Nitrogen Transformation In Sewage-Sludge Amended Soil As Influenced By Addition Of Zinc, Cadmium And Nickel

An incubation experiment was conducted to study the effect of Zn, Cd and Ni alone and in combination on the N transformation in loamy-sand soil amended with sewage-sludge. Nitrification decreased significantly at Zn, Cd and Ni concentration > 1.5, greater than or equal to 0.25 and greater than or equal to 1.5 meq/kg, respectively at the end of 16 weeks incubation. However, the decrease was more marked during later period of incubation. Ammonification was affected at much higher rates of metal addition. The threshold metal concentration for 5% decrease in the total mineralized N was 6 (196.1 ppm), 0.25 (14.1 ppm) and 6 (176.1 ppm) meq/kg Zn, Cd and Ni, respectively. The effect of Zn and Ni on the N transformation was comparable, whereas Cd was 6-8 and 4-6 times more inhibiting than Zn and Ni. Addition of metals combinedly caused severe reduction in the nitrification and ammonification. The effect was additive on the nitrification up to 6, 1 and 6 meq/kg Zn, Cd and Ni, respectively when used in combination. Further increase in the metal concentration resulted in mon than additive inhibitory effect except the 9, 1.5 and 9 meq/kg Zn, Cd and Ni, respectively showing less than additive effect when combinedly added. Addition of metals combinedly caused 9.5-86.1 and 14-44.2% decrease in the nitrification and ammonification, respectively after 16 weeks of incubation.