Organic stimulants for enhancing phytoremediation of crude oil polluted soil: A study on cowpea

Highlights: • Soil contamination with petroleum hydrocarbon inhibits cowpea shoot and yield production. • Pawpaw seed powder and moringa seed powder enhance cowpea phytoremediation potentials on crude oil polluted soil. • 150 g m^-2 pawpaw seed powder gave the highest (78.2%) removal rate of petroleum hydrocarbon from crude oil polluted soil. • Petroleum hydrocarbon may be translocated and accumulated in fresh pod yield of cowpea. Petroleum hydrocarbon (PH) contamination of soils remains a major threat to environmental health and food security. A two-years phytoremediation study was conducted on a crude oil polluted soil to assess changes in soil total petroleum hydrocarbon concentration (TPH_soil) following use of pawpaw seed powder (PSP), moringa seed powder (MSP) and their combination (PSP + MSP) as organic stimulants in cowpea cultivation. The stimulants were tested at different application rates (100, 150, 200 and 250 g m⁻²), with the control (No stimulant) for their effectiveness in reducing TPH_soil and accelerating the removal rate (R) of PH from soil. The TPH_soil did not differ significantly (p < 0.05) among the treatments in year 1, but was highest in the control (11,600 mg kg⁻¹) and least in 200 g m⁻² PSP (7400.0 mg kg⁻¹). In year 2, mean TPH_soil varied significantly (p < 0.05) and remained highest in control (7100 mg kg⁻¹) but lowest in 150 g m⁻² PSP (2700 mg kg⁻¹). Application of 150 g m^-2 PSP gave the highest R (78.2%), followed by 150 g m^-2 PSP+MSP (77.4%), and least by the control (42.7%) over two years of study. The average fresh pod yield of cowpea over two years was highest in 250 g m⁻² PSP (2416.67 kg ha⁻¹), followed by 150 g m⁻² PSP (2173.34 kg ha⁻¹) and least in control (1302.22 kg ha⁻¹). There was significant negative association between TPH_soil and fresh pod yield (r = −0.403; p < 0.01). However, application of 150 g m⁻² PSP appeared most effective for enhanced phytoremediation of crude oil polluted soil and improvement of cowpea yield.