Influence of farmyard manure on some morphological and biochemical parameters of cowpea (Vigna unguiculata) seedling grown in cadmium-treated soil.

The present study aims to assess the effects of the two kinds of farmyard manure (poultry and pig manures) as amendments for soil on cadmium (Cd) toxicity in plants using cowpea seedlings as plant model. Cd toxicity was evaluated by assessing the effect of the metal on the growth rate and antioxidant status as well as the ability of the plant to metabolise xenobiotic. There was a significantly (p < 0.05) increased concentration of Cd in the root, stem and leaves of cowpea seedlings grown in all the treated soils relative to control. Addition of poultry manure to the soil significantly (p < 0.05) decreased the level of Cd in these component parts of the seedlings and their corresponding bioaccumulation factor in a dose-dependent manner as compared with treatments with Cd pollution without manure addition and Cd pollution with pig manure addition. There was restoration of Cd-induced effect on growth rate parameters to levels comparable to controls in cowpea seedlings grown in Cd-treated soil augmented with poultry manure but not in cowpea seedlings in cadmium-treated soil with pig manure amendments. Similarly, augmentation of Cd-treated soil with pig manure did not alter the Cd-induced effect on the levels of superoxide dismutase (SOD) and lipid peroxidation (LPO) in leaf, stem and roots, as SOD remained significantly (p < 0.05) decreased and LPO increased relative to control. On the other hand, the levels of SOD and LPO in these parts of cowpea seedlings grown in Cd-treated soils amended with poultry manure were restored to a level not significantly (p > 0.05) different from control. Like in the case of SOD, the Cd-induced inhibition of the activity of xenobiotic metabolising enzymes, aldehyde oxidase and sulphite oxidase remained significantly (p < 0.05) decreased in the organs of seedling grown in Cd-treated soil amended with pig manure. Conversely, the Cd-induced effect on the activities of these enzymes was reversed in the organs of seedlings exposed to Cd and poultry manure as it was not significantly (p > 0.05) different from control. In conclusion, the findings of the study revealed that supplementation of Cd-treated soils with pig and poultry manures reversed effects of Cd on cowpea seedlings. However, poultry manure was more effective than pig manure in ameliorating the effects of Cd.