhizosphere using culture-independent techniques.