Abstract

The effect of heavy metals (cadmium (Cd), nickel (Ni) and lead (Pb)) on organic carbon mineralization (OCM) was studied in the laboratory. Top soil (Typic Paleudult) containing 1.0 g organic carbon in the form of milled dry Cajanus cajan leaves was incubated at field capacity with heavy metals (Cd, Ni and Pb) at four levels (0, 100, 200 and 300 mg/kg soil) for 30 days in a 3 × 4 factorial experiment. Each soil contained one level and one type of heavy metal. Carbon dioxide-carbon evolved was measured every five-day period. The quantity of CO₂-C was highest in the first five-day period of incubation than the subsequent periods. All the rates of the heavy metals studied gave significantly (P < 0.05) lower mean periodic CO₂-C than the control. There was significant (P < 0.05) metal × rate interaction. With the exception of Ni, when applied at 100 mg/kg soil, application of heavy metals at all the rates studied, led to lower cumulative CO₂-C as compared to the control. Elovich kinetic model satisfactorily described the rate of organic carbon mineralization in these soils as evidenced from the high correlation coefficient (r) values. The heavy metals in the concentrations applied were found to decrease the organic carbon mineralization rate (OCMR) over the control. The mean OCMR ranged from 194.46 mg CO₂-C per kg soil in the control to 145.28 mg CO₂-C per kg soil, in the soils with heavy metal at 300 mg/kg soil. The order of the ability of the heavy metals to decrease OCMR was found to be Cd>Ni>Pb. Since OCMR is an index of soil productivity, it was therefore concluded that heavy metals decreased soil productivity.

Key words: Heavy metals, soil productivity, organic carbon mineralization.